Ordering of charge/spin/orbital degrees of freedom in complex materials accompanies domains and domain walls associated with the directional variants (Z_m) and also antiphases (Z_n). It has been recently realized that nontrivial Z_mxZ_n topology can exists in large-scale real-space configurations of domains and domains walls of complex materials. Furthermore, the vertices where domain walls merge can be considered as topological defects with well-defined vorticities (Z_l vortices). We will discuss the recently-discovered examples of Z_mxZ_n domains and Z_l vortices in complex materials.

I will discuss a recently proposed form of hidden order -- the magnetoelectric monopole -- and its relationship to a material's magnetoelectric response. Using density functional calculations for the Li transition metal phosphate series, LiMPO₄, with M = Mn, Fe, Co and Ni, I will show that materials with the same overall antiferromagnetic ordering can have distinct ferromonopolar or antiferromonopolar orderings, that lead to different, and in principle measureable, magnetoelectric responses. The current status and open questions in both the theoretical formalism and experimental verification will be outlined.
Monopole-based formalism for the diagonal magnetoelectric response, N. A. Spaldin, M. Fechner, E. Bousquet, A. Balatsky and L. Nordstrom, Phys. Rev. B 88, 094429 (2013).
Magnetic field generated by a charge in a uniaxial magnetoelectric material, M. Fechner, N. A. Spaldin and I. E. Dzyaloshinskii, Phys. Rev. B 89, 184415 (2014).

Room temperature multiferroics has been a frontier research field by manipulating spin-driven ferroelectricity or charge-order-driven magnetism. A new family of charge-transfer crystals based on electron donor and acceptor assembly - exhibiting simultaneous spin ordering - are drawing significant interests for the development of all-organic magnetoelectric multiferroics. Here, we report that a remarkable anisotropic magnetization and room temperature multiferroicity can be achieved through assembly of thiophene donor and fullerene acceptor. The crystal motif directs the dimensional and compositional control of charge-transfer networks that could switch magnetization under external stimuli, thereby opening up a new class of all-organic nanoferronics.

Layered perovskites are emerging as an interesting player in the field of multiferroicity and beyond. Several examples of exotic behavior have been predicted [1-4] in the family A_nB_nO_3n+2, which exhibits a natural internal stacking in blocks of n octahedra along a (110) direction.
The n=4 case is usually a band insulator, frequently ferroelectric by way of a unusual uncompensated-rotation mechanism; magnetic doping or substitution thus offers an opportunity for multiferroicity. We exemplify this case with a) the weak ferromagnet La₂Mn₂O₇, with its anomalously large linear magnetoelectric coupling [2], and b) V-doped La₂Ti₂O₇ [2,3], a ferroelectric ferromagnet exhibiting magnetization inversion upon polarization inversion - the quintessential magnetoelectric effect.
The n=5 material is a strongly anisotropic metal which seems to sustain a polarization in some cases. Our showcase is the first known native ferroelectric metal, Bi₅Ti₅O₁₇, a n=5 system, which exhibits coexisting metallicity and spontaneous polarization, as well as -with some tweaking via delta doping still being investigated- a depolarizing field in a finite system [1]. (we are considering the possibility that this material may possibly turn out to be an example of hyperferroelectric.)
Finally, the layered structural pattern naturally lends itself to the combination of, for example, n=4 and n=5 materials to form functional. We are currently exploring the relation and interaction of the various degrees of freedom in a n=4/n=5 superlattice where both constituents are polarized.
[1] A. Filippetti, V. Fiorentini, F. Ricci, P. Delugas, and J. Iniguez: Prediction of a native ferroelectric metal, submitted.
[2] M. Scarrozza, M. B. Maccioni, G. M. Lopez, and V. Fiorentini, Topological multiferroics, Phase Trans.88, doi:10.1080/01411594.2014.986731 (2015)
[3] M. Scarrozza, A. Filippetti, and V. Fiorentini: Ferromagnetism and orbital order in a topological ferroelectric, Phys. Rev. Lett. 109, 217202 (2012)
[4] M. B. Maccioni, A. Filippetti, V. Fiorentini, F. Ricci, and J. Iniguez: Multiply-polarized perovskite superlattices, in preparation

The functionality of any ferroic material depends on its domains. Consequently, their shape and manipulation in external fields are of major interest. In compounds uniting magnetic and electric order in the same phase, the magnetoelectric coupling on the level of the domains is, however, largely unexplored. For such so-called multiferroics it is therefore not known how exactly the electric or magnetic fields affect the multiferroic domains and their walls. In my talk I will discuss this issue and focus on the influence of the multiferroic order on the ferroelectric state and its domain walls. Examples will be: (i) multiferroics with geometric ferroelectricity like hexagonal YMnO₃ where topological requirements lead to domain walls with anisotropic conductance [1]; (ii) multiferroics with magnetically induced ferroelectricity like MnWO₄ or TbMnO₃ where the electric polarization within the wall is expected to rotate instead of passing through zero, as in conventional displacive ferroelectrics [2, 3]; (iii) multiferroics with strain-induced ferroelectricity like SrMnO₃ where the interplay of strain and oxygen vacancies leads to polar state in which domain walls act as insulating boundaries to the conducting domains [4].
[1] D. Meier et al., Nature Materials 11, 284 (2012)
[2] N. Leo et al., Nature Comm. 6, 6661 (2015)
[3] M. Matsubara, S. Manz et al., Science 348, 1112 (2015)
[4] C. Becher et al., Nature Nanotech. 10, DOI: 10.1038/NNANO.2015.108 (2015

In recent years, attempts towards electric-field control of magnetism has aroused significant interest in the field of spintronics dealing with various systems such as metals, diluted magnetic semiconductors, and multiferroics, because of its potential for future energy efficient electronic devices. In the case of magnetoelectric multiferroics, the electric-field control of magnetism has been demonstrated in various composite systems such as ferromagnetic(FM)/piezoelectric, FM/magnetoelectric, and FM/multiferroic heterostructures. Furthermore, the electric-field control of magnetic properties has also been demonstrated in several single-phase multiferroics. However, such a control can be realized only far below room temperature in most of single-phase multiferroics.
As for single-phase magnetoelectrics, only a few compounds (e.g., Cr₂O₃) exhibit magnetoelectric effects at room temperature. In 2010, however, it has been found that polycrystalline samples of Z-type hexaferrite Sr₃Co₂Fe₂₄O₄₁ showing a complex conical spiral magnetic structure exhibit a direct magnetoelectric (ME) effect, i.e., the change in electric polarization by a magnetic field, up to 400 K. More lately, it has been reported the converse ME effects, that is, the change in magnetization by applying an electric field, in single crystals of Z-type (Ba,Sr)₃Co₂Fe₂₄O₄₁ around room temperature. The microwave permeability changes due to the application of DC electric fields have also been observed in Sr₃Co₂Fe₂₄O₄₁ hexaferrite slabs at room temperature. Thus, the ME effect in single-phase magnetoelectrics is now going to the room-temperature operation. These results suggest a potential for electric-filed-control electronic devices using the converse ME effect in single-phase magnetoelectrics. In this presentation, we report on our recent activity to pursue single-phase room-temperature magnetoelectrics.
This work has been done in collaboration with K. Haruki, K. Okumura, A. Iyama, Y. Yoshimori, H. Ueda, K. Kimura, S. Hirose, and Y. Tanaka.

Perovskite type structured ferroelectric materials such as BaTiO₃ and Pb(Zr,Ti)O₃ are used for memory, sensor and various applications since they are found around 70 years ago. For good or but, these materials have been still used because of their superior ferroelectricity, piezoelectricity and their related multiferroic properties. Overcoming this situation, we suggest novel ferroelectric/ferrimagnetic ScFeO₃ which has GaFeO₃-type (ε-type) crystal structure classified as polar Pna2₁ space group. ε-ScFeO₃ epitaxial thin films were prepared on (111)SrTiO₃ single crystal substrates by pulsed laser deposition. Growth of metastable ε-ScFeO₃ phase was controlled by film preparation technique. Polarization-electric field loops were measured at room temperature using Pt/ε-ScFeO₃/SrRuO₃ capacitor structure. Saturation polarization of 5 mu;C/cm₂ was observed at 100 Hz. We also carried out first principle calculation for investigation of switching mechanism in GaFeO₃-type structure. Novel polarization switching system was considered through paraelectric phase with lower activation energy of 0.1-0.15 eV than that with reported value of 0.5 eV at Pnna phase. This result suggests that polarization switching should be occured by electric field. Magnetic and multiferroic properties of ε-ScFeO₃ will be discussed.

Multiferroic hexagonal manganites possess intriguing domain patterns and potential applications. The improper ferroelectrics YMnO₃ has six domain variants, which can cycle around vortex and antivortex cores, so called “topological defects”. The vortex, antivortex, and connections between them form two types of domain networks, type-I without and type-II with applied electric fields. Here we employ the phase-field method to investigate the vortex-antivortex evolution in type-I networks and the transition from type-I to type-II networks. The predictions are shown to have excellent agreements with experimental observations. It is found that type-I networks process log-normal statistical distributions (the logarithms of the variable is normally distributed), whereas Type-II shows scale-free power-law distributions with the exponent of ~2. A “proportionate growth” mechanism during the transition between two types of networks is shown to be responsible for the emergence of the scale-free network.

BiMnO3 is the only transition-metal perovskite oxide that is insulating and shows strong ferromagnetism in bulk. This distinctive behavior would make it a promising candidate as a magnetoelectric multiferroic if it was also a polar material, but experiments have shown that bulk BiMnO3 has either a very small polarization (below 0.1mu;C/cm2) or, most likely, that it is a paraelectric. There is also experimental evidence that the polarization in BiMnO3 films grown on SrTiO3 can be as high as 20mu;C/cm2. Despite the interest in these behaviors, the diagram of BiMnO3 as a function of epitaxial strain has remained largely unexplored. Here, we use first-principles to predict that, both under enough compressive and tensile epitaxial strain, BiMnO3 films are ferroelectric with a giant polarization around 100mu;C/cm2. The phases displayed by the films are similar to those experimentally found for BiFeO3 in similar conditions—at compressive strains, the film is supertetragonal with a large component of the polarization pointing out of plane, while at tensile strains the polarization points mostly in plane. As in BiFeO3 films, these phases are antiferromagnetic—the orbital ordering responsible for ferromagnetism in BiMnO3 is absent in the polar phases. Our calculations also show that the band gap of some of these BiMnO3 films is substantially smaller than gaps typically found in ferroelectric oxides, suggesting it may be a suitable material for photovoltaic applications.

In this work, HoCrO₃ and Fe substituted HoCrO₃ (HoCr_0.7Fe_0.3O₃) powders and thin films were synthesized via a solution route. The phase purity and structural properties were examined by Raman spectroscopy and Rietveld refinement of the x-ray diffraction data. The dc magnetic measurement indicates that the ordering temperatures of Cr³⁺ are 140 K and 174 K for HoCrO₃ and HoCr_0.7Fe_0.3O₃, respectively. By fitting the temperature dependent susceptibility data of the powder samples in the paramagnetic region with the Curie-Weiss fit, the effective magnetic moment were determined to be 11.67mu;_B and 11.30mu;_B for the HoCrO₃ and HoCr_0.7Fe_0.3O₃ samples respectively, and those are in good agreement with theoretically calculated values. The ac magnetic measurements of the present HoCrO₃ and HoCr_0.7Fe_0.3O₃ powder samples not only confirmed the Cr³⁺ ordering transitions obtained using dc magnetic measurements but also clearly showed the Ho³⁺ ordering at ~10 K. The isothermal magnetization measurements were also performed to obtain the magnetic behavior and temperature dependence of coercive field and remnant magnetization data. Further, the magnetocaloric properties of HoCrO₃ and HoCr_0.7Fe_0.3O₃ powders samples were studied, indicating their potential for applications in magnetic refrigeration. The structural, magnetic, and dielectric data of these samples will be presented in details.

The hexagonal RFeO₃ (R=Y, Yb and Lu) has attracted attention due to its novel multiferroic properties.[1] However, RFeO₃ is metastable, thus bulk sample synthesis is quite difficult. Very recently, it was found that the hexagonal phase can be stabilized in the region around x=0.5 in Lu_1-xSc_xFeO₃ and can be synthesized by a conventional solid state reaction method.[2] It was also reported that bulk Lu_0.5Sc_0.5FeO₃ exhibits a magnetic transition at T_N=162 K. Here, we present evidence from a neutron scattering experiment on a powder sample of Lu_0.5Sc_0.5FeO₃, that two successive antiferromagnetic transitions occur instead, one at T_N1 = 162 K and another one at T_N2 ~50 K. From inelastic measurement, we observed that the scattering intensity under the magnetic (100) reflection change from elastic to inelastic as a function of temperature. The magnetic (100) reflection most likely arises from the Γ₄ magnetic structure, which has the spin orientation arranged in a 120° configuration in the a-b plane. Critical magnetic scattering appears at temperatures well above T_N, and disappears below as the system globally orders. The broad inelastic scattering, which appears in the same region in momentum space as the (100) reflection, shows an asymmetric shape below T_N indicating that the magnetic fluctuations are confined in two dimensions. This is consistent with our previous results on LuMnO₃, in which the magnetic frustration is most likely present well below T_N with fluctuations confined in the plane.[3] This seems to be a quite common feature in this class of materials.
References:
[1] W. Wang et al., Phys. Rev. Lett. 110, 237601 (2013).
[2] A. Masuno et al., Inorg. Chem. 52, 11889 (2013).
[3] S. Yano et al., J. Phys. Soc. Jpn 83, 024601 (2014).

The coexistence of competing order parameters in multiferroics is of great interest. The hexagonal manganites AMnO3 (A = Y, Lu, Ho and Yb) with the P6₃cm space group exhibit a ferroelectric transition, T_C, at very high temperatures, typically ~1000 K, while the antiferromagnetic (AFM) transition, T_N, occurs at ~ 100 K. Earlier studies on YMnO₃ and LuMnO₃ using neutron scattering on single crystals provided evidence for strong diffuse scattering superimposed on the nuclear and magnetic structures. Diffuse scattering is present around the (100) nuclear Bragg peak. The (100) reflection is where the minimum of the spin wave dispersion occurs. From our neutron scattering results on LuMnO#8323; we show how the scattering intensity under the (100) and (200) reflections changes from elastic to inelastic as a function of temperature in going through the transition. Critical magnetic scattering appears at temperatures well above the AFM transition, and precipitously disappears below as the system globally orders. The inelastic scattering, although broad, appears in the same region in momentum space as the (100) and (200) reflections. Its shape is symmetric as it follows a liquid-like structure factor above T_N, but becomes asymmetric in going through T_N before it disappears at base temperature. The evolution of the commensurate magnetic phase transition follows a mean-field temperature dependence with a critical exponent, β, of about 0.2, suggesting that the AFM interactions are two dimensional in nature as well, and confined in the plane. We recently investigated the effects on the specific reflections under an electric field. The results will be discussed.

Epitaxial PbTiO₃ thin films can show different ferroelectric characteristics depending on the orientations of the thin films. Domain configurations in the ferroelectric thin films are determined by alignments of the thin films. Various ferroelectric twins give each different polarization as a result multi polarization switching behaviors. In order to control the domain configurations in the epitaxial thin films, several factors can be regulated such as grains sizes or thicknesses. In this paper, (001)- and (111)-oriented PbTiO₃ thin films are studied to compare ferroelectric characteristics in each thin films. PbTiO₃ thin films were deposited on conducting LaNiO₃ by using pulsed laser deposition. To understand local ferroelectric characterizations in the thin films, we use piezoelectric force microscopy. Domain configurations in the thin films were obtained by measuring piezoresponse microscopic images and origin of ferroelectric characteristics were studied based on these results. Local ferroelectric switching behaviors were obtained and poling behaviors with time dependence were also obtained. With different poling process, we can observe various domain dynamics in the (111)-oriented thin films. From this point of view, we can compare ferroelectric characteristics and domain dynamics of (001)- and (111)-oriented PbTiO₃ thin films and we also report a frustrated domain formation and conductivity across the domains in the (111)-oriented ferroelectric thin films.

Pb(Zr,Ti)O₃ (PZT) thin films are of technological interests owing to the good dielectric and piezoelectric properties. The overall piezoresponse can be significantly enhanced through the movement of the 90^#9702; domain walls in ferroelastic PZT, and particularly, through local electric training (i.e. a cyclic application of bipolar voltage bias) [1].
Herein, the phase-field modeling approach is employed to study domain switching behavior during the electric training process. It is discovered that thick a domains which form due to the relaxation of elastic energy cannot be fully switched back after the application of a reverse bias [2]. First, new a domains nucleate and grow near the newly-formed 180^#9702; domain walls after the application of a negative bias. Then, the thick a domains tends to be pushed away from the tip under positive bias where high electric field along z is expected while the thin a domains are switched. The newly-grown a domains can be stabilized even when the bias is removed, leading to a higher population of a domains as well as 90^#9702; domain walls after each training. The piezoresponse of the film after the electric trainings indicates that the piezoresponse increases 31% after only five set of trainings. A dramatic increase near the 90^#9702; domain walls is observed, which is asymmetric for the two 90^#9702; domain walls. The “soft” side of the domain wall has a much higher response than the “hard” side. It is also revealed that after the trainings, not only the 90^#9702; domain wall density increases but also some domain walls has much higher piezoresponse, leading to a dramatic increase of the average piezoresponse coefficient.
[1] S. Bühlmann and P. Muralt, Adv. Mater. 20 (16), 3090-3095 (2008).
[2] Z. Hong, J. Britson, J. M. Hu, L. Q. Chen, Acta Mater. 73, 75-82 (2014).

The hexagonal manganite YbMnO₃ is one of multiferroic materials exhibiting both ferroelectric and antiferromagnetic properties. According to the previous studies, the appearance of the ferroelectricity in this material is directly associated with the P6₃/mmc-to-P6₃cm structural transition, which is driven by an instability of the K₃ mode at the zone-boundary K point. However, the K₃ mode itself cannot produce a net electric polarization. The ferroelectricity is then understood to be due to a coupling between the K₃ and zone-center polar Γ_2- modes. Another striking feature is that the ferroelectric state exhibits a cloverleaf domain structure with a pseudo-six-fold symmetry. Because the structural transition does not accompany a loss of the six-fold axis, the point-group change in the transition should not be responsible for the appearance of the cloverleaf domain structure. To understand the origin of its appearance, thus, we have examined the crystallographic features of the cloverleaf structure in YbMnO₃ by transmission electron microscopy.
From the experimental data obtained in this study, it was confirmed that the ferroelectric state was identified as a triple-q state with <001> polar vectors, which is characterized by a superposition of three modulated waves with q =<1/3 1/3 0> for the K point. As for the ferroelectric domain structure, the cloverleaf structure was observed even in dark field images with electron beam incidences perpendicular to the polar directions. In this sample orientation, however, there is no reason why the cloverleaf domain structure appears in terms of the point-group change. We then paid attention to the nature of a domain boundary, and took high resolution electron micrographs of areas including a boundary. The analysis of obtained micrographs indicated that a domain boundary could be identified as a discommensuration with a phase slip of π/3. This suggests that the appearance of the cloverleaf domain structure should originate from the break of the translational symmetry.

The multiferroic material BiFeO₃ with the simple perovskite structure has been reported to show ferroelectric and antiferromagnetic properties. When Bi³⁺ ions in BiFeO₃ were replaced by Sm³⁺ ions, in the mixed-oxide system Bi_1-xSm_xFeO₃, the R3c phase for BiFeO₃ was found to be changed into the Pnma one around x = 0.14. The interesting feature is that the R3c/Pnma phase boundary is identified as a morphotropic phase boundary (MPB), in the vicinity of which a remarkable piezoelectric response has been reported so far. Because of this, the presence of a ferroelectric domain structure with a nanometer scale can be expected near the MPB, just as in the case of Pb(Zr_1-xTi_x)O₃. We have thus investigated the crystallographic features of the ferroelectric state for 0 le; x le; 0.20 in Bi_1-xSm_xFeO₃, mainly by transmission electron microscopy. In this study, the solid-state-reaction method was used for the preparation of Bi_1-xSm_xFeO₃ samples.
From our observation made by transmission electron microscopy, it was first confirmed that Bi_1-xSm_xFeO₃ samples for 0 le; x le; 0.12 and 0.16 le; x le; 0.20 have the R3c and Pnma structures, respectively. In the vicinity of the MPB around x = 0.15, on the other hand, we could detect a microstructure consisting of two kinds of regions with an average size of about 300 nm. Although one region has the R3c structure, the other could not be identified as regions with the Pnma symmetry. The notable feature of electron diffraction patterns in the latter is that superlattice reflections with q = [1/2 0 0]_o and q = [0 1/2 0]_o, are present, in addition to the reflections due to the Pnma structure, where the subscript o denotes the orthorhombic system. From the extinction rules of the superlattice reflections, they could be explained as being due to two transverse lattice modulations, which may be related to an antiferroelectric property.

Physics phenomena at interface of complex oxide hetero-structures has stimulated intense research activity due to the occurrence of a broad range of electric and magnetic functionalities that do not pertain to any of the constituents. In the case of the interface superconductivity, the interatomic structure relaxation and charge transfer play a key role.
In this work, we combine atomic-resolved quantitative STEM imaging with analytical STEM-EELS/EDX analysis to investigate the local lattice distortion, as well as cation and electron hole distributions in Sr-δ-doped La₂CuO₄ superlattices exhibiting T_c up to 40 K, despite its non-superconducing constituents. The simultaneously acquired HAADF and ABF images allow us to quantitatively analyze the local lattice and copper-apical-oxygen distortions. The interatomic structure analysis shows that the copper-apical-oxygen distance, which is known to be strongly related with T_c, has a remarkable variation near the δ-doped position. In the investigated superlattice, hole doping was achieved on the two sides of Sr-δ-doped plane by charge redistribution (or accumulation) and Sr doping, which are spatially separated yet both contribute to the superconductivity of the superlattices. The correlation between charge redistribution and local atomic structural change that may account for the occurrence of the high-T_c superconductivity in the Sr-δ-doped La₂CuO₄ superlattices will be discussed.
Acknowledgements: The research leading to these results has received funding from the European Union Seventh Framework Program [FP/2007-2013] under grant agreement No.312483 (ESTEEM2). Financial support for the ARM200CF project by the Max Planck Society is gratefully acknowledged.

In this work we report an improper mechanism responsible for piezoelectric LiRTiO₄ (R=Rare earth) layered perovskite family, which previously were reported to be centrosymmetric [1],[2],[3]. In these layered perovskites, inversion centers are removed by the TiO₆ octahedral rotations represented by a^-b^oc^o/b^oa^-c^o [4]. First-principles phonon calculations, supported by synchrotron x-ray diffraction, neutron diffraction, optical second harmonic generation measurements and piezo-response force microscopy, help us reveal this phenomena. These compounds demonstrate the low temperature non-centrosymmetric phase of P-42₁m and the high temperature centrosymmetric phase of P4/nmm (where no octahedral rotations exist). The centrosymmetric to non-centrosymmetric phase transition temperature (T_ac) is ~850K for LiSmTiO₄ and gets higher as the rare earth ions get smaller (for Eu, Gd, Dy). While the octahedral rotation in LiRTiO₄ is of the same type as in the recently reported NaRTiO₄ [5], the T_ac is higher for this new series. The work reveals a general improper mechanism for creating acentric layered perovskite structures.
Reference:
[1] K. Toda, S. Kurita, and M. Sato, “New Layered Perovskite Compounds, LiLaTiO₄ and LiEuTiO₄,” J. Ceram. Soc. Japan, vol. 104, no. 1206, pp. 140-142, 1996.
[2] S.H. Song, J. Alonso, J.-G. Cheng, and J. Goodenough, “Magnetic phase transformation induced by electrochemical lithium intercalation in Li1 + x EuTiO4 and Li2 + 2x Eu2Ti3O10 (0 le; x le; 1) compounds,” J. Solid State Electrochem., vol. 18, no. 7, pp. 2047-2060, 2014.
[3] S.H. Song, K. Ahn, M. G. Kanatzidis, J. A. Alonso, J.-G. Cheng, and J. B. Goodenough, “Effect of an Internal Electric Field on the Redox Energies of ALn TiO 4 ( A = Na or Li, Ln = Y or Rare-Earth),” Chem. Mater., vol. 25, no. 19, pp. 3852-3857, Oct. 2013.
[4] A. M. Glazer, “The classification of tilted octahedra in perovskites,” Acta Crystallogr. B, vol. 28, pp. 3384-3392, 1972.
[5] H. Akamatsu, K. Fujita, T. Kuge, A. Sen Gupta, A. Togo, S. Lei, F. Xue, G. Stone, J. M. Rondinelli, L.-Q. Chen, I. Tanaka, V. Gopalan, and K. Tanaka, “Inversion Symmetry Breaking by Oxygen Octahedral Rotations in the Ruddlesden-Popper NaRTiO4 Family,” Phys. Rev. Lett., vol. 112, no. 18, p. 187602, May 2014.

Single phase magnetically driven ferroelectrics, such as TbMnO₃, are a class of magnetoelectric multiferroic materials that can exhibit high magnetoelectric coupling. However, their application toward devices is hindered due to the typically low magnetoelectric transition temperatures (< 40 K). Recently, the rare-earth chromites (RCrO₃), with orthorhombically distorted perovskite structure, have been proposed as magnetically driven ferroelectrics at comparatively higher temperatures (120 to 250 K depending upon rare-earth ion) than rare-earth manganites. The latest reports show a polarization onset at their Néel temperature (Cr³⁺ ordering), which would suggest that the polarization is related to the magnetic order in the material. It is well known that the Cr³⁺ sublattice magnetically orders in a canted G-type antiferromagnetic state. However, the existence and onset temperature of the ferroelectric order is still widely debated in these materials. In this work, the structural, magnetic, and electronic properties of Ho substituted DyCrO₃ bulk and thin-films are studied. The synthesis and structural characterization, through the use of techniques including Raman spectroscopy and x-ray diffraction, of bulk pellets and thin films will be presented. DC magnetic characterizations revealed that the magnetic moment and magnetic coercivity of the samples were significantly enhanced with the Ho substitution. Temperature dependent dielectric properties with the presence of magnetic field will also be presented in detail. This work will lead to a better understanding of the nature of magnetism and ferroelectricity in the rare-earth chromites such as Ho substituted DyCrO₃.

The electronic states of the simple perovskite manganite Sr_1-xSm_xMnO₃ (SSMO) is characterized by a three-dimensional highly-correlated electronic system. When Sr²⁺ ions in SrMnO₃ with only Mn⁴⁺ ions are partially replaced by Sm³⁺ ions, e_g electrons are introduced into its electronic system via the appearance of Mn³⁺ ions. The e_g-electron doping then leads to the fascinating electronic states such as the C-type orbital-ordered (COO) state. According to the previous studies on the COO state in SSMO, the state obtained by cooling from the disordered cubic (DC) state has the tetragonal-I4/mcm symmetry, and its crystal structure involves both the Jahn-Teller distortion for orbital ordering and the R₂₅-type rotational displacement of oxygen octahedra. However, a role of the rotational displacement in the formation of the COO state has not been understood yet. In this study, thus, we have investigated the crystallographic features of the COO state appearing in prepared SSMO samples with 0.10 le; x le; 0.25, mainly by transmission electron microscopy. Thin specimens for our observation were prepared by the Ar-ion thinning technique.
The experimental data obtained in this study revealed that the formation of the COO state from the DC state took place in the following two steps. The first step is the introduction of the R₂₅-type rotational displacement about one of the <100>_DC directions into the DC state. In the second step, the COO state is formed by the appearance of the Jahn-Teller distortion as a response of a lattice system to C-type orbital ordering. The notable point is that, because the introduction of the rotational displacement is responsible for the symmetry change into the I4/mcm one, the Jahn-Teller distortion can be identified as a dilatational distortion with no symmetry change. In spite of this, the appearance of the dilatational Jahn-Teller distortion leads to a nanometer-scale banded structure, which is characterized by an alternating array of two tetragonal variants with different c/a values. It was also found that the formation of the COO state accompanied the remarkable time-relaxation phenomenon.

YCrO₃ (YCO) is known to be a biferroic with orthorhombic, Pnma, structure with center of inversion. However, the local structural inhomogeneity in this compound is believed to give rise to ferroelectric behavior. In this study we explore high temperature Raman investigations of YCO and observed that one of its Raman mode B3g (3) (CrO₆ Octahedral tilt mode) softens around the structural phase transition. This indicates the plausible ferroelectric nature of the sample and in addition the absorption studies reveal the electronic transitions that could hinder any macroscopic electrical studies of YCO. In addition, we investigated the role of bismuth (Bi) in order to understand structural and magnetic properties of Y_1-xBi_xCrO₃. X-Ray powder diffraction studies reveals the orthorhombic Pnma phase. The additional Raman mode at lower wavenumber indicates the substitution of Bi in A-site. Optical absorption studies reveals the reduction in optical band gap with increase of Bi content. However, the effective magnetic moment calculated by fitting the susceptibility data increases with increase of Bi content, with no major variation in the magnetic transition temperatures. The bulk ceramic results were studied and compared with that of polycrystalline thin films of YCO grown by pulsed laser ablation.

Transition metal oxide superlattices show a variety of interesting phenomena, the details of which depend on the layering parameters. For example, the magnetic behaviour and conductivity of LaNiO₃ can be tuned by controlling the layer thickness. Various methods for growing these structures exist; pulsed laser deposition (PLD) and molecular beam epitaxy (MBE) are both popular. The growth of superlattices, especially by PLD, is a rich area of study because of the large number of controllable variables, each with its own impact on the resulting structure.
In this study we compare specimens of LaNiO₃-LaAlO₃ (LNO-LAO) grown by PLD on two different substrates, (LaAlO₃)_0.3(Sr₂TaAlO₆)_0.7 (LSAT) and SrTiO₃ (STO), to nominally identical samples grown by oxide MBE. Investigations done in an aberration-corrected JEOL JEM-ARM200F TEM at 200kV revealed that the PLD-LSAT sample had a compositionally altered layer at the substrate surface; the MBE-LSAT sample did not. The PLD-STO sample [1] had NiO precipitates (particles) at the substrate-layer interface, while the MBE-STO sample did not [2].
The appearance of particles can be influenced by PLD process variables, such as the kinetic energy of the impinging particles [3,4], the background gas pressure [5], or the type of target-laser interaction [3,6]. The substrate material [1] and temperature [4] can also have an effect. Detemple et al. attributed the NiO precipitates to the non-polar nature of the STO substrate, since LNO/LAO grown on polar substrates had no precipitates [1]. However, Zhu et al. [7] grew LNO on STO by PLD and did not observe precipitates; we will explore reasons for why our MBE-STO sample also has none.
Some of the same variables also influence surface alteration, e.g. the particle kinetic energy [5,8], or the substrate material and surface [9]. In our case, the high particle kinetic energy in PLD will be explored as the most likely factor for the altered surface of the PLD-LSAT sample.
Since it is known that the perfection of a superlattice has a bearing on its performance it is important that defects be characterized and their origins understood. These results discuss the characterization of these defects and with this data begin to explore their origins and how they are affected by the different growth process variables. [10]
[1] E. Detemple et al., J. Appl. Phys. 112 (2012) 013509
[2] F. Wrobel et al., in preparation
[3] G. Rijnders, D.H.A. Blank, in Pulsed Laser Deposition of Thin Films, R. Eason (ed.) (Wiley, Hoboken NJ, 2007) Chap. 4
[4] H.M. Christen, G. Eres, J. Phys.: Condens. Matter 20 (2008) 264005
[5] D.P. Norton, in Ref. 4, Chap. 1
[6] C.B. Arnold, M.J. Aziz, Appl. Phys. A 69 (1999) S23
[7] J. Zhu et al., Mat. Chem. Phys. 100 (2006) 451
[8] J. Gottmann et al., doi: 10.1117/12.308606
[9] K. Ohashi et al., Microscopy 63 (2014) i20
[10] This research has received funding from the EU Seventh Framework Program [FP/2007-2013], grant agreement no. 312483 (ESTEEM2).

Mn₃Cu_1-xGd_xN with antiperovskite structure was synthesized by using the solid - state reaction. The magnetic measurement showed a typical transition from paramagnetic (PM) to ferromagnetic (FM) state at Curie temperature T_C asymp; 143 K in undoped material. In addition, with Gd doping in matrix, another metamagnetic transition from FM to a complex magnetic state including FM and antiferromagnetic (AFM) was found in lower temperature below T_C, indicating the appearance of the AFM component with Gd doping. Furthermore, the lower metamagnetic transition temperature Tm decrease and T_C increase with increasing the amount of Gd, which makes the temperature section of stable magnetic state broadened. In metastable magnetic state below Tm, the step - magnetization was observed in high doped material from isothermal magnetization curve.

In the highly-correlated electronic system Sr_1-xR_xMnO₃ with R=Nd and Sm, the replacement of Sr²⁺ ions by R³⁺ ions leads to the state change from the tetragonal C-type orbital-ordered (COO) state to the monoclinic A-type (AOO) state around x = 0.40. It was found that their crystal structures involved the R₂₅-type rotational displacement of oxygen octahedra, in addition to the Jahn-Teller distortions for orbital orderings of e_g electrons. As for the difference, the COO state is characterized by the R₂₅-type displacement about one of the <100>_c directions in the pseudo-cubic notation, while the displacements about two directions are involved in the AOO state. Because of this, the state change between these two states is likely to occur continuously. However, the detailed features of the state change have not been understood yet. We have thus examined the crystallographic features of prepared Sr_1-xR_xMnO₃ samples with 0.350.45 near the state boundary, mainly by transmission electron microscopy.
The present experimental data revealed that the (COO → AOO) state change with increasing the R content did not occur directly. That is, the disordered state was found to be present between the COO and AOO states. In the case of Sr_1-xNd_xMnO₃ with x=0.43, for instance, electron diffraction patterns obtained from the disordered state exhibited diffuse scattering both at q = <1/2 1/2 1/2>_c-type positions and around fundamental reflections due to the simple perovskite structure. The point to note here is that we could not detect the splitting of fundamental reflections indicating the appearance of the COO and AOO states. In addition, a so-called tweed pattern was observed in dark field images taken by using fundamental reflections. Because the diffuse scattering at q = <1/2 1/2 1/2>_c originates from a structural fluctuation of the R₂₅-type displacement, it is thus understood that the disordered state is characterized by a nanometer-scale coexistence state, in which nanometer-scale regions involving the R₂₅-type displacement are distributed uniformly in a cubic matrix.

The highly-correlated electronic system Ca_1-xPr_xMnO₃ (CPMO) with the simple perovskite structure has been reported to exhibit electronic and magnetic states that are associated with degrees of charge, orbital, and spin freedoms for 3d electrons in Mn ions. When Ca²⁺ ions in the end material CaMnO₃ was partially replaced by Pr³⁺ ions, the C-type antiferromagnetic state was found to appear for 0.10 le; x le; 0.20 in CPMO. Although the appearance of the C-type antiferromagnetic state should be related to orbital ordering, the detailed features of the corresponding C-type orbital-ordered (COO) state including its formation have not been understood sufficiently. We have examined the crystallographic features of the COO state in CPMO samples with 0.10 le; x le; 0.20, mainly by transmission electron microscopy.
In this study, CPMO samples with 0.10 le; x le; 0.20 were prepared by a usual solid-state-reaction technique. To examine the crystallographic features of the COO state, their electron diffraction patterns, and corresponding bright- and dark-field images were taken in the temperature range between room temperature and 87 K, by using JEM-1010 and H-800 transmission electron microscopes. Thin specimens for our observation were prepared by an Ar-ion thinning technique.
Based on the experimental data obtained in this study, the disordered state for 0.10 le; x le; 0.20 in CPMO was confirmed to have the orthorhombic Pnma structure. When the temperature was lowered from the disordered state, in electron diffraction patterns taken from prepared samples, the splitting of reflections indicating the appearance of the COO state with the point group of the monoclinic-C2/m was found to occur, for instance, around 100 K for x = 0.14. The striking feature is that characteristic diffuse scattering appeared in the vicinity of each reflection just before the appearance of the COO state on cooling. In addition, the COO state exhibited a characteristic banded structure with a nanometer scale. The similar nanometer-scale banded structure has already been found in the COO states in other manganites Sr_1-xR_xMnO₃ with R=Nd³⁺ and Ce⁴⁺, whose disordered states have the cubic Pm-3m symmetry.

Multiferroic relaxors [Pb(Fe_0.5Nb_0.5)O₃ (PFN), Pb(Fe_0.5Ta_0.5)O₃, Pb(Fe_2/3W_1/3)O₃] has attained great interest due to their anomalous structural and physical properties. We have investigated the correlation between structure, phase transition temperature (dielectric maxima - T_max), cation ordering and phonon modes of (1-x)Pb(Fe_0.5Nb_0.5)O₃ - xPb(Sc_0.5Nb_0.5)O₃ solid solutions. Phase pure Pb[(Fe_0.5-xSc_x)Nb_0.5]O₃ [x = 0 to 0.5)] ceramics have been synthesized via wolframite solid state reaction route. Structural studies confirm that the synthesized powders are phase pure without any secondary phase. Rietveld refinement of x-ray diffraction pattern reveals the presence of a structural transformation at x = 0.3 from monoclinic (Cm) to rhomobohedral (R3m). Ferroelectric phase transition temperature (T_max) increases initially at lower Sc content (upto x le; 0.25), and further drops beyond x ge; 0.3. Such behavior of T_max in these compounds is due to the onset of B', B" local cation ordering at x = 0.3. Room temperature Raman spectra clearly reveals that the B-site cation ordering begins at x = 0.3 and increases beyond this composition, evolution Pb-O coupled vibrational mode [F_2u, ~ 350 cm^-1) confirms the beginning of local cation ordering. In addition, high temperature Raman spectra of all samples have been recorded from room temperature to 723 K at different intervals. Two phonon modes originated from polar nano regions (PNRs) i.e., B-localized mode [F_1u, ~ 250 cm^-1) and BO₆ octahedral rotational mode [F_1g, ~ 200 cm^-1] show unusual behavior unlike with other phonon modes with temperature. F_1u mode show soft mode like behavior, which softens until ferroelectric phase transition temperature (T_m) and becomes harden beyond phase transition without any over damping across the transition. Whereas, F_1g mode behaves like a normal phonon mode in compounds with monoclinic symmetry (Cm) and like a soft mode in compounds with rhombohedral symmetry (R3m). Thus a strong correlation between phonon modes F_1u, F_1g (originated from PNRs) with local cation ordered regions have been observed in compounds with rhombohedral symmetry (0.3le; xle; 0.5). The behavior was also compared with polycrystalline thin films of PFSN ceramics.

Multiferroics in nanoscale dimensions are promising for novel functional device paradigms, such as magnetoelectric memory, due to intriguing cross-coupling between coexisting ferroelectric and (anti-)ferromagnetic order parameters. However, the ferroic order is inevitably destroyed below the critical dimension of several nanometers. Here, we demonstrate a new path toward realization of ultimately-small multiferroics while resolving the controversial origin of dilute ferromagnetism that unexpectedly emerges in nanoparticles of nonmagnetic ferroelectric PbTiO₃. Systematic exploration using state-of-the-art hybrid Hartree-Fock density functional calculations as well as the DFT+U calculations with a theoretical Hubbard U derived from the constrained random phase approximation (cRPA) successfully identifies that oxygen vacancies formed at surfaces/grain boundaries induce ferromagnetism due to local non-stoichiometry and orbital symmetry breaking. The localized character of emerged magnetization allows an individual oxygen vacancy to act as an atomic-scale multiferroic element with a nonlinear magnetoelectric effect that involves rich FM-AFM-NM phase transitions in response to switching spontaneous polarization. Moreover, we also demonstrate that the local (anti-)ferromagnetism at oxygen vacancies can be controlled through the elastic strain engineering. Therefore, defects in ferroelectric oxides can behaver as atomic-scale low-dimensional multiferroics. Engineering these multiferroic features opens a new avenue to the design of ultrahigh-density integration for atomic-scale or quantum multiferroics.

Ferroelectric materials exhibit intrinsic coupling of spontaneous polarization and strain. Their ferroelectric/ferroelastic domains can be reversibly switched with electric bias and/or stress. This characteristic has been used for high density non-volatile memory. However, both ferroelectric and ferroelastic backswitching have been reported which will cause significant data lose. In this presentation, we will report our in-situ transmission electron microscopy investigation results on separated and combined effects of electric bias and stress on the domain structures in a ferroelectric material 67%Pb(Mg_1/3Nb_2/3)O₃-33%PbTiO₃. Our results show that electric bias produces unstable domain structures. The unstable domain structures can be stabilized through one mechanical loading cycle or a small negative bias. A advanced writing and reading process for non-volatile memory is proposed based on combined application of stress and electric field. This method can prevent data lose from the ferroelectric/ferroelastic backswitching. The mechanism behind the experimental observation will be briefly discussed based on phase-field modelling.

It is recognized that ferroelectrics and multiferroics exist in several other material chemistries, crystal classes and stoichiometries than the well-known oxides perovskites such as the fluorine-based compounds, including polymers and ceramics [1-3]. Given the low polarizability of the B-F bond, we have highlighted that the mechanism for ferroelectricity in fluorine-based crystals is distinct from the one found in oxides [4]. Here, we show from first-principles calculations that a stable ferroelectric and magnetoelectric ground state can be achieved in Pnma NaMnF₃ films under epitaxial strain. We show that ferroelectricity in this fluoroperovskite has a totally different origin with respect to the one in oxides where a geometric steric effect is observed rather than a charge transfer origin [4]. We also analyze the ferroelectric and magnetic properties and show that the crystal has a strong non-linear polarization/strain coupling such as the response under epitaxial strain is counter-intuitive with respect to the one reported in ferroelectric oxides [5]. We will show that this non-linear coupling affects the piezoelectric and the magnetoelectric responses. Thus, this makes the study of ferroelectricity in ionic ABF₃ of high interest for the physics of ferroelectrics and very attractive to explore novel electric crystal responses from a theoretical and an experimental point of view. Experimental efforts in NaMnF₃ thin films are on their way in order to confirm the multiferroic properties theoretically predicted.
[1] J.F. Scott, R. Blinc, J. Phys. Cond. Matter, 23, 113202 (2011).
[2] A. J. Lovinger, Science, 220, 1115 (1983).
[3] G. Pilania, T. Lookman, Phys. Rev. B 90, 115121, (2014).
[4] A. C. Garcia-Castro, N. A. Spaldin, A. H. Romero, and E. Bousquet, Phys. Rev. B 89, 104107 (2014).
[5] C. Ederer, N. Spaldin, Phys. Rev. Lett. 95, 257601, (2005).

Multiferroic oxides that exhibit more than one ferroic phase have garnered interest for applications in memory storage and solid-state sensors. However, probing ferroelectric and magnetic properties at nano to atomic scales requires new characterization techniques. Here, we investigate ferroelectric fields and its associate local dipole fields that give rise to the nanoscale domains in BiFeO₃ by advancing Lorentz-STEM (Scanning Transmission Electron Microscopy) to record the entire electron diffraction pattern with fast acquisition and high dynamic range. The challenge for quantitative Lorentz-STEM, like holography and DPC-STEM (differential phase contrast STEM), is to accurately decouple the deflections resulting from crystallographic orientation, local atomic potentials, and the long-range strain and electromagnetic fields.
Lorentz-STEM is a technique where the deflections of the transmitted beam due to the Lorentz forces intrinsic to BiFeO₃ is recorded. The novelty of our experiment stems from the ability to quantify the Lorentz forces by recording the full electron diffraction pattern at each scan position using a custom high-speed mixed-mode pixel array detector (MMPAD) with exceptionally high dynamic range. The custom MMPAD is capable of recording up to 1000 images per second with single electron sensitivity and a full well of > 10⁶ electrons per pixel per image, such accuracy and sensitivity needed for characterization of local fields and domains. Fast MMPAD readout also allows for Lorentz-STEM imaging at atomic resolution. Since the entire electron diffraction pattern is recorded, all standard STEM imaging modes can be extracted post hoc - e.g. annular dark field (ADF) and bright field. In BiFeO₃, atomic resolution images of domain boundaries can be extracted and over a larger field of view, ferroelectric domains can be seen clearly showing long range and short-range contributions.
Naively, these deflections could be converted directly to electric or magnetic fields. However, the spontaneous polarization of the ferroelectric material is accompanied by lattice distortion and crystal tilt. The magnitude of the beam deflection from the field of interest is comparable to those caused by crystal mistilt, thickness variations, atomic potentials, and long-range electric fields, so methods for decoupling the contributions from each of them must be developed. The high-dynamic range PAD offers exciting opportunities for Lorentz-STEM - where the multiple signal channels over a wide range of scattering angles provide additional information beyond traditional magnetic contrast electron imaging methods to account for crystal orientation, pm-level strains, local atomic potentials, polarity and electromagnetic fields.

Phase retrieval methods in X-ray diffraction (XRD) such as Coherent Bragg Rods Analysis (COBRA) [ref. 1] provide an alternate approach to obtain 3D electron density inside the material by reverse Fourier transformation of the structure factor constructed from the recorded diffraction data. However the current COBRA analysis codes have serious shortcomings, which we have addressed in this work. We first generalize the COBRA routines from its current constraint of handling only tetragonal symmetry, to handling any symmetry of the film and substrate. Secondly, the performance of our new analysis routines was tested to be capable of handling large dataset simultaneously (>150 rods) without compromising the accuracy.
Our new COBRA analysis codes on a strained series of CaTiO₃ films, which have polarization as well as oxygen octahedral tilts in all three directions. [ref. 2] However, the atomic structure transition and the role of oxygen octahedron tilt incorporated in the strained CaTiO₃ remains ambiguous. We demonstrate high accuracy (~0.02#8491;) 3D atomic structure obtained by COBRA analysis at both room temperature and 30K. From the analysis of CaTiO₃/NdGaO₃(110) (1% tensile strain) system, the evolution of the positions of Ca, Ti and O atoms in each unit cell from the substrate interface to sample surface were clearly visible, suggesting a single domain orthorhombic ferroelectric state with both in-plane and out-of-plan polarization component. Changes in the film structure with various strain states of the film will be presented. This will be the first example where COBRA is applied to oxides with complex tilt systems and large tilt angles.
Reference:
1. Y. Yacoby, et al., J. Phys.: Condens. Matter 12 (2000)
2. C.-J. Eklund, et al., PHYSICAL REVIEW B 79, 220101R (2009)

In this presentation composite magnetoelectrics (ME) implemented as thin film heterostructures are discussed in view of their applicability as highly sensitive magnetic field sensors. Composite materials benefit from the advantage that their constituting phases can be optimized almost independently. A precondition is compatibility of individual process steps during device fabrication. Here, either PZT or AlN serve as piezoelectric component. Whereas PZT layers are made by chemical solution deposition, a room temperature pulsed DC sputtering method is used to grow AlN layers. In addition, thin film deposition of the magnetostrictive component enables utilization of not only single (Fe₉₀Co₁₀)₇₈Si₁₂B₁₀ layers but also more complex layer systems such as exchange biased multilayers. The unidirectional anisotropy imposed by exchange coupling can be applied to shift the magnetostriction curve and to set the optimum working point condition - the maximum of the piezomagnetic coefficient - to zero bias field [1]. All functional layers are deposited with thicknesses of a few micrometers on Si cantilever structures with typical lateral dimensions of 3 mm x 25 mm.
To improve the minimum detectable magnetic field one could either rise the sensitivity given by the ME coefficient or lower the sensor system noise governed by dielectric losses, irregular magnetization changes, acoustic interference and noise of the read-out electronics. For the cantilever design highest sensitivity is observed for external magnetic fields with frequencies matching the mechanical resonance frequency. Under such condition ME coefficients as large as 6900 V/cmOe and a low limit of detection (LoD) below 2 pT/Hz^1/2 can be measured [2]. The currently best result demonstrates a LoD = 500 fT/Hz^1/2 at 958 Hz frequency using a set of two sensors for noise suppression. A frequency conversion technique is proposed in order to broaden the applicability of resonant ME sensors to a wider frequency range [3]. Here, the sensor sensitivity is periodically modulated by an electrical or a magnetic field. Again, exchange biased multilayers are applied to tailor magnetic domain activity in the magnetostrictive component thereby reducing sensor noise and improving the LoD for non-resonant frequencies.
[1] E. Lage, C. Kirchhof, V. Hrkac, L. Kienle, R. Jahns, R. Knöchel, E. Quandt, D. Meyners, Nature Mater. 11, 523 (2012).
[2] V. Röbisch, E. Yarar, N. O. Urs, I. Teliban, R. Knöchel, J. McCord, E. Quandt and D. Meyners, J. Appl. Phys. 117, 17B513 (2015)
[3] R. Jahns, H. Greve, E. Woltermann, E. Quandt, R. Knöchel, Sensors and Actuators A 183, 16 (2012).

The coexistence of electric polarization and magnetization in multiferroic materials provides great opportunities for realizing magnetoelectric coupling, including electric field control of magnetism, or vice versa, through a strain mediated magnetoelectric coupling in layered magnetic/ferroelectric multiferroic heterostructures. Strong magnetoelectric coupling has been the enabling factor for different multiferroic devices, which however has been elusive, particularly at RF/microwave frequencies. In this presentation, I will cover the most recent progress on new integrated multiferroic devices for sensing, memory, RF and microwave electronics, including novel RF NEMS magnetoelectric resonators with picoTesla sensitivity for DC magnetic fields, and novel GHz magnetic and multiferroic inductors with a wide operation frequency range of 0.3~3GHz, and a high quality factor of close to 20, and a voltage tunable inductance of 50%~150%. At the same time, we will demonstrate other voltage tunable multiferroic devices, including voltage tunable multiferroic bandpass filters, tunable bandstop filters, tunable phase shifters, magnetoelectric random access memory, etc. These novel integrated multiferroic devices show great promise for applications compact, lightweight and power efficient sensing, memory, RF and microwave integrated electronics.
[1]. N.X. Sun and G. Srinivasan, SPIN, 02, 1240004 (2012);
[2]. J. Lou, et al., Advanced Materials, 21, 4711 (2009);
[3]. M. Liu, et al. Advanced Functional Materials, 21, 2593 (2011);
[4]. T. Nan, et al. Scientific Reports, 3, 1985 (2013);
[5]. M. Liu, et al. Advanced Materials, 25, 1435 (2013);
[6]. Z. Zhou, et al. Nature Communications, 6, 6082 (2015).

In this work, we experimentally demonstrate deterministic electrically-driven, strain-mediated domain wall (DW) rotation in ferromagnetic Ni rings fabricated on piezoelectric [Pb(Mg_1/3Nb_2/3)O₃]_0.66-[PbTiO₃]_0.34 (PMN-PT) substrates. While imaging the Ni rings with X-ray magnetic circular dichroism photoemission electron microscopy, an electric field is applied across the PMN-PT substrate which induces strain in the ring structures that drives DW rotation around the ring toward the dominant PMN-PT strain axis by the inverse magnetostriction effect. The DW rotation we observe is analytically predicted using a fully coupled micromagnetic/elastodynamic multi-physics simulation which verifies that the experimental behavior is caused by the electrically-generated strain in this multiferroic system. Finally, this DW rotation is used to capture and manipulate micron-scale magnetic beads in a fluidic environment to demonstrate a proof-of-concept energy-efficient pathway for multiferroic-based lab-on-a-chip applications.
Published: H. Sohn, M.E. Nowakowski, et al. ACS Nano, doi: 10.1021/nn5056332 (2015).

Complex perovskite oxides exhibit a rich spectrum of functional responses, including magnetism, ferroelectricity, highly correlated electron behavior, superconductivity, etc. The basic materials physics of such materials provide the ideal playground for interdisciplinary scientific exploration. Over the past decade we have been exploring the science of such materials (for example, colossal magnetoresistance, ferroelectricity, etc) in thin film form by creating epitaxial heterostructures and nanostructures. Among the large number of materials systems, there exists a small set of materials which exhibit multiple order parameters; these are known as multiferroics. Using our work in the field of ferroelectric(FE) and ferromagnetic oxides as the background, we are now exploring such materials, as epitaxial thin films as well as nanostructures.
We are exploring two pathways to : (i) use electric field to control the direction of magnetization in a ferromagnet using a Multiferroic-ferromagnet heterostructure; (ii) use a piezoelectric-ferromagnet heterostructure to control the state of magnetization. Over the past year, we have made significant progress in both directions. I will present our results to date in these directions.

Efficiently controlling magnetism in the small scale presents significant problems in our society. In most cases we rely on an electrical current to manipulate the magnetic state but this is extremely energy inefficient. For example, state of the art spin transfer torque STT requires approximately 100 fJ of energy to reorient a bit of memory with an energy barrier of 0.3aJ, i.e. 0.0003% efficiency. An efficiency of this level should be considered unacceptable and new approaches are needed.
One approach is voltage-induced strain to alter the local magnetic anisotropy of nanoscale magnetoelastic elements. Here piezoelectric materials are mechanically coupled to nanoscale magnetoelastic elements to transfer the energy. The coupling coefficient (energy transferred) in available piezoelectric materials (e.g. PZT) can be ~0.8 and the coupling coefficient in available magnetoelastic materials (e.g. Tb_0.3Dy_0.7Fe_1.92 Terfenol-D) is of similar magnitude. Thus the amount of energy to overcome a 0.3aJ bit barrier is estimated to be optimisitacally ~0.5 aJ or an efficiency of ~60% neglecting line losses, i.e. orders of magnitude improvement compared to STT. However, strong magnetoelastic materials such as thin film (~50 nm) Terfenol-D are difficult to fabricate as well as few analytical predictions are available to suggest the response in a single magnetic domain structure.
This presentation focuses on reviewing our DC magnetron sputtering of Terfenol-D onto both Si and PMN-PT substrates as well as analytical predictions using coupled micromagnetics and elastodynamic to control magnetism. The Terfenol-D thin film was produced with various sputtering parameters and two different crystallization methods. Several characterization techniques including WDS, XRD, TEM, AFM, SQUID and MOKE was used to determine the physical and magnetic properties. Results show that tailoring the residual stresses with both sputtering and annealing parameters is extremely important for voltage control of magnetism. TEM studies reveal that the film deposited on heated substrates has large grains grown along the film thickness producing relatively rough surfaces while the film crystallized by post-annealing method shows uniformly distributed small grains producing a smooth surface. The Terfenol-D film was also deposited onto (011) cut PMN-PT single crystal substrate to demonstrate the strain dependence of magnetic anisotropy with a large change of 1553 Oe in coercivity and lambda;s = 859 x 10-6. Analytical results indicate that the single domain state in nanoscale structures is extremely sensitive to the exchange constant and accurate measurements are needed. Analytical predictions of write energies using Terfenol-D film deposited onto thin film PZT are orders of magnitude larger than STT systems presently being used.

Current-induced spin-orbit torques originating from the spin-Hall effect enable various applications including magnetization switching and steady-state oscillations with low power¹. Another promising approach to manipulate the magnetization is to use multiferroic heterostructures with large magnetoelectric coupling arisen from strain-, interfacial charge- or exchange bias-mediated effects^2,3. Here we demonstrate tuning of spin-orbit torques with applied voltage in a CoFeB/Pt bilayer thin film on a ferroelectric (PMN-PT) substrates. The effective spin-Hall angle theta;_SH is extracted from both line-shape analysis and DC-current-induced resonance linewidth modulation using spin-torque ferromagnetic resonance (ST-FMR)^4,5. In the virgin state, theta;_SH asymp;0.10 for CoFeB/Pt was estimated which is comparable to other reported Pt-based spin-Hall devices. With the voltage-induced uniaxial strain from the PMN-PT substrate, a large modulation of the magnetic anisotropy field was observed, and theta;_SH increased to asymp;0.12 simultaneously. ST-FMR was conducted at several in-plane applied field angles to understand the anisotropy of spin-orbit torques induced by the anisotropic piezo-strain. Such findings with the enhanced spin-orbit torques offer new opportunities for applications such as voltage-assisted magnetization switching and tunable spin-Hall oscillators. The in-situ control of the spin-Hall angle with voltage may also provide fundamental insights into spin-orbit torques that are otherwise inaccessible.
¹ A. Brataas and K.M.D. Hals, Nat. Nanotechnol. 9, 86 (2014).
² J. Ma, J. Hu, Z. Li, and C.W. Nan, Adv. Mater. 23, 1062 (2011).
³ N.X. Sun and G. Srinivasan, Spin 2, 1240004 (2012).
⁴ L. Liu, T. Moriyama, D.C. Ralph, and R. a. Buhrman, Phys. Rev. Lett. 106, 1 (2011).
⁵ T. Nan, S. Emori, C.T. Boone, X. Wang, T.M. Oxholm, J.G. Jones, B.M. Howe, G.J. Brown, and N.X. Sun, Phys. Rev. B 91, 214416 (2015).

Faster magnetic domain walls normally require larger driving electric currents that may generate excessive heat. An electric field can move magnetic domain walls with negligible heat dissipation through piezostrain actuation, but an electrically driven unidirectional domain wall motion remains scarce. Here we computationally demonstrate a fast electric-field-driven unidirectional domain wall motion in a magnetic nano-ring via piezostrain. In an amorphous Co₄₀Fe₄₀B₂₀ nano-ring, a peak average magnetic domain wall velocity of above 550 m/s is predicted upon applying a voltage to a (001)-oriented Pb(Mg_1/3N_b2/3)_0.7Ti_0.3O₃ nano-island underneath. This velocity value is comparable to those in state-of-the-art technology of current-driven magnetic domain wall motion. An analytical model is developed to understand the simulation data, based on which a new method of determining the effective domain wall mass is proposed. Our results suggest an energy-efficient route to achieve fast and unidirectional magnetic domain wall motion for domain wall spintronic devices.

Electron microscopy has been playing an increasing role in condensed matter research. Its unique feature of high spatial resolution and large scattering cross-sections with matter, especially with the recent advances in electro-optical aberration correction, has yielded unparalleled capabilities in characterizing nanoscale structures, such as interfaces and defects that control the properties of materials, with atomic precision. In this presentation, I will give an overview of our recent work on the interplay between lattice, charge (polarization) and spin (magnetization) order in ferroelectric, ferromagnetic and multiferroic systems. Dynamic behavior of ferroelectric vortices in hexagonal ErMnO3 and ferromagnetic vortices in Py/Cu/Py tri-layered heterostructures under, respectively, electric biasing and GHz resonance excitation will be given as examples. I will show that in the former case, the switching of ferroelectric order is directly related to the antiferromagnetic order via apical oxygen tilt, while in the latter the switching of the magnetic order of the indirect exchange coupled dual vortices can be attributed to the RKKY coupling mechanism. The measurement of non-adiabatic spin torque effect using spin current will also be presented. Work supported by DOE/BES-MSD under Contract DE-AC02-98CH10886. Collaborations with S.W. Cheong, D. Arena, and K. Buchanan are greatly acknowledged.

Recent experimental results have shown that enhanced functionality and emergent behavior can be obtained through interfacing ferroic films at nano- and/or meso-scales. Epitaxial nanocomposite films, in either lateral superlattice or vertical aligned forms, provide a new platform to produce novel properties that cannot be obtained in the individual constituents. In this talk, we will discuss our efforts to understand the role of interfaces on the competing interactions of complex ferroic metal-oxide films. The first example will be the induced interface magnetism in La_0.7Sr_0.3MnO₃/BiFeO₃ superlattices extending from the interface through several atomic layers of the BiFeO₃ (BFO). Using controlled synthesis, advanced probing, and theoretical modeling, we provide a framework to address some key challenges of understanding emergent behaviors across the interfaces. We will then use vertically aligned epitaxial nanocomposite films (such as BiFeO₃:Sm₂O₃ and La_0.7Sr_0.3MnO₃:ZnO) as other examples to illustrate the significant role of interfaces on the functionalities of ferroic metal-oxide films.

Magnetoelectric composites have attracted increasing attention due to the control of magnetic anisotropy by electric field through strain-mediated magnetoelectric interaction. Devices based upon such effect could be both fast and energy efficient, overcoming some of the intrinsic limitations of conventional microwave devices, while also providing new functionalities. However, in order to overcome the strict requirements of RF/microwave devices, namely low insertion losses and narrow FMR linewidths, all while introducing wide-range frequency tunability, high-quality epitaxial magnetoelectric heterostructures composed of low-loss, commercially-viable ferrimagnetic materials must be realized. To this end, our work focuses on developing unique physical vapor growth facilities that enable the growth of such structures. We have started by focusing on two commercially-viable microwave ferrite materials, yttrium iron garnet, and (Ni_0.65Zn_0.35)(Fe_1.2Al_0.8)O_4shy; (Al:NiZnFe0₄). Here, report on lrm;the growth, microstructure, and magnetic properties of high-quality epitaxial YIG and Al:NiZnFe0₄ using ultra-high vacuum pulsed laser deposition, and our transition towards growth on PMN-PT substrates. The strained-layers are atomically smooth, and demonstrate excellent resonance characteristics. YIG has an X-band linewidth of ~1.6 Oe and Gilbert damping coefficient of ~1.5x10^-4; comparable to the highest-quality submicron-thick YIG reported in recent years. Films exhibit significant uniaxial anisotropy along the out-of-plane axis, such that the effective demagnetizing field is >800 Oe greater than the saturation magnetization 4πM_s. We will also present the results of the effects of strain states on Al:NiZnFeO₄ grown on several substrates and show how we believe the magnetoelectric heterostructures using this material are promising candidate for next-generation frequency-agile RF/microwave components.

Magnetoelectric (ME) composites exploiting high coupling between piezoelectric and magnetostrictive phases have demonstrated high sensitivity and are considered very promising candidates for magnetic field sensing. At electromechanical resonance, several orders of magnitude increase in ME coupling and larger voltage output can be achieved. Here we investigated free-standing CoFe thin-film doubly clamped stress reconfigurable resonators as a function of magnetic field and pressure. Magnetic hysteresis loops revealed a large uniaxial anisotropy with the easy magnetization axis aligned along the length of the beams, resulting from residual tensile stress. Operation of the beams in a vacuum chamber with reduced air pressure results in a marked increase in the quality factor of the driven resonators. This enhancement led to improved signal to noise ratio, and the intrinsic magnetic noise is predicted to be reduced by a factor of 6 potentially reaching as low as ~ 25 pT/radic;Hz at 1 Torr. A significant reduction in the intrinsic magnetic noise is also predicted, making these stress reconfigurable sensors a promising candidate for near DC magnetic field sensing. Stress reconfigurable sensors operating under vacuum could thus further improve the limit of detection and advance development of magnetic field sensing technology.

Multiferroic magnetoelectric nanocomposites or heterostructures are made up of individual ferroelectric and ferromagnetic phases. They are being explored to significantly improve the performances of or/and to add new functionalities to many existing or emerging devices such as memories, tunable microwave devices, sensors, etc. The multiferrocity of such multiphase materials is achieved through the mechanical, electric, and or magnetic interfacial coupling across the interphase interfaces, and hence their magnetoelectric effects are expected to strongly depend on the microstructures of nanocomposistes or the geometrical configurations of heterostructures. In this presentation, we will report the recent applications of the phase-field method to modeling and predicting the magnetoelectric responses of multiferroic nanocomposites and heterostructures. In particular, we will discuss strategies of achieving 180 degree magnetic switching through the application of an applied electric field or electric field pulses and of optimizing the magnetoelectric responses of magnetoelectric composites or the performances of heterostructural devices

Multiferroic nanocomposites of ferrites and ferroelectrics are of interest for studies on strain mediated magneto-electric (ME) coupling since the ME coupling strength in nanocomposites is expected to be strong due to a large surface area-to-volume ratio. We recently synthesized core-shell nickel ferrite and barium titanate nanoparticulate composites by click-reaction aided chemical self-assembly by creating covalent attachments between the particles [1,2]. A shortcoming of this technique, however, is that the attachments are non-dynamic (cannot be reorganized once assembled) and create nanocomposites that are limited in size due to low coupling efficiency for the click reaction. Here we discuss a new approach based on biological tenets which utilizes DNA covalently attached to nanomaterials to effect assembly of nanocomposites based on specific base-pairing interactions between complimentary sequences. This approach possesses great promise since the strength of bonding, specificity of interaction, and distance between nanoparticles can be controlled using custom designed DNA. Furthermore, the thermal reversibility of binding allows a reorganization of the morphology of the nanocomposites (aided by electric or magnetic fields) and the ability to assemble mm-size or greater structures based on nm periodicity. In this work we discuss the synthesis of oligomeric DNA-functionalized ferroelectric and ferromagnetic nanoparticles, 600 nm barium titanate (BTO) and 200 nm nickel ferrite (NFO), respectively, which are components of multiferroic composites. Under high salt conditions, the modified particles exhibit self-self interactions forming superparticles ranging in sizes from 0.5 mm (BTO) to 100 mm (NFO). The superparticles disassemble in the absence of saline. Mixing BTO and NFO particles, possessing complementary DNA sequences, results in formation of multiferroic heteronanocomposites held together by DNA hybridization. The composites were imaged by scanning electron microscopy and scanning microwave microscopy. The presence of heteroassemblies along with core-shell architecture is clearly observed. Magneto-dielectric measurements indicate strong ME coupling between NFO and BTO in the composites. Such structures are of importance for realizing mm-sized nanocomposites with theoretically predicted strong ME coupling for applications in useful technologies.
1. G. Sreenivasulu, et.al, Appl. Phys. Lett. 104, 052901 (2014) .
2. M. Popov, et. al., AIP Advances 4, 097117 (2014).

Magnetoelectric composites pose a promising candidate for room temperature sensing of low frequency, low amplitude signals e.g. from biomagnetic sources. They are characterised by a strong enhancement of the ME effect in mechanical resonance. MEMS structures usually yield comparably high mechanical resonance frequencies e.g. in the kHz regime. However by operating the sensor as a mixer using frequency conversion, the mechanical resonance amplification can still be exploited for the detection of low frequency signals below 100 Hz. Depending on the choice of the constituent materials, different paths to apply the carrier signal exist. Usually the mixing is preferred in the magnetic phase by applying an off-resonance AC magnetic field as carrier signal, where the sum or difference frequency with the signal to be measured is set in such a way to correspond to the mechanical resonance. The integration into sensor arrays of this technique is not easy, additionally noise specific to the magnetic layer is introduced. In a different, energy efficient implementation using frequency conversion the carrier signal is electrically applied to a piezoelectric phase, modulating the magnetostrictive layer, without the need for external bias magnetic fields. In this presentation results of Si-based thin film cantilevers employing amorphous FeCoSiB as magnetostrictive phase, Aluminium nitride (AlN) and lead zirconate titanate (PZT) are presented and discussed in view of low frequency detection of biomagnetic signals.
Financial support by the DFG is gratefully acknowledged.

Ferroelectric characteristics are determined by structural formation and orientations of the ferroelectric thin films are a dominant factor to determine ferroelectric characteristics. There are many attempts to achieve (001) oriented films and recently studies of (111) oriented films attract interest because of strong structural and electronic coupling. BiFeO₃ as a famous multiferroic material shows various switching behaviors depending on the orientation of the thin films. PbTiO₃ as a ferroelectric material has a tetragonal structure and because of asymmetry in the structure there would be many ferroelectric twins in the (111)-oriented thin films. In this paper, we fabricated epitaxial PbTiO₃ thin films with two different orientations, (001) and (111). In addition, we use LaNiO₃ as a bottom electrode and new electronic phases were obtained because of different electronic arrangement in PbTiO₃ and LaNiO₃. In order to control the orientations of the thin films, (001) and (111) oriented SrTiO3 were used as substrates. X-ray diffraction patterns were used to determine crystallinity and its strained nature of the thin films. In addition, reciprocal space mapping was also obtained and showed clearly strained structures. In order to analyze structural ordering in atomic resolution, we used transmission electron microscopy. Because of different atomic arrangement in (001) and (111) planes, different growth mechanisms are developed in each thin film.

Based on phase field modeling and thermodynamic analysis, purely electric-field-driven magnetization reversal was shown to be possible in a multiferroic heterostructure of a square-shaped amorphous Co₄₀Fe₄₀B₂₀ nanomagnet on top of a ferroelectric layer through electrostrain. The reversal is made possible by engineering the mutual interactions among the built-in uniaxial magnetic anisotropy, the geometry-dependent magnetic configuration anisotropy, and the magnetoelastic anisotropy. Particularly, the incorporation of the built-in uniaxial anisotropy made it possible to reverse magnetization with one single unipolar electrostrain pulse, which is simpler than previous designs involving the use of bipolar electrostrains and may alleviate ferroelectric fatigue. Critical conditions for triggering the magnetization reversal are identified.

Spin filtering leads to the difference in tunneling probability of spin up and spin down of electrons, which has a similar effect as ferromagnetic electrodes and can generate and detect spin signals very effectively. Therefore spin-filtering materials can substitute magnetic electrodes in spintronic devices. In our experiments, we studied epitaxial magnetic tunnel junctions formed with epitaxial FeCo/MgO/EuS as the active region, and deposited on (100)-Si in a high vacuum electron beam evaporation system. We used MgO as the buffer layer to help relax the lattice mismatch between Si and FeCo, and to avoid alloying and interdiffusion at the interface. We use a composite MgO and EuS barrier in order to couple the large symmetry filtering effect of MgO with the spin filtering effect from EuS. The devices were created with in situ shadow masks and the effective junction area is 30x30µm². The top electrode is formed with polycrystalline 15nm Ti followed with 15nm TiN and 6nm Al₂O₃ as the protective layer. The high quality epitaxial heterostructures have been verified by X-Ray diffraction and off-axis pole scans. Mild surface roughness, characterized with ex situ AFM, is associated with the epitaxial layers&’ formation and is attributed as the major cause of pinholes across the junctions. Bottom electrode oxidation is also present due to the MgO deposition and the extra shadow masking layers. Magnetoresistance of these junctions were measured and large tunnel magnetoresistance up to 64% was achieved at 4k.

Perovskite-spinel nanocomposite thin films are of great interest for multiferroic devices due to the coexistence of ferroelectric and ferrimagnetic property and their magnetoelectric coupling. Ferrimagnetic spinel (CoFe₂O₄) phase grows epitaxially as pillars within an immiscible ferroelectric perovskite phase (BiFeO₃) and this nanocomposite represents a good combination of magnetostrictive CoFe₂O₄ (CFO) and piezoelectric and ferroelectric BiFeO₃ (BFO), leading to strong coupling between the two phases. It is well known that magnetic properties such as coercivity and anisotropy of the CFO pillars in the nanocomposite are governed by the pillar shape, size, strain state and coupling with the perovskite matrix.
The ability to adjust the magnetic hysteresis of the multiferroic nanocomposite film is necessary to optimize its performance, for example in low power memory or logic devices. This can be done by adjusting the composition or geometry of the pillars, which can be accomplished by changes in growth conditions such as the temperature or the relative arrival rates of the species during the deposition process. However, it has been challenging to produce spinel pillars with a through-thickness composition gradient due to the cation interdiffusion during growth. Another approach to modifying the CFO hysteresis is to alter the CFO pillar shape. There has been little exploration of shape-modulated pillars, though we previously showed examples of pillars with a through-thickness modulation in width made by placing a blocking layer to partly cover the pillars, or by changing temperature.
In this work, the effects of pillar width modulation on the strain state and magnetic properties of CFO-BFO nanocomposites are described. In-depth structural and strain analysis of nanocomposites was performed using high resolution X-ray diffraction, since strain and epitaxy are important in determining the structure and properties of self-assembled nanocomposites.

Nanocomposites consisting of multi-phase complex oxides exhibit an amazing range of functional properties and have received great interest for high-temperature superconductivity, colossal magnetoresistance, ferroelectricity, and multiferroicity. Most self-assembled vertical nanocomposites are made of oxides with perovskite and spinel structures in which one phase grows as pillars within the other, but there are now reports of epitaxial nanocomposite thin films consisting of combinations of other oxides.
Elastic interactions between the two phases are very important in determining their properties. For example, strain transfer at the vertical interface affects the magnetic and ferroelectric properties dramatically and leads to magnetoelectric coupling. On the other hand, chemical interactions are also important in nanocomposite growth and properties. It has been reported that interdiffusion between spinel and perovskite phases are almost negligible. However, we have reported that there is significant interdiffusion within spinel pillars during deposition, leading to a homogeneous pillar composition even if the composition of the arriving flux is changed partway through growth.
In this study, self-assembled epitaxial BiFeO3-MgO and BiFeO3-MgAl2O4 nanocomposite thin films were grown on SrTiO3 substrates by pulsed laser deposition to investigate the chemical diffusion between perovskite and the MgO or MgAl₂O₄ phases. Phase separation of BiFeO3 and MgO or MgAl2O4 was observed within a very small growth window due to substantial chemical diffusion. Diffusion of iron from BiFeO3 to MgO or MgAl2O4 formed a magnetic spinel ferrite, which led to the formation of magnetic nanocomposites from codeposition of non-magnetic MgO or MgAl2O4 and antiferromagnetic BiFeO₃. A BiFeO3-MgO nanocomposite was used as a blocking layer to prevent interdiffusion between a nanocomposite consisting of magnetically hard CoFe2O4 pillars in BiFeO3 and one consisting of soft MgFe2O4 pillars in BiFeO3, but the MgO did not grow selectively on the CoFe2O4 and it led to the merger of neighboring pillars. Chemical reactions between the four phases were investigated by X-ray photoelectron spectroscopy and transmission electron microscopy to understand the evolution of the (BiFeO3-CoFe2O4)(BiFeO3-MgO)(BiFeO3-MgFe2O4) nanocomposite film.

Multiferroics have potential for many applications such as actuators, magnetic field sensors or new types of electronic memory devices. Magneto-electric effect (ME) was first observed in inorganic single crystals but their use on device applications was not successful since these materials exhibit weak ME effect. In order to overcome these limitations, the use of composite materials is promising. In this context, our project aims at developing new flexible artificial multiferroic materials exhibiting the highest possible magneto-electric coupling through a smart tailoring of the hybrid interface between a ferroelectric polymer (poly(vinylidene fluoride), PVDF) and inorganic ferromagnetic nanoparticles.
Here, we report the design by soft chemistry of self-standing hybrid films which consisted of 10-12 nm sized ferrimagnetic and magnetostrictive cobalt ferrite (CoFe₂O₄) nanoparticles (NPs)¹ dispersed into a ferroelectric and piezoelectric PVDF beta phase film. We first prepared CoFe₂O₄ nanoparticles (NPs) using the polyol process. Half of the NPs were functionalized to improve their miscibility in PVDF, in order to promote interactions between NPs and PVDF. Then, flexible CoFe₂O₄-PVDF based hybrid films were prepared by dispersing NPs in a solution of PVDF and melt processing at 80-100 °C². Interestingly, functionalized nanoparticles led to better dispersion in hydrophobic PVDF promoting its crystallinity.
The stress-induced self-crystallization of piezoelectric polymer phase is also obtained by mechanical stretching³ of films containing originally non polar alpha phase resulting in films mostly in the electro-active β-phase.This latter is achieved by coupling tensile tests to X-ray diffraction (XRD) for monitoring the crystalline phase modifications. Microstructural changes are observed by near-field microscopy (AFM) pointing a granular-fibrillar transformation highlighting α to β phase transition.
The resulting composites were characterized by Transmission Electron Microscopy (TEM), Vibrating Sample Magnetometer (VSM), Thermogravimetric Analysis (TGA), Zeta potential, Attenuated Total Reflectance FTIR (ATR-FTIR), Differential Scanning Calorimetry (DSC), X-ray diffraction (XRD) and near-field microscopy (AFM).
[1]F. Mammeri, F. Mamèche, Z. Kataya, N. Yaacoub, A. Slawska-Waniewska, N. Menguy, J.-M. Grenèche, S. Ammar. MRS Proceedings. 2011, 1359, mrss11-1359-nn08-07 doi:10.1557/opl.2011.883.
[2]L. Ourry, S. Marchesini, M. Bibani, S. Mercone, S. Ammar, F. Mammeri. PhysicaStatus Solidi A. 2015, 212, 252-258.
[3]B. Mohammadi, A.A. Yousefi, S.M. Bellah, Polymer Testing, 2007, 26, 42-50.

Soft magnetic films with high damping performance in high ferromagnetic resonance frequency range are wildly used in noise absorbed area. In order to enlarge the applicability of materials, other than high saturation magnetization 4πM_s, suitable anisotropy Hk, the position and width of the resonance peak should be artificially adjustable for the different working frequency as well. In this present work, soft-magnetic multilayers with or without SiO₂ interlayer were fabricated on Si (111) substrates using magnetron sputtering. The static magnetic and high frequency electromagnetic propertiesof FeCoB(60 nm)/SiO₂(6 nm)/FeCoSiB(t nm) or FeCoB(60 nm)/FeCoSiB(t nm)were systematically investigated. The experimental results showed that all multilayers had an obvious in-plane uniaxial magnetic anisotropy and the interlayer of SiO₂ could decouple the different magnetic phases leading to steps in hysteresis loops. Two resonance peaks of complex permeability spectra were observed obviously for FeCoB(60 nm)/SiO₂(6 nm)/FeCoSiB(t nm) films. Compared to the samples without SiO₂ interlayer, the presence of dielectric interlayer could effectively decouple the different magnetic phases in the multilayers leading to double resonance peaks and broaden band in gigahertz range, which makes the properties of each magnetic layer be controlled independently as well. The method to broaden band in our system was simple and controllable, which may be utilized for industrial applications.

Multiferroic composite materials consisting of both magnetic property and ferroelectric performance have drawn a lot of attention due to the strong magnetoelectric (ME) coupling. Most recently, many multiferroic materials working in high frequency range have been reported, which give rise to a number of novel multiferroic RF devices, including thin film stress sensors, phase shifters, and integrated tunable RF inductors. ^[1] FeGaB/PMNPT multiferroic heterostructure has been reported to have giant tunability of 450Oe, with an acceptable peak to peak linewidth of 70Oe, yet in which fine polished PMNPT with minimized surface roughness was required. Lou and coworkers ^[2] have reported a large ME coupling at microwave frequencies in a FeGaB/Si/PMN-PT multiferroic laminates, which exhibited a tunability of 58% with E-field from -6 to +2kV/cm while the linewidth was around 20Oe. Based on this idea, in this work, we induce DRIE (Deep Reactive-Ion Etching) MEMS process to remove Si substrate completely after FeCoSiB deposition over SOI wafer with in-film bias field of 200Oe, then FeCoSiB (100nm)/Si (10mu;m)/SiO₂ (50nm) is epoxy bonded to 0.5mm (011) PMN-PT slab, with easy axis parallel and perpendicular to (011) direction respectively. Electric field induced change in magnetic hysteresis loops were measured by vibration sample magnetometer. The magnetization ratio shows a large change when the electric field is varied from 0 to ±8kV/cm. The field-sweep FMR spectra under different electric fields were tested by electron spin resonance spectroscopy. In the sample with easy axis parallel to (011) direction, a large shift of resonance field of 159.66Oe, with a narrow linewidth of 38.46Oe can be observed with 8kV/cm, indicating a large ME coupling. On the contrary, the resonance field shifts to the opposite direction with tunability of 65.56Oe and linewidth of 49.92Oe for the sample with easy axis perpendicular to (011) direction. Good butterfly shape shift-ac field behavior was also demonstrated during measurement. On the comparison, in the direct deposited FeCoSiB (100nm) over the same non-polished PMNPT substrate, the resonance field shift is similar but the linewidth can be as large as 283.03Oe. Therefore, this work provides both the giant ME coupling and narrow linewidth, hence offers great opportunity for Si-base integrated electrostatically tunable RF devices (inductors, sensors, etc.) and circuits.
[1] Y. Gao et al., Microwave Symposium (IMS) 2014 IEEE MTT-S International (2014)
[2] J. Lou et al., Adv. Mater., 21, 4711 (2009)

In the past decade, strong magnetoelectric (ME) coupling has been demonstrated in magnetostrictive/piezoelectric composites.[1]One of most important applications of ME composites is weak magnetic field sensor. In such application, giant ME coupling coefficient, i.e. magnetic-field induced electrical charge, of hundreds even thousands of V/Oe was reported for ME composites if one selected ferroelectrics with very high piezoelectric coefficient. However, the parasitic pyroelectric effect causes significant noise for magnetic field detection. In addition, for applications which require spatial resolution by sensor arrays, the bulk ME composites are not suitable despite their fascinating properties. So thin film ME composites consisting of piezoelectric not ferroelectric AlN (or ZnO) and amorphous magnetostrictive layers were fabricated using MEMS techniques.[2] However, it turns out very challenging in sensing weak DC and low frequencies magnetic fields for these film stacks due to the relatively low ME coefficient and the large 1/f noise.
In current work, instead of collecting magnetic-field induced electrical charge which decreases with shrinking the volume of ME sensor, we propose to utilize surface acoustic wave resonator to detect the shift of the center frequency with the change of magnetic field. In essence, the magnetoelectric surface acoustic wave (MESAW) properties of ZnO/FeSiB half space were studied based on a stable scattering matrix method. Only the first Rayleigh mode was found with phase velocity between 2200m/s and 2650m/s, and the maximum electro-mechanical coupling coefficient about 1%. It is found that the center frequency of ZnO/FeSiB is highly sensitive on the change of magnetic field, up to 396 MHz/Oe, thanks to the giant delta(E) effect of Metglas@2605S3. The monitoring of center frequency of MESAW allows a broad band magnetic field from DC to several kHz, since the delta(E) effect is unrelated to the magnetic field frequency. In addition, no bias magnetic field is needed.
Reference
[1] C. W. Nan, M. I. Bichurin, S. Dong, D. Viehland, and G. Srinivasan, J. Appl. Phys. 103, 031101 (2008).
[2] H. Greve, E. Woltermann, H. Quenzer, B. Wagner, and E. Quandt, Appl. Phys. Letts. 96, 182501 (2010); C. Kirchhof, M. Krantz, I. Teliban, R. Jahns, S. Marauska, B. Wagner, R. Kneuro;ochel, M. Gerken, D. Meyners, and E. Quandt, Appl. Phys. Letts. 102, 232905 (2013)
[3] L. Huang, D. D. Wen, Z. Zhong, H. W. Zhang and F. M. Bai. Magnetoelectric surface acoustic wave resonator with ultrahigh magnetic field sensitivity. arXiv:1405.4076 [cond-mat.mtrl-sci]

One of the challenges today is the constantly growing demand for energy and a significant amount thereof is used for industrial and private cooling applications. Therefore new cooling devices with a higher efficiency are needed. Solid-state cooling cycles relying on magnetic field induced phase transitions have been proposed [1]. Promising materials are Heusler alloys such as Ni-Co-Mn-X (X=Ga, In, Sb, Sn) which show an inverse magnetocaloric effect with a structural and magnetic phase transition between a ferromagnetic austenite at high temperature and a weak magnetic martensite at low temperature. Additionally to high external magnetic fields, the transformation can also be induced by the application of mechanical stress. This can be achieved by the deposition of a magnetocaloric thin film on a ferroelectric substrate. Thin films are of particular interest since their high surface to volume ratio allows fast heat transfer and high cycling frequencies which leads to higher cooling power using less material.
We present in-situ studies on sputter deposited epitaxial Ni-Mn-Ga-Co thin films on ferroelectric Pb(Mg_1/3Nb_2/3)_0.72Ti_0.28O₃ (PMN-PT) substrates. The transition temperature can be tuned with a variation of the composition. Temperature dependent texture and magnetic measurements show the structural and magnetic phase transition in the material. From a series of M(T) measurements at external magnetic fields of 0.1, 2, 5 and 9 T a T(H) phase diagram has been determined and a maximum entropy change of 3.6 J/(kgK) for magnetic fields µ₀H > 1 T was calculated [2]. We demonstrate with X-ray diffraction measurements, that an applied voltage induces up to 0.13 % strain in the thin film and that the phase transition in Ni-Mn-Ga-Co can not only be influenced by a magnetic field or temperature [3], but also by mechanical stress. In-situ synchrotron XRD measurements at PETRA III 02.1 (DESY, Hamburg) have been used to obtain a temperature-strain phase diagram by in-situ application of an electric voltage to the multiferroic stack. This work is supported by DFG through SPP 1599 www.FerroicCooling.de.
[1] S. Fähler, U. K. Rössler, O. Kastner, J. Eckert, G. Eggeler, H. Emmerich, P. Entel, S. Müller, E. Quandt and K. Albe, Caloric Effects in Ferroic Materials: New Concepts for Cooling. Adv. Eng. Mat., 14, 10-19 (2012)
[2] B. Schleicher, R. Niemann, A. Diestel, R. Hühne, L. Schultz and S. Fähler, Epitaxial Ni-Mn-Ga-Co thin films on ferroelectric substrates for multicaloric applications. J. Appl. Phys. in revision (2015)
[3] A. Diestel, R. Niemann, B. Schleicher, S. Schwabe, L. Schultz and S. Fähler, J. Appl. Phys. accepted (2015)

Magnetoelectric (ME) coupling is a promising mechanism for controlling magnetism by electrical field in multiferroic heterostructures, and exhibits extensive applications in low-power spintronic devices such as magnetoelectric random access memories (MERAM), magnetic field sensors, and tunable radio frequency (RF)/microwave devices. The major challenge is to find an energy efficient way to switch the distinct magnetic states in a stable, non-volatile, and reversible manner. We investigated the interfacial charge-mediated ME coupling in ultrathin NiFe/PLZT heterostructures using ferromagnetic resonance (FMR) measurements. The E-field-induced magnetic surface anisotropy showed a strong dependence on the thickness of the magnetic layer, which was discussed based on the interfacial charge screening effect. Non-volatile and reversible tuning of effective magnetic field was obtained due to remanent polarization in the ferroelectric layer. The measured FMR field shift is robust, non-volatile, and repeatable, allowing voltage impulses, instead of a constant voltage, to manipulate magnetism. These results present a framework for realizing charge-mediated ME coupling in ultrathin multiferroic heterostructures, demonstrating great potential for delivering compact, lightweight, reconfigurable, and energy-efficient electronic devices.

The ability to create atomically sharp interfaces of perovskite oxides has resulted in functional interfaces, and the degree of oxygen octahedral rotations and tilts are central in determining the interface properties. Interfaces between (111)-oriented perovskites are in this regard interesting, and expected to have large electronic and orbital reconstructions. Here we present a detailed study of epitaxial (111)-oriented ferromagnetic La_0.7Sr_0.3MnO₃ (LSMO)/ antiferromagnetic LaFeO₃ (LFO) based heterostructures, with focus on emerging magnetism at the LSMO/LFO interface. To do this, we rely on epitaxial heterostructures deposited by pulsed laser deposition on (111)-oriented SrTiO₃. AFM analysis reveals a step-and-terrace morphology, and XRD investigations show that the thin films are epitaxial. High-resolution STEM studies confirm the crystalline quality and sharp interfaces. Thin LSMO/LFO heterostructures have an increased Curie temperature compared to single layers of LSMO(111), and the effect is increased for thinner layers of both components. VSM investigations reveal an increased saturation magnetic moment, larger than for bulk LSMO, for ultra thin LSMO/LFO(111) heterostructures. In order to elucidate on this extra magnetic moment, we performed x-ray magnetic circular dichroism studies. By comparing the Mn and Fe edges we conclude that there is an induced ferromagnetic moment at Fe in the antiferromagnetic LFO that couples anti-ferromagnetically to LSMO. The appearance of ferromagnetism in the LFO layer is also confirmed by spin-polarized neutron reflectivity investigations. EELS measurements across the interface reveal no charge transfer, and that Fe has a constant oxidation through the LFO layer. The data will be discussed with respect to possible structural interface coupling between the rhombohedral LSMO and orthorhombic LFO, and corroborated with DFT calculations. We will especially focus on the interplay between structural changes and orbital ordering effects.

As one of the fundamental components for integrated circuits, inductors are widely used in different applications. Voltage tunable inductors, if made possible, will enable another dimension of frequency tuning in L-C tank circuits, and lead to a much larger tunable frequency range. However, compact and power efficient electronically tunable inductors have not been available until multiferroic inductors that utilize voltage induced permeability change in a strain-mediated magnetic-ferroelectric heterostructure¹. Most recently, we have reported integrated voltage tunable inductance in solenoid inductors with a magnetic core with large tunability at RF frequencies². In this work, we report integrated GHz inductors based on solenoid structures with FeCoB/Al₂O₃ multilayer films, which exhibit significantly enhanced inductance and quality factor (Q) at GHz compared with air core. Compared with air core counterparts, these inductors show excellent high-frequency performance with a wide frequency range of 0.5-5GHz, and over 100% inductance enhancement. Multiferroic inductors with a 0.1mm and 0.3mm thick ferroelectric (011) lead magnesium niobate-lead titanate PMN-PT slab were fabricated, which the range of tunable inductance shows over 100% with a low voltage of < 50V. These kind of voltage tunable GHz inductors with giant tunability show great potential for reconfigurable radio frequency integrated circuits.

Due to its close to room temperature phase transition from antiferromagnetic (AFM) to ferromagnetic (FM) holds great promise for future applications [1]. For example, the giant entropy change during the field-induced magnetostructural transition can potentially be used for efficient magnetocaloric devices. However, the magnetocaloric effect peaks within a narrow temperature range at the magnetic phase transition temperature. This poses a serious limitation to applications and finding viable options to tailor the active temperature range is imperative [2, 3].
Here we show that strain-doping by He implantation into epitaxial films is an effective means to control the center and the width of the magnetic transition in phase pure FeRh. Low-energy helium ions are implanted ex situ at room temperature using an ion source. The internal lattice stress induced by the He doping results in a unit cell volume increase that significantly reduces the magnetic ordering transition temperature. By finely controlling the number of helium ions in the film, it is possible to control magnetic transition temperatures with unprecedented precision. Our data shows that the total refrigeration capacity of the magnetocaloric remains constant upon He doping. The magnetocaloric active temperature range can thereby be tuned simply by choosing a specific He dose without losing any efficiency. An additional advantage is that the He depth can be controlled by the ion acceleration voltage during the implantation process. This opens the possibility to create specific He ion distribution profiles within the FeRh films and will thus allow for the manufacturing of magnetocaloric devices with working temperature ranges designed for specific applications. We use polarized neutron reflectrometry (PNR) to determine the depth profile of He implanted FeRh films. The data unambiguously demonstrates the relation between the He concentration in the film and the magnetic properties.
Supported by the US DOE Office of Basic Energy Sciences, Materials Sciences and Engineering Division and under US DOE grant DE-SC0002136.
[1] S.P. Bennett et al., Scientific Reports 5, 9142 (2015)
[2] R. Barua et al., Journal of Applied Physics 115, 17A903 (2014)
[3] J. Liu et al., Nature Materials 11, 620 (2012)

Manipulation of electronic properties at a single molecular scale by subjecting the molecules to external stimuli like electric or magnetic field holds promises for fascinating science and large range of applications. Recent years have witnessed huge surge in interest for ferroelectric tunnel junctions (FTJ) based on complex oxide materials showing giant tunneling eletroresistance (TER) effect. The FTJs can write and read information using an applied voltage and retain data without any power supply and therefore can be used as non-volatile, energy efficient data storage elements.
Here we report a robust, spontaneous, and electrically switchable TER at room temperature in few nanometer thick, spin-coated films of ferroelectric co-polymer P(VDF-TrFE) (70:30) on different conducting oxide and metallic electrodes. A room temperature conductive-tip atomic force microscopy (AFM), piezo-force microscopy (PFM) and tunneling AFM (TUNA) measurements on the P(VDF-TrFE) thin films on conductive Indium Tin Oxide (ITO), La_0.67Sr_0.33MnO₃ (LSMO), Nb-doped SrTiO₃ (Nb-STO) and Au substrates clearly demonstrate that large TER arises due to electrically induced polarization reversal in P(VDF-TrFE) molecules. Time dependent measurements reveal written domains are most stable on the Nb-STO bottom electrodes followed by LSMO, ITO and Au bottom electrodes. This finding is in contradiction to the existing knwledge that ferroelectric domains are most stable on highly conducting electrodes and to find a satisfactory answer to this puzzle we have made in-depth study of the system changing morphology and thickness of the polymer films. The results show that depolarizing field effect is negligible in ultrathin co-polmer films. This result is of utmost importance for non-volatile resistive memory elements working under ambient conditions. Also easy, large area and low temperature processing technique of the polymer holds promises for additional advantages of low cost, printable, greener technology solutions for future.

Recently, multiferroic heterostructures composing of both ferromagnetic and ferroelectric phases have received much attention due to the electrical tunability of magnetic properties and their potential applications such as spintronic, sensors, or tunable microwave devices.^1,2 In general, the electrical tuning of magnetic properties can be realized by strain-mediated and/or charge-mediated magnetoelectric (ME) coupling in multiferroic composites.^3,4 It is interesting to investigate the combined strain- and charge- mediated ME coupling, and to distinguish these two ME coupling mechanisms in multiferroic heterostructures.⁴ In this work, ME coupling in both Fe₇₀Co₃₀ and Fe₇₀Co₃₀/Cu(5nm) thin films, which were deposited on PMN-PT single crystal substrate, respectively, were investigated. The magnetic and electric properties, ferromagnetic resonance (FMR) of the two thin films were measured when applying different electric fields on PMN-PT substrates. The FeCo/PMN-PT heterostructure exhibited an electric-field induced effective magnetic field change of 216 Oe and magnetization difference up to 7.8%. The ME coupling mechanisms were discussed based on the experimental results of the two multiferroic heterostructure thin films.
Reference
1. M. Liu, and N. X. Sun, Phil. Trans. R. Soc. A 372, 20120439 (2014).
2. Y. Lee, Z. Q. Liu, J. T. Heron, J. D. Clarkson, J. Hong, C. Ko, M. D. Biegalski, U. Aschauer, S. L. Hsu, M. E. Nowakowski, J. Wu, H.M. Christen, S. Salahuddin, J. B. Bokor, N. A. Spaldin, D. G. Schlom, and R. Ramesh, Nat. Commun. 6, 6959 (2015).
3. M. Weisheit, S. Fahler, A. Marty, Y. Souche, C. Poinsignon, and D. Givord, Science, 315, 349 (2007).
4. T. X. Nan, Z. Y. Zhou, M. Liu, X. Yang, Y. Gao, B. A. Assaf, H. D. Lin, S.Velu, X. J. Wang, H. S. Luo, J. M. Chen, S. Akhtar, E. Hu, R. Rajiv, K. Krishnan, S.Sreedhar, D. Heiman, B. M. Howe, G. J. Brown, and N. X. Sun, Sci. Rep. 4, 3688 (2014).

Multiferroic-magnetoelectric materials that combine ferromagnetism and ferroelectricity and the coupling between them are one of the most active research materials because of their potential applications in memory devices. These materials give the possibility of controlling electric with a magnetic field or vice versa in a single material. The rich magnetoelectric coupling between the ferromagnetic and ferroelectric orders is expected to produce new phenomena, such as magnetocapacitance, magnetooptic, etc. that would exploit the best features of ferroelectricity and magnetism with electrical-writing and magnetic-reading in random access memories. However, most of the single-phase multiferroic materials exhibit low Curie temperature and weak magnetoelectric coupling near or above room temperature; that limits their utilization for practical applications. For these reasons, we studied (Pb_0.975Co_0.025)(Zr_0.53Ti_0.47)_0.9(Fe_0.5Ta_0.5)_0.1O₃ (PCZTFT) thin films synthesized by pulsed laser deposition technique. We have grown films with different thicknesses from 3 to 250 nm of PCZTFT on La_0.67Sr_0.33MnO₃/(LaAlO₃)_0.3(Sr₂AlTaO₆)_0.7 (LSMO/LSAT) (001) substrate. The x-ray diffraction patterns of the heterostructures showed only (00l) reflections corresponding to the LSAT substrate, PCZTFT and LSMO layers. The Atomic force microscopy of PCZTFT/LSMO/LSAT heterostructures showed that the average surface roughness decreases from 2.7 to 0.3 nm for PCZTFT films with thickness from 250 to 3 nm respectively. Well saturated ferroelectric and magnetic loops were observed in PCZTFT-250 nm films at room temperature, exhibiting a remanent polarization of sim;65 mu;C/cm² and a saturated magnetization of ~50 emu/cm³. Piezoresponse force microscopy measurements for 3, 5, and 7 nm ultrathin PCZTFT films showed a clear and reversible out-of-plane phase contrast above ± 3V, which indicates the ferroelectric character in ultra-thin films. The magnetic domain patterns were obtained utilizing magnetic force microscopy in all samples. The ferromagnetic to paramagnetic transition was investigated utilizing a physical property measurement system (PPMS, Quantum Design). These and the temperature dependent dielectric properties will be discussed for 20, 50, 100 and 250 nm thick PCZTFT films.

The electrical and magnetic characteristics of thin films can be significantly altered by strain effects from the substrate. Vanadium dioxide (VO₂) exhibits a reversible metal-insulator transition at ~341 K in the bulk, transforming from a low-temperature insulating phase to a high temperature metallic phase. We have studied the effect of strain on the electrical properties of VO₂ thin films. Strain is induced in the film by depositing them on a piezoelectric Pb(Mg1/3Nb2/3)0.72Ti0.28O3(001) (PMN-PT) substrates and applying an electric field. In order to reduce the inter-diffusion between film and substrate, we have used a buffer layer of 100 nm RF sputtered silicon dioxide on the PMN-PT substrate. Then a high quality polycrystalline film of VO₂ is grown on SiO₂/PMN-PT using a low pressure chemical vapor deposition system. The resistance versus temperature measurement shows a three order of magnitude change in the resistivity through the metal-insulator transition, prior to measurement of the strain effect. To investigate the effect of strain, thin film electrodes of Pt are deposited on top of the VO₂ film as well as to the back of the substrate, and the substrates are poled. Bipolar sweeping of the bias voltage shows a butterfly-like change in the resistance of the sample resulting from strain. Hysteresis loops are obtained by unipolar sweeping of the voltage, with repeated measurements providing consistent hysteresis plots. The observed change in the resistance is more than 25% when the bias voltage crosses zero, which gives two possible resistance states after removing the bias voltage. We will discuss the temperature dependence of the strain-induced resistance changes, and suggest alternate geometries for increasing these effects.

Once structural glass is formed, the relaxation time (inversion of structural relaxation rate) needed to crystallize would increase exponentially with decreasing temperature, thus the glass has little chance to transform into crystal upon further cooling to zero temperature. Similar amorphous ferroics can also form through quenching the defect-rich ferroics from high temperature. However, a spontaneous amorphous to crystalline transition has been observed experimentally in doped ferroic systems within certain range of point defect concentrations. The origin of this transition may have different physical explanations; a universal thermodynamic principle is still missing. Here we highlight our thermodynamics analysis on this and the ferroelastics is used as a stereotype. Our results clearly elucidate that the dramatically different thermodynamic behaviors, i.e., diffusive or cooperative, would influence the temperature-dependent relaxation behaviors of structural glass and ferroelastic system. For structural glass, the activation energy barrier between glass and crystal decreases upon cooling but remains nonzero even at zero Kelvin, leading to continuous decrease in relaxation rate, which makes the spontaneous transformation experimentally impossible; For cooling process in ferroelastic system, although activation energy also decreases, the relaxation rate, which is affected by both activation energy and temperature, would actually increase at the low temperature range, leading to an abnormal amorphous to crystalline transition.

Magnetoelectric-multiferroic materials have drawn much attention of the scientific community due to the interest in the physics of coupling mechanisms between ferroelectric and magnetic ordering as well as for application in novel multifunctional devices. Multiferroic materials with combined robust electric and magnetic polarizations at room temperature with effective magneto-electric coefficients are promising for device applications in memory and spintronics. BiFeO₃ (BFO) is a room temperature single phase multiferroic, which exhibits both ferroelectricity and ferromagnetism (i.e., T_C ~ 1100 K, T_N sim; 643K). As both T_C and T_N in BFO are above room temperature, it is of great interest from the technological application point of view. But due to high leakage current, high dielectric loss, low magnetoelectric coupling constant, it is not suitable for device applications. A and B site co-substitution in BFO can lead to reduction in leakage current, increase in resistivity and enhance the multiferroic and magnetoelectric properties. Here, we report, the role of La and Gd co-substitution on the structural, electrical, magnetic and magnetoelectric properties of BFO. Bi_0.90La_0.10Fe_1-yGd_yO₃ (y=0.01, 0.03) (BLFGO) samples were synthesized by conventional solid state reaction method. Stoichiometric amount of oxide powders by addition of 10% Bi₂O₃ excess to compensate Bi volatilization were mixed and milled by low energy ball milling for 24 h then calcined and sintered at 850 °C and 940 °C for 4h each, respectively. XRD patterns revealed that the BLFGO ceramics with 3% and 1% Gd samples are rhombohedraly distorted perovskite structures accompanying acceptable amount of secondary phase at around 2theta;=27.7° arising from excess Bi which is added to avoid Bi deficiency and oxygen vacancies. Enhanced weak ferromagnetic behavior was observed in 3% Gd substituted sample while that of 1% Gd exhibits antiferromagnetic behavior at room. Detailed studies on Raman, dielectric, ferroelectric, magnetic, and magnetoelectric properties will be presented at the meeting.

In recent years, single phase BiFeO₃ (BFO) showed promising potential towards photovoltaic applications due to its superior multiferroic properties and small bandgap. Among other wide bandgap Fe-PV materials (i.e. BaTiO₃, LiNbO₃ and Pb(Zr,Ti)O₃), BFO exhibits large open circuit voltage (V_OC), tunable output, and switchable photodiode effect. Herein, we report improved multiferroic and photovoltaic properties in (Sm, Hf) co-substituted (Bi_0.9Sm_0.1)(Fe_0.97Hf_0.03)O₃ [BSFHO] thin films. BSFHO thin films with the thickness of ~320 nm are deposited on LaNiO₃ (LNO)-buffered Si (100) substrate using pulsed laser deposition (PLD) technique. An improved ferroelectric behavior is observed in Pt/BSFHO/LNO/Si hetrostructure with a significant decrease in leakage current mainly due to the suppression of oxygen vacancies by donor type aliovalent dopant (Hf⁴⁺) at B-site. We observed well saturated P-E hysteresis loop with a maximum remanent polarization of ~52 mu;C/cm² at an applied electric field of ~600 kV/cm. Polarization switching is observed from the piezoelectric phase imaging after applying different poling voltages through the conducting tip of PFM on the film surface. Switchable photocurrent and photovoltage can be observed with changing the polarization direction by applying electrical poling of different voltages (±2 to ±10 V). The open circuit voltage (V_OC) ~ 0.25 (-0.30) V and short circuit current density (J_SC) ~ -80 (115) mu;A/cm² were found after downward (upward) poling at ±10 V. Switchable diode characteristics and ferroelectric resistive switching are also observed in BSFHO thin films. The mechanism of switchable photovoltic and diode effect in our BSFHO films can be explained by polarization modulated Schottky barrier at Pt/BSFHO interface. Our results provide deep insights into the major role of polarization flipping on switchable photovoltaic effects in multifunctional ferroelectric films.

Exploiting electric fields to modulate magnetocrystalline anisotropy of a ferromagnet is a promising way to control the magnetization orientation. Using voltage rather than current (that is required to generate a spin transfer torque) can potentially alleviate the energy dissipation bottleneck of the existing magnetic memory technology. For transition-metal ferromagnets and their alloys the applied electric field affects the surface (interface) anisotropy due to the spin-dependent electrostatic screening being confined to the surface (interface). The electric field changes the relative population of the d-orbitals, which governs the surface magnetocrystalline anisotropy energy. The magnitude and sign of this effect is extremely sensitive to the electronic structure of the interface on the scale of meV/atom, and thus depends dramatically on the atomic structure of the interface (e.g., oxygen stoichiometry), electron doping (alloying) of the ferromagnet, formation of the interface states, etc. In addition, there is a notable effect resulting from atomic displacements driven by an applied electric field which modulates bond lengths between ferromagnet and insulator atoms at the interface, contributing through interface orbital hybridizations. This talk will overview our recent efforts, based on first-principles density-functional calculations, to understand the mechanisms responsible for the voltage controlled magnetic anisotropy and to explore ways to enhance this effect, such as applying voltage across a high-permittivity dielectric or ferroelectric material.

Voltage control of magnetism has the potential to substantially reduce power consumption in spintronic devices, while offering new functionalities through field-effect operation [1-4]. Magneto-electric coupling has usually been achieved using complex oxides such as ferroelectrics, piezoelectrics, or multiferroic materials. Here I describe a new approach to voltage control of magnetism based on solid-state electrochemical switching of the interfacial oxidation state [2-4] in thin metallic ferromagnets. In ultrathin ferromagnet/oxide bilayers, perpendicular magnetic anisotropy (PMA) arises from interfacial hybridization between the ferromagnetic 3d and oxygen 2p orbitals. By using GdOx as a gate oxide with high oxygen ion mobility, we show that O^2- can be reversibly displaced at a Co/GdOx interface with a small gate voltage, leading to unprecedented large, non-volatile changes to interfacial PMA by > 0.75 erg/cm² [4]. We present high-resolution cross-sectional transmission electron microscopy (TEM) and in-situ electron energy-loss spectroscopy (EELS) during bias application [4], which directly reveals the reversible motion of oxygen ions at the Co/GdO_x interface. By optimizing the device structure and geometry, we show that these effects can be achieved at room temperature and achieve a reduction in switching time from ~100 seconds [2-4] to <100 microseconds [4]. Finally, we describe the integration of magneto-ionic gates into spintronic devices based on the controlled motion of magnetic domain walls in magnetic nanowires, which function as low-power spintronic memory and logic devices. In collaboration with U. Bauer, A. J. Tan, P. Agrawal, S. Emori, and H. L. Tuller L. Yao and S. van Dijken.
[1] U. Bauer, et al., Nano Lett.12, 1437 (2012)
[2] U. Bauer, et al., Appl. Phys. Lett.100, 192408 (2012)
[3] U. Bauer, et al., Nature Nano.8, 411 (2013)
[4] U. Bauer, et al., Nature Mater.14, 174 (2015)

Magnetoelectric (ME) composites are subject of great interest due to their high ME coefficients that are larger than natural multiferroics and their high sensitivity to magnetic fields at room temperature without the need of cooling. Such composites, consisting of magnetostrictive and piezoelectric materials, mainly exploit the product property of their constituent phases through elastic coupling of the strains which is greatly reduced by substrate clamping. Release of the sensors from the underlying Si substrates might allow the realization of sandwich-type sensors that are independent of the mechanical resonance.
FeCoSiB, being an amorphous magnetostrictive material, undergoes a crystallization process at the conventional AlN deposition temperatures (300-400 °C) which deteriorates the soft-magnetic properties of the material. This obliges the deposition of AlN as the first layer thus limiting any possible variation concerning the sensor design. Therefore a process flow that allows the deposition of AlN without intentional substrate heating was developed which in turn enables the fabrication of inverse-bilayer and sandwich structures. Deposited AlN films displayed high longitudinal (d_33,f = 5.3±0.2 pm/V) and transversal (e_31,f = -1.38±0.02 C/m²) piezoelectric coefficients for 1.5 µm thick films. Dielectric losses much below 0.1% were achieved while a near constant permittivity was observed for different films thicknesses. Performed TEM investigations displayed quasi-epitaxial AlN growth starting from the AlN-seed layer interface.
Furthermore the process compatibility between the constituent depositions was investigated and the miniaturization as well as the release of the composite sensors from the underlying substrate were attempted via combined lithographic and wet/dry etching techniques. Prepared composite sensors were characterized and the obtained results were compared against conventional composites prepared on thick Si substrates.
Financial support by the DFG is gratefully acknowledged.

Magnetoelectric (ME) coupling has received tremendous research effort due to potential application of various multifunctional devices. However, the strain-mediated ME heterostructures suffers from substrate clamping effect. Strong charge-mediated ME effect has been reported in (La,Sr)MnO₃, Ni and Ni₇₉Fe₂₁ ultra-thin films. Piezoelectric single crystals are characterized with predefined spontaneous polarization and ferroelectric domains. Large saturate polarization can only be obtained along spontaneous polarization directions. In this study, we selected piezoelectric single crystals with different orientation, thus different polarization values. Much stronger charge-mediated ME coupling can be observed along the spontaneous polarization directions. These results provide an important method to improve the charge-mediated ME coupling effect.

Strain-mediated magnetoelectric coupling between ferromagnetic films and ferroelectric substrates permits electrical control of magnetism. I intend to present PEEM images that illustrate the local nature of these magnetic changes in both unpatterned films and patterned elements. For a recent example of relevant PEEM images, see DOI: 10.1002/adma.201404799, where we concomitantly imaged electrically driven changes of ferromagnetic and ferroelectric domains, using a Ni film on a barium titanate substrate. This high-resolution magnetoelectric imaging was performed at Diamond Light Source, UK.

Exchange coupling between antiferromagnetic order in multiferroic BiFeO₃ and a ferromagnetic overlayer has until now only been shown in multidomain BiFeO₃ films in which domain walls, either for their ferromagnetic or ferroelastic properties, are considered essential to the coupling. Difficulties controlling domain wall properties as well as devices challenges of reproducibility and scaling suggest an alternative approach may be necessary for practical exchange-coupled spintronic devices based on BiFeO₃. Here we show a room temperature, deterministic and robust “intrinsic” (without domain wall) exchange coupling in monodomain BiFeO₃¹. In situ ferroelectric switching of monodomain BiFeO₃/Co heterostructures peformed during photoemission electron microscope (PEEM) and magnetooptic Kerr effect (MOKE) measurements show a coherent ~70° clockwise rotation of the Co magnetization upon poling, and counterclockwise 70° rotation upon reverse poling, reproducibly for thousands of cycles. The Co magnetization rotation is strictly correlated to single-domain 71° ferroelectric switching and reorientation of the spin-cycloid plane, from in-plane to nearly vertical. Our experimental data demonstrate that the spin-cycloid properties in these strained films are different from bulk single crystal BiFeO₃, suggesting that the internal magnetoelectric coupling is changed by the crystal symmetry lowering caused by strain, and that is likely responsible for creating the strong intrinsic coupling at the BiFeO₃/Co interface.
*This work has been done in collaboration with W. Saenrang, B. A. Davidson, F. Maccherozzi, J. P. Podkaminer, K. Reierson, J. Irwin, J. W. Freeland, C. A. F. Vaz; L. Howald, J. C. Frederick, S. Ryu, M. Veenendaal, S. Dhesi, M. S. Rzchowski.
**This work was supported by Army Research Office under Grant No. W911NF-13-1-0486.
¹S.H. Baek et al. Ferroelastic switching for nanoscale non-volatile magnetoelectric devices. Nature Materials9, 309 (2010).

The integrated non-reciprocal tunable bandpass filter with a NiZn feritte film deposited via a spin sprayed process is presented. With external in-plane magnetic fields from 100 to 400Oe, the central frequency was tuned from 3.78 to 5.27GHz, with an insertion loss of 1.73 to 3.42dB and an in-band isolation of more than 13 dB, which was attributed to the non-reciprocity characteristics of the magnetostatic surface wave. By attaching the BPF on a PMN-PT slab, it can perform a voltage tunable behavior by magnetoelectric coupling. Frequency tunabilities of 500MHz/100Oe and 55MHz/1(kV/cm) are achieved. This design with dual functionality of integrated tunable bandpass filters and an UWB isolator can lead for compact, low-cost and power-efficient RF communication systems on RFIC and MMIC.

Magnetoelectric sensors based on cantilever like composites posses high sensitivities up to 20 kV/cmOe [1] using the mechanical enhancement of the cantilevers resonance. Magnetic ac -fields in the picotesla regime can be measured by this enhancement [2].
Many applications like magnetoencelography and -cardiography require however to measure quasi static magnetic fields from 1 to 100 Hz, thus different groups have explored measurement techniques to utilize the high sensitivities at low frequencies.
Jahns et al. [3] developed 2012 a magnetic frequency conversion technique using the nonlinearity of the magnetostrictive curve. Hereby a magnetic modulation field is applied which mixes the low frequency magnetic field into the mechanical resonance. Another technique [4] uses the change in elastic modulus (ΔE-effect) of the magnetostrictive phase depending on its magnetization. By applying an electrical modulation field the change in resonance frequency is utilized. In this work we compare the techniques on one type of ME sensor and discuss their expandability. The sensors consist of 50 µm thick silicon cantilevers (length 3 mm, width 1 mm) made from SOI Wafers. The active materials are 2 µm ALN as piezoelectric phase and 2 µm FeCoSiB as the magnetostrictive phase.
[1] C. Kirchhof, M. Krantz, I. Teliban, R. Jahns, S. Marauska, B. Wagner, R. Kno#776;chel, M. Gerken, D. Meyners, and E. Quandt, Giant magnetoelectric effect in vacuum, Appl. Phys. Lett.102, 232905 (2013).
[2] R. Jahns, A. Piorra, E. Lage, C. Kirchhof, D. Meyners, J.L. Gugat, M. Krantz, M. Gerken, R. Knöchel, and E. Quandt, Giant Magnetoelectric Effect in Thin-Film Composites, J. Am. Ceram. Soc. 96, 1673 (2013).
[3] R. Jahns, H. Greve, E. Woltermann, E. Quandt, and R. Knöchel, Sensitivity enhancement of magnetoelectric sensors through frequency-conversion. Sensors and Actuators A: Physical183, 16 (2012).
[4] B. Gojdka, R. Jahns, K. Meurisch, H. Greve, R. Adelung, E. Quandt, R. Knöchel, and F. Faupel, Fully integrable magnetic field sensor based on delta-E effect, Appl. Phys. Lett.99, 223502 (2011).

Magnetoelectric (ME) effects in composite ferromagnetic-piezoelectric (FM-PE) structures arise due to combination of magnetostriction of the FM layer and piezoelectricity of the PE layer. The direct ME effect exhibits itself as generation of electrical voltage u across the structure under action of a magnetic field H, while the converse ME effect exhibits itself as variation of the structure magnetization M caused by an electrical field E. Under high amplitudes of magnetic fields the nonlinear dependence of the FM layer magnetostriction on magnetic field results in a variety of nonlinear ME effects such as detection of ac magnetic fields, generation of higher harmonics, mixing of magnetic fields etc.
This review paper presents results of experimental investigations of nonlinear ME effects in composite planar structures containing FM layers of various magnetostrictive materials (nickel, galfenol, permendur, amorphous alloys) and various PE layers (ceramic lead zirconate-titanate, single-crystal langatate, piezo-fiber composite, polymer piezoelectrics). The dependences of nonlinear ME interaction efficiency on the excitation field amplitude h in the range up to 100 Oe and on the dc magnetic field H up to 4 kOe in the frequency band 1 mHz -200 kHz at the room temperature have been studied. It has been demonstrated that the efficiency of nonlinear ME interactions increases in the resonance regime under excitation of bending or in-plane acoustic oscillations in the structures.
A simple theory describing analytically the effects observed at relatively low excitation fields and allowing for numerical calculation of the effects&’ characteristics for arbitrary amplitudes of the fields is presented. Possible applications of nonlinear ME effects for design of the high-sensitivity dc magnetometers, wide-band ac magnetic field sensors, low-frequency modulators and filters have been demonstrated.

Magnetoelectric coupling forms the central question for the studies of multiferroics. In particular, the electric field control of magnetism has drawn great attention due to the rich physics involved as well as the promising potential applications in spintronic. In the past one decade, there has been significant progress in this vein. For instance, the electric field control of the antiferromagnetic axis has been observed in multiferroic (ferroelectric and antiferromagnetic) BiFeO₃, which is further employed to achieve the electric field control of magnetization through the exchange coupling with ferromagnetic layers (e.g. CoFeB, La_1-xSr_xMnO₃); and the electric field can be directly coupled with the ferromagnetic materials as well with the carrier and lattice modulations through the charge screening and ferroelastic coupling respectively. In this talk, I will present our recent study of a nonvolatile electric field control of magnetic ground state. We demonstrate that the electric field can be used as a tuning knob to switch on and off of an antiferromagnetic state at room temperature. Comparing with the electric field control of antiferromagnetic axis, which requires delicate domain manipulation, this approach holds the advantages of simplicity and determinacy.

Multiferroics, combining multiple order parameters, offer an exciting way of coupling phenomena such as electronic and magnetic orders and have been one of the central focuses in the research of condensed matters. Attention on BiFeO₃ is considerably increasing because its high order temperatures and strong coupling between ferroelectricity and antiferromagnetism make it possible to control magnetism by an electric field, a quality that is vital to future spintronic circuits. While ferromagnetism has been observed in BFO thin films due to the Dzyaloshinskii-Moriya (DM) interaction, the magnetic moments in BFO are very small (several emu/cc). A recent study on the strain-driven rhombohedral/tetragonal mixed phase in BFO showed that an enhanced spontaneous magnetization emerges from the highly strained rhombohedral (R) phase as a consequence of piezomagnetic coupling to the adjacent tetragonal phase as well as the epitaxial constraint. In this talk, strong magnetization in uniformly strained BFO thin films is revealed. This is achieved by 1) a highly distorted rhombohedral phase to suppress the antiferromagnetic Neel temperature and consequently reduce the strength of super-exchange interaction, and 2) an external electric field applied to rotate the ferroelectric polarization axis, which is shown to enhance DM interaction based on the density function theory (DFT) calculation. The enhanced ferromagnetism and strongly coupled behaviors is observed by photoemission electron microscopy. Although BFO sets an ideal template of manipulating the spin degree of freedom via electric field, before the realization of new devices, several key issues have to be solved. The primary control parameter is the ferroelectric switching. Solving the ferroelectric reliability issues, such as imprint, retention, and fatigue has to be made prior to realizing a practical device. For example, retention can be addressed to thermodynamic instability of the domain. Asymmetric free energy landscapes between polarizations directed away and toward the substrates result in at least one unstable polarization state. Such an asymmetry is mainly due to different out-of-plane electric boundary conditions. Effects of depolarization fields in the unstable domain become significant when the polarization bound charges are not fully screened. In order to shed light on the retention problem, we intend to use the elastic energy as a key parameter to improve ferroelectric retention of BFO, since the ferroelectric switching of BFO typically involves a ferroelastic deformation. In this talk, self-assembled BFO mesocrystal will serve as a model system. We observed the elastic coupling between BFO mesocrystal and surrounding matrix plays an important role to diminish the retention of BFO. The achievement of great improvement on the functionalities in this system via structural coupling will open a new avenue to the application in nonvolatile memory and spintronics.

Magnetic nanowire sensors require nanometer control of dimensions, while incorporating various metals and alloys. To realize this control, our 7- to 200-nm diameter nanowires are synthesized within insulating matrices by direct electrochemistry, which negates sidewall damage, such as that caused by lithographical patterning of vacuum-deposited structures. Our nanowires can easily have lengths 10,000x their diameters, and they are layered with magnetic (Co, Fe, FeGa, FeNi, Ni) and non-magnetic (Ag, Cu, Au). This talk will reveal synthesis secrets for nm-control of layer thicknesses, even for difficult alloys, which has enabled studies of magnetization reversal, magneto-elasticity, giant magnetoresistance (GMR), and spin transfer torque (STT) switching. In addition, this lithography-free synthesis yields 10-nm diameter nanowires that have resistivities of only 5.4mW.cm (nearly that of bulk copper) due to negligible sidewall roughness. Therefore, these nanowires will mitigate the ITRS Roadmap&’s “Size Effect” Grand Challenge which identifies the high resistivities in small interconnects as a barrier to continued progress along Moore&’s Law (or better). Ten-nm diameter trilayers of [Co(15nm)/Cu(5nm)/Co(10nm)] have also met or surpassed all of the criterion for the world&’s smallest read heads with 30 Omega; resistance and 19% magnetoresistance. High magnetoresistance is also possible in other multilayered nanowires that exhibit excellent properties for mulit-level nonvolatile random access memory (RAM) using STT switching at very low current densities (~kA/cm²).

With the development of new devices miniaturizing trend, there is growing interest in combining magnetic and electronic properties into multi-functional thin film materials for potential applications in various devices. In recent years, the research interest in multiferroics has focused more and more in thin films area with the development of thin film growth techniques, which enable deposition under epitaxial engineering and non-equilibrium conditions. Magnetoelectric (ME) composite thin films among multiferroic ones are promising candidate materials for applications in new and novel multifunctional devices. Among the most widely studied two-phase multiferroic composite thin films, self-assembled epitaxial BiFeO₃-CoFe₂O₄ (BFO-CFO) nanocomposite thin films have attracted tremendous research interest. This kind of thin films are known to self-assembly grow and form (1-3) nanostructure with CFO nanopillars of rectangular shape embedded in BFO matrix by deposited on (001) SrTiO₃ (STO) substrates.
In our work, by utilizing that specific nanostructure, we pre-deposited one BFO layer on the bottom, and post-deposited another BFO layer on the top of that above mentioned BFO-CFO (1-3) thin film on (001) STO substrate. Hence, we successfully obtain a new quasi (0-3) heterostructures, which have second phase CFO nanoparticles embedded in a primary BFO matrix phase. These new nanocomposite heteroepitaxial films not only overcome the clamping effect from the substrate, but also significantly reduced possible leakage current paths through the ferromagnetic phase. An indirect ME effect has been confirmed for this new magnetoelectric quasi (0-3) film, which owing to a better coupling from good connectivity amongst the piezoelectric and magnetostrictive phases. Phase-field simulation has been conducted to simulate the enhancement of the piezoelectric strain and then confirmed the experimental results. This new architectured magnetoelectric heterostructures may reveal new approaches to the application of ME films in micro-devices.

Two different types of charge-transfer (CT) states, namely optical CT states and magnetized CT states, based on thin-film devices with the architecture of ITO/ TPD:BBOT/TPD/Co/Al is studied by magneto-dielectric measurement. The magnetized CT states are formed at the Co/TPD interface, generating a magneto-dielectric response with a broad non-Lorentzian line-shape. The optical CT states are generated at the TPD:BBOT interfaces under photoexcitation, leading to a magneto-dielectric signal with a narrow Lorentzian line-shape. We find that combining the optical CT states and magnetized CT states yields a new magneto-dielectric signal with distinctive line-shape and amplitude. The magneto-dielectric analysis indicates that there exists a coupling between optical CT states and magnetized CT states through the interactions between the magnetic Co/TPD interface and the optically excited bulk TPD:BBOT. Furthermore, we show that the coupling between optical CT states and magnetized CT states experiences the long-range Coulomb and spin-orbital interactions by changing (i) the density of optical CT states and (ii) the separation distance between optical CT states and magnetized CT states. Clearly, the coupling between optical CT states and magnetized CT states provides a new approach to mutually tune magnetic and electronic properties through thin-film engineering by combining magnetic and organic materials.

Polarization and related ferroelectric properties in M-type lead hexaferrites (PbFe₁₂O₁₉) will be presented. The remnant polarization of the PbFe₁₂O₁₉ ceramic reaches as high as 104mu;C/cm², exhibiting large spontaneous polarization at room temperature. Subsequent annealing of the PbFe₁₂O₁₉ ceramics in oxygen atmosphere plays a key role on the saturation of its polarization hysteresis loop due to the great enhancement of its electric resistance. Two current peaks in I-V curve reveal the switching of polarization, which provides an effective evidence for the ferroelectricity of the PbFe₁₂O₁₉ ceramics. Its temperature dependent dielectric constant demonstrates a colossal change near the vicinity of the transition temperature (518°C) of ferro- to papra- phase, which follows modified Curie Weiss law, verifying its relaxor ferroelectric characterization. The ceramics also exhibit strong ferromagnetic characterization. These combined functional responses in PbFe₁₂O₁₉ ceramics present an opportunity to create electric devices that actively couple the magnetic and ferroelectric orders. The off-center feature of the iron atoms away from the FeO₆ octahedron will be discussed in the Fourier space of the HRTEM images. Therefore the origin of the polarization of PbFe₁₂O₁₉ could be derived out. Magnetoelectric coupling effect of the PbFe₁₂O₁₉ ceramics after annealing in the pure oxygen atmosphere will also be presented.

Ferro- or ferrimagnetic oxides are of great interest for a wide variety of applications. Insulating spinel oxides (e.g. ferrites like CoFe₂O₄) are attractive for magneto-electronic devices as well as for high frequency and high power applications. They can show multiferroic properties and can be fabricated to be transparent, making them potentially useful for opto-electronics and photovoltaics. In spintronics, LaMnO₃ and La_1-xSr_xMnO₃ are now among the most commonly used materials for electrodes.
However, the deposition of such magnetic oxide layers usually requires sophisticated instrumentation. We present a new approach to effectively prepare smooth thin films of metal oxides by spin coating from aqueous precursor solutions followed by annealing. The fabrication process is reliable and easily tunable for different material compositions while the use of purified water instead of organic or halogenated solvents makes it sustainable for the environment.
Here, we present the characterization results for thin films of CoFe₂O₄ prepared in this manner. The full dielectric tensor (including the magneto-optical off-diagonal elements) of the material was determined from spectroscopic ellipsometry and magneto-optical Kerr effect (MOKE) spectroscopy measurements for a spectral range of 1.7 to 5.0 eV. The occurrence of characteristic spectral features and line shapes proves the correct compound composition. Additionally, MOKE magnetometry measurements reveal a typical, very broad magnetization hysteresis. Electrical characterization was carried out by 4-point probe I-V-measurements and the morphology of the films was investigated by cross sectional scanning electron microscopy. We especially focus on the evolution of the investigated properties with the temperature of the post-spin coating annealing procedure.

Multiferroic materials with simultaneous ferroelectric and magnetic order have and continue to be the focus of scientific research due to their intriguing innate physics and potential for application in next-generation sensors, logic, and memory devices. Among multiferroic materials, BiFeO₃ is particularly interesting because of its room temperature multiferroism, which could allow for electric-field control of magnetism. In order to understand coupling and ultimate utilize this material, it is critical to understand how the domain walls, misfit strain, and crystal orientation of BiFeO₃ films influence and drive exchange coupling to ferromagnetic layers and potential routes to achieve large intrinsic magnetization in BiFeO₃. In this presentation, we will first report on the observation of 180° stripe nanodomains in (110)-oriented BiFeO₃ thin films grown on orthorhombic rare-earth scandate substrates. From there, we will explore exchange coupling studies based on Co_0.9Fe_0.1/BiFeO₃ heterostructures which reveal exchange bias and exchange enhancement in heterostructures based-on BiFeO₃ with 180° domain walls and an absence of exchange bias in heterostructures based-on BiFeO₃ with 71° domain walls; suggesting that the 180° domain walls could be the possible source for pinned uncompensated spins that give rise to exchange bias. This is further confirmed by X-ray magnetic circular dichroism studies, which demonstrate that films with predominantly 180° domain walls have larger magnetization than those with primarily 71° domain walls. Finally, we will show that strain can be used to control the antiferromagnetic spin axis in (110)-oriented BiFeO₃ films and to tune the magnetic anisotropy of the coupled Co_0.9Fe_0.1 layer. Furthermore, we find that the Fe L-edge X-ray linear dichroism is fully reversed for (110)-oriented BiFeO₃ #64257;lms grown on two substrates with reversed lattice mismatch. Careful analysis of the angular dependence of X-ray linear spectra allow for determination of the magnetic spin axis in films which have only one or two domain variants.

We investigate the structure and properties of the 1:1 superlattice composed of LaVO₃ and SrVO₃ using the density functional theory plus U (DFT+U) method. We find two low-energy charge-ordered Mott-insulating antiferromagnetic phases with distinct ordering patterns. In one of these insulating phases, the spontaneous polarization normal to the interface is nonzero. When the structures are fully relaxed, the energy of this polar state relative to the lower-energy nonpolar state is found to be only 2.3 meV/5 atom unit cell. The epitaxial strain dependence of the energy difference between the two phases is calculated. Under tensile strain, the two phases become equal in energy, which suggests that that the polar state could be induced by applied electric field, and, depending on the switching process, a ferroelectric hysteresis loop could be observed.
Support: ONR N00014-11-0666

We present a solid-state synthesis of lithium niobate doped with lanthanum nanoparticles (La_0.05Li_0.85NbO₃) with their corresponding structural aspects, electrical and ferroelectric properties. La_0.05Li_0.85NbO₃ was prepared using lithium carbonate, niobium oxide and lanthanum oxide as precursors. After alloying, the samples were calcinated at 9000C in order to remove carbonate residues and a second milling of 8 hours was done in order to obtain nanoparticles of an average size of 70±5 nm. X-ray diffraction indicates the formation of ferroelectric phase obtained in air atmosphere with spherical shape as confirmed by TEM micrographs. The particles were binded with PVA and then pressed at 105 kg/cm² to conform pills with a diameter of 11.7 mm and a distance between plates of h=1 mm. Then sintered at 1000, 1100, 1120, 1150 ⁰C for 5 hours. Samples sintered at 1120⁰C showed better densification and were further studied at 1 and 3 hours. Raman and X-ray fluorescence spectroscopy were performed in order to indicate new vibrations modes and elemental analysis quantification respectively. Permittivity and dissipation factor were measured in a frequency range of 100 Hz and 1 MHz. The results show and abnormal behavior between 200 and 400 KHZ when compared to pure lithium niobate. Hysteresis loop was determinate with the objective to determinate the behavior ferroelectric properties with the influence of the La doped. The hysteresis loops show a P_s= 0.235 mu;C/cm², P_r= 0.141 mu;C/cm² and E_c= 1.35 kV/cm values for La_0.05Li_0.85NbO₃ sample. And the values for P_s= 0.0701 mu;C/cm², P_r= 0.0382 mu;C/cm² and E_c= 391 kV/cm when compared to pure lithium niobate.

BiFeO₃, more than essentially any other material, has attracted intense interest as a multiferroic material because of its robust ferroelectric polarization, room-temperature magnetic order, and a range of other intrinsic, exotic phenomena. Practical application of BiFeO₃, however, has ultimately been limited by one critical property - the material typically exhibits large leakage currents in device structures. Chemical dopants and post-processing techniques, such as annealing, have been widely studied as potential routes to minimize leakage in epitaxial BiFeO₃ thin films, but less work has explored the effects of self-doping in BiFeO₃. The ability to deterministically control both the cation and anion chemistry and chemical homogeneity of such complex materials is key in both understanding and ultimately controlling these deleterious effects.
In this presentation, we explore a detailed study of how the growth process effects the stoichiometry, conductivity, and ferroelectric performance of epitaxial BiFeO₃ thin films. High-quality, phase-pure, epitaxial thin films of BiFeO₃ were grown via pulsed-laser deposition as a function of a number of growth parameters. Using extensive Rutherford backscattering spectrometry (RBS) studies, it has been found that the laser repetition rate (i.e., growth rate) and fluence can provide deterministic control of the average Bi:Fe ratio from Bi-excess to Bi-deficiency of over 10 atomic percent. Furthermore, decreasing the growth rate results in the formation of bismuth gradients as large as 8 atomic percent across the thickness of a 100-150 nm thick film. Routes to produce stoichiometric films will be shown. Despite considerable variations in film chemistry, X-ray diffraction and piezoresponse force microscopy studies show the ability to produce phase-pure films with similar two- and four-variant domain structures expected for BiFeO₃. Subsequent capacitor-based electrical measurements further highlight the importance of film chemistry. In particular, we study the chemistry-dependent evolution of electrical leakage and ferroelectric hysteresis loops. Leakage current density is found to be reduced in slightly Bi-deficient films and in films with small (~3%) gradients in the bismuth content across film thickness. Further non-stoichiometry and gradient magnitude (~6%) results in increased leakage current density. Similar trends in the ability to access low-frequency hysteresis loops are observed. As part of this presentation, we will discuss the defect chemistry; the relative conductivity of intrinsic, oxygen vacancy-dominated, and Bi-vacancy dominated versions of BiFeO₃; and the implications of these effects on the leakage mechanism. Ultimately, we observe that there is an ideal range of off-stoichiometry for enhanced leakage performance and that gradients in the bismuth content can give rise to changes in the leakage mechanism at the electrode interfaces.

The multiferroic Ni₃TeO₆ exhibits a non-hysteretic colossal magnetoelectric effect.¹ The absence of hysteresis in the magnetic-field dependences of the magnetization and dielectric permittivity precludes losses for magnetoelectric applications. A collinear antiferromagnetic order appears below 55 K, giving rise to a spin-induced ferroelectric order. In the current work, we investigated the spin and lattice excitations of Ni₃TeO₆ ceramics and single crystals. Infrared, time-domain THz and Raman spectroscopy experiments were conducted in a temperature range from 5 to 300 K. Time-domain THz spectroscopy in external magnetic field was carried out at selected temperatures below and close to the antiferromagnetic phase transition. The THz spectra revealed a dynamic magnetoelectric coupling, as attested by changes in the spectra with magnetic field. Simultaneously infrared- and Raman-active spin excitations correspond to electromagnons, highly sensitive to the magnetic field.

“Geometric improper” mechanisms are being increasingly used to induce spontaneous polarizations in cation ordered oxides and provide a robust route towards the discovery of novel multiferroics. In this work, we seek to extend this framework to the family of perovskite-derived brownmillerite oxides (chemical formula ABO_2.5, or, alternatively, A₂B₂O₅), which are well studied for ionic conductivity applications owing to the presence of ordered oxygen vacancy “channels” at the unit cell level. These parallel rows result in alternating layers of BO₆ octahedra and chains of BO₄ tetrahedra; each of the chains can cooperatively rotate into either a left- or right-handed configuration. The combination of these chain arrangements dictate whether a brownmillerite phase will possess inversion symmetry or exist in a polar space group required for a switchable electric polarization. We hypothesize that these ordered chain structures would be susceptible to epitaxial strain, as supported by recent experiments [1], and may be controlled in magnetic oxides to realize multiferroic ground states.
We first disentangle the structural and energetic effects controlling the stability of these different polymorphs using first-principles density functional theory calculations. Through an investigation of the G-type antiferromagnetic Sr₂Fe₂O₅ and Ca₂Fe₂O₅ phases, we find that the observed ground state is governed by complex interactions between several structural descriptors, including ionic size, distortions of nominally regular oxygen octahedra, and in- and out-of-plane separation of tetrahedral chains. Furthermore, these same effects also control the preferred oxygen vacancy orientation under epitaxial strain. In agreement with available experimental synthesis studies, we find that both Sr₂Fe₂O₅ and Ca₂Fe₂O₅ exhibit non-polar space groups (Pbcm and Pnma, respectively); furthermore, strain has a large effect on the electronic structure, with a change of almost 1 eV observed. With this structural understanding, we then describe how to induce polar phases in these materials using A-site cation ordering, a mechanism which has been applied with much success in perovskite oxides to produce magnetic improper ferroelectrics [2]. We show how this ordering can remove inversion symmetry, resulting in modest spontaneous electric polarizations with coexisting long-range magnetic order. Our study demonstrates that control over multiple oxygen polyhedral distortions in addition to chemical ordering is a promising strategy to realize room temperature multiferroics.
JY and JR were supported by the Penn State NSF-MRSEC Center for Nanoscale Science under Grant No. DMR-1420620 and the U.S. DOE, Office of Science under Contract No. DE-AC02-06CH11357.
1. Inoue et al., Nature Chem. 2 213 (2010)
2. Young et al., J. Phys.: Condens. Matter (2015), forthcoming topical review

Exchange bias with perpendicular anisotropy is of great interest for potential applications such as read heads in magnetic storage devices with high thermal stability and reduced dimension. Here we report a novel approach to achieving perpendicular exchange bias (PEB) by orienting the ferromagnetic/antiferromagnetic interface coupling in the vertical geometry through a unique vertically aligned nanocomposite (VAN) design. Robust PEB phenomena have been demonstrated in micrometer-thick films by employing a prototype multiferroic system of antiferromagnetic BiFeO₃ and ferromagnetic La_0.7Sr_0.3MnO₃. Enhanced and tunable exchange bias fields have been obtained by tailoring the film composition and interface coupling orientations. The characteristic response of exchange bias to perpendicular magnetic field reveals the existence of exchange coupling along the vertical heterointerfaces, which exhibits strong dependence on the interface structure and strain state. Further, the VAN approach to achieve PEB effects is realized in self-assembled LSMO:NiO nanocomposite films. Instead of using the spin polarized tunneling across the interface, a unique approach based on the magnetic exchange coupling along the interface is employed to control magnetotransport properties. The exchange coupling at the vertical interfaces enables a dynamic and reversible switch of the resistivity between two distinct exchange biased states. The above work demonstrates that the VAN architecture provides a viable route to manipulate exchange bias and magnetotransport for advanced spintronic applications.

(Ba_0.2Sr_0.8)₃Co₂Fe₂₄O₄₁ crystallizes in the Z-type hexaferrite structure and it belongs to a rare group of room-temperature multiferroics.[1-3] Time-domain THz transmission, IR reflectivity and Raman scattering measurements were combined with studies of static magnetic susceptibilities, magnetization and dielectric studies in external magnetic field H. The ceramic samples were leaky at room temperature, but it was possible to measure the static magnetoelectric coupling below 50 K. Ferroelectric polarization arises with applying a magnetic field of a few mT, rises up to 1 T and disappears above H = 2 T due to changes in the magnetic structure. The phonon frequencies do not exhibit any significant shifts with temperature, only their damping decreases on cooling. This gives evidence that the crystal structure does not change with temperature below T_C = 500 K. The most interesting changes occur in the THz transmission spectra below room temperature. A new mode activates on cooling; its frequency increases and damping significantly decreases upon cooling. This mode also dramatically shifts down with magnetic field and disappears near H = 2 T. This is a typical sign of spin excitations (magnons). We assume that we observe an electromagnon, which is activated in the THz spectra by the dynamic magnetoelectric coupling; in fact, in polarized spectra of single crystals, it is active only if the electric vector of the THz radiation is parallel to the c crystal axis. We observed the same electromagnon in Raman scattering spectra. The ferrimagnetic structure changes with magnetic field, therefore the electromagnon frequency and IR activity are strongly dependent on the applied magnetic field. The magnon density of states measured using inelastic neutron scattering did not reveal any maximum at the electromagnon frequency. Based on this we propose that the electromagnon is activated by the inverse Dzyaloshinskii-Moriya interaction. Moreover, we observed a ferrimagnetic resonance which appears in the THz spectra near 100 GHz at H = 4 T and whose frequency linearly increases with magnetic field.
[1] Y. Kitagawa et al., Nature Mat., 9, 797 (2010)
[2] S.H. Chun, et al., Phys. Rev. Lett. 108, 177201 (2012).
[3] J. Wu, et al., Appl. Phys. Lett.101, 122903 (2012).

Multiferroic BiFeO₃ (BFO) system shows many promising functionalities emerging from its intercoupled ferroelectric polarization, transport, mechanical and magnetic properties. Mixed-R/T (rhomboheral/tetragonal) phase BFO thin films grown on LaAlO₃ substrates near a strain-induced morphotropic phase boundary (MPB) have attracted particular interests for piezoelectric applications due to their large field-induced strains and shape-memory effects. However, these effects, though being reversible, are hysteretic (i.e., the field-induced state has to be annihilated by applying an opposite field) due to the intricate mixed-phase microstructure and thus are undesirable in applications of e.g., tunable electroacoustic devices. For such purpose, R-phase BFO is to be considered instead.
In this work, we investigated the local Rminus;T phase transition phenomena, driven by applied tip biases, in epitaxially strained (001)-BiFeO₃ (rhombohedral) ferroelectric thin films from nanoscale sample volumes using band-excitation piezoresponse force spectroscopy/microscopy. Probing phase transition dynamics at such a fine length scale in real space, as opposed to those traditionally accessed via e.g. X-ray/neutron scattering and macroscopic physical property measurements, has enabled us to reveal complex dynamic interplay between the lattice polarization rotation mechanism and heterogeneous domain microstructures in the sample materials. Phase-field modeling was also performed to help understand the local polarization switching and rotation behavior of the system. Furthermore, we discussed the potential applicability of BiFeO₃ in tunable devices based on our observations.
This research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division of U.S. Department of Energy. Support was provided by U.S. DOE, Basic Energy Sciences, Materials Sciences and Engineering Division through the Office of Science Early Career Research Program (Q.L., N.B.).

Recent experiments have demonstrated precise one-to-one match of magnetic and ferroelectric domains in layered heterostructures with magnetic thin films directly grown on ferroelectric substrates, where the magnetic domain can be precisely manipulated by electric-field control over its elastically coupled ferroelectric domain wall, offering new opportunities for the creation of periodic magnetic domain patterns used for spintronic devices. In the present work, a phase-field model is developed to study the dynamics of such local elastic coupling between magnetic and ferroelectric domains in multiferroic heterostructures. Taking a CoFe film grown on a BaTiO₃ single crystal substrate as an example, we simulate the evolution of both types of domains upon applying an electric field, including field-driven changes in domain morphology and domain wall propagation.
Ferroelectric stripe domains are directly imprinted onto the overlaying magnetic domains through elastic coupling in the as-grown multiferroic heterostructure. By driving the elastically coupled ferroelectric domains with a vertical AC electric field, repeated writing and erasure of the magnetic stripe domains are achieved. An alternating occurrence of coherent local magnetization rotation and magnetic domain wall motion coupled to ferroelectric domain walls is observed during an electric field loop. Studies on the dynamics of electric-field manipulation of magnetic domain evolution reveal closely coupled domain wall motions of magnetic and ferroelectric domain walls with almost identical velocities.

ε-Fe₂O₃ is a metastable intermediate phase of iron (III) oxide, between maghemite (γ-Fe₂O₃) and hematite (α-Fe₂O₃). Epsilon ferrite has been investigated essentially because of its ferrimagnetic ordering with a Curie temperature of circa 500 K. However, given its orthorhombic crystal structure that belongs to the non-centrosymmetric and polar space group Pna2₁, it should exhibit ferroelectric behavior along with magnetoelectric coupling of the two orders (potentially making it one of the few room temperature multiferroic materials). Moreover, the material is characterized by strong magnetic anisotropy, resulting in a ferromagnetic resonance (FMR) frequency in the THz range in the absence of magnetic field and at room temperature. This is of particular interest given its potential use in short-range wireless communications (e.g. 60GHz WiFi) and ultrafast computer non-volatile memories.
Due to its metastable nature, ε-Fe₂O₃ needs to be stabilized at room temperature: to date such feature has been obtained mainly by synthesizing it by sol-gel as nanoparticles embedded inside a SiO₂ matrix, with the stabilization mechanism being either pressure or size confinement (or both). Recently however, deposition of epitaxial thin films of ε-Fe₂O₃ on SrTiO₃ (111) was demonstrated; in this case the stabilization is thought to be due to both epitaxial strain and interface interaction between the substrate and the film.
We report the growth by Pulsed Laser Deposition of epitaxial thin films of ε-Fe₂O₃ and ε-Al_xFe_2-xO₃ on different single crystal substrates, both oxides (SrTiO₃, LaAlO₃, LSAT, Al₂O₃, YAlO₃, and YSZ) and non-oxides (single crystal Silicon) and discuss the influence of the chosen substrate and of aluminum doping on the structural, magnetic and dielectric properties. In particular, we focused our attention on the effect of Al inclusion inside the ε-Fe₂O₃ lattice, which should result (i) in the improvement of the electric properties, given the ferroelectric nature of the isostructural AlFeO₃, and (ii) in a lowering of the FMR frequency due to non-magnetic nature of Al. Electric and magnetic properties were probed both macroscopically and locally through ferroelectric measurements/vibrating sample magnetometer and piezoelectric/magnetic force microscopy (PFM - MFM), while measurements of the ferromagnetic resonance were conducted by direct FMR probing and through THz radiation absorption.
The ability of growing thin films of epsilon ferrite both on surfaces with 3-fold symmetry like STO and LAO (111) and on surfaces with 4-fold symmetry like cubic YSZ and YSZ-buffered Si (100) will help to understand the twinning mechanism which was observed given the lower symmetry of the deposited film with orthorhombic symmetry in comparison with the cubic symmetries of the substrates. Moreover, depositions over vicinal STO and YSZ substrates have been performed to promote the growth along only one crystallographic direction.

At the interface between two oxide materials and a length scale of only a few nanometers exists a wide breadth of new physics and chemistry. For example, superconductivity has been observed within only one CuO₂ plane and changes in magnetic ground states in manganites can be isolated to only a few nanometers. Here, we present two short stories of tuning the interfacial electronic structures of different oxide materials through ferroelectric field effect control. The first story explores the interface between ferroelectric PbZr_0.2Ti_0.8O₃ (PZT) and the ferromagnetic metal La_0.8Sr_0.2MnO₃. Using the depth sensitive technique of polarized neutron reflectometry, we demonstrate that the magnetization at the interface is enhanced due to a hole-accumulated state. Replacing PZT with a non-ferroelectric layer produces a manganite with the so-called magnetic dead-layer, which verifies that the enhancement we observe is induced by the field effect. Applying the knowledge of the field effect in the manganite-PZT system, we explored the layered cuprates, La_2-xSr_xCuO₄, whose superconducting properties are known to be highly sensitive to hole doping. Through extensive X-ray absorption spectroscopy and transport measurements, we show that the field effect is an efficient means to effectively tune the hole concentration and thus the superconductivity. These results emphasize the universal nature of field-effect doping as a parameter for tuning the phase diagrams of a wide range of materials.
Acknowledgements The work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division.

We have studied the structure of magnetoelectric antiferromagnetic Cr₂O₃ thin films for voltage-controlled magnetic tunnel junctions using transmission electron microscopy (TEM). Cr₂O₃ films were grown by molecular beam epitaxy (MBE) on Al₂O₃ (0001) surface. Good epitaxy is observed for Cr₂O₃ films, as well as some periodic columnar grain boundaries along the growth direction. The selected area diffraction pattern, the spacing and orientation of moiré fringes in the TEM images, and the Fourier filtered TEM image reconstruction indicate that the columnar boundaries are formed by Cr₂O₃ grains which are rotated 60° about the c-axis. Most of the rotated Cr₂O₃ grains begin at the interface and keep their columnar shape toward the top surface of the Cr₂O₃ films. The columnar boundaries may serve as an easy pathway for electrons, which may explain in part the lower dielectric breakdown field for thin films Cr₂O₃ compared to bulk Cr₂O₃. Cr₂O₃ grown on Piranha-etched Al₂O₃ substrates has a larger grain size and lower boundary density, presumably due to lower nucleation density on the etched surface. Preliminary density function theory calculations for all three possible termination of Al₂O₃ surface, the single metal layer -Al-O3, the double metal layer -Al-Al-O3 and the oxygen layer -O3 surface termination show that the -Al-O3 surface of Al₂O₃ substrate has a higher energy barrier for the formation of 60°rotated Cr₂O₃ grains, which in turn, could result in larger grain size and lower boundary density. The microstructure of Cr₂O₃ films on Pd electrode layers is also studied. The Cr₂O₃ films largely inherit the grain structure of the Pd, so high-quality, large-grain Pd films are important for the achieving high-performance Cr₂O₃-based devices.

Structures that simultaneously combine electrical polarization and magnetization may be used in memory storage devices [1, 2]. Unfortunately, these properties are ineffectually implemented in any single material simultaneously. This is due to the incompatibility of chemical and electronic bonds. The properties of a multiferroic may be effectively combined by creating multilayer heterostructures, each layer of which possesses the requisite properties [3], or by reducing the spacial symmetry through load on the interface.
Experimental research has been conducted upon a range of heterostructures with even and odd numbers of monolayers YFeO₃ and LaFeO₃ on a base of DyScO₃. The total thickness of the multilayer is 160 Nm. It has been demonstrated that at a photon energy level of 1.41 eV there occurs a resonance intensification of the magnetic dipole contribution of the generation of the second optical harmonic.
Using symmetrical analysis of the azimuthal dependencies of generation of the second optical harmonic it has been demonstrated that the nonlinear optical signal is caused by the magnetic dipole contribution. The magnitude of the polarization signal was observed to be dependent on the number of monolayers, and the maximum signal was recorded among heterostructures having an odd number of monolayers. The magnetically-induced generation of the second optical harmonic observed in the structures likewise demonstrates a dependency on the quantity of monolayers. When the quantity of monolayers decreases the coercive field increases. Thus, by changing the quantity of layers we can change (and improve) the properties of the structure.
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REFe₂O₄, (RE: rare earth elements), which is a topic of discussion among various electronic properties like charge-ordering, ferroelectricity arising from charge-ordering and multiferroic properties, is promising a magnetoelectrics material. In this work, we synthesized its single-phased epitaxial thin films and fabricated electric double layer transistor (EDLT) with carefully chosen ionic-liquid electrolyte as a gate dielectric to attain high conductivity modulation.
A single crystalized YbFe₂O₄ thin film was prepared on YSZ (111) substrate by pulsed-laser deposition, and their resistivity characteristics was well agreed with the bulk report indicating occurrence of the charge ordering. As for ionic-liquid electrolyte, it is known that the molecular size strongly affects on the capacitance, and it was recently reported that imidazolium salts shows larger capacitance than anmonium salts because of their inter-molecular coulomb interaction of p-electrons. Based on this rule on the capacitance, ionic liquids of DEME-TFSI, DEME-BF₄, HMIM-TFSI, EMIM-TFSI, EMIM-BF₄, EMIM-MeSO₄, were investigated so that EMIM-MeSO₄ exhibited larger capacitance of 6.4 mF/cm² than that of 3.6 mF/cm² for often used DEME-TFSI. We attempted to dope carriers into YbFe₂O₄ channel in electrostatic manner through EDLT with EMIM-MeSO₄ with small molecular size. The electrostatic operation in YbFe₂O₄/EMIM-MeSO₄ EDLT enabled us to attain 2% degree of conductivity modulation at the maximum.
# Present affiliation of Dr. Kohei Fujiwara: The Institute for Materials Research#12289;Tohoku Univ., Japan

Oxide heterostructures of transition metals offer a unique opportunity to tailor and tune the physical properties by exploiting local symmetry breaking, epitaxial strain, frustration or charge transfer between the materials, giving rise to very unexpected emergent phenomenon. Of particular importance, the BiFeO₃ (BFO), a multiferroic thin film in contact with a soft ferromagnetic material such as La_0.7Sr_0.3MnO₃ (LSMO) has garnered a tremendous scientific curiosity such as magnetoelectric coupling and interface ferromagnetism. However, to our knowledge there has been no report on exploring the magnetic properties of BFO when it is in contact with a hard ferromagnetic metal such as SrRuO₃ (SRO), which forms the subject of present study. As confirmed from XRD and TEM, the BFO/SRO films were epitaxially grown on Si (100), by introducing epitaxial layers of SrTiO₃ (STO)/MgO/TiN using PLD, similar to BFO/LSMO heterostructures reported^1-3 recently. HRTEM images showed BFO/SRO interface is abrupt without noticeable interdiffusion. Characteristic butter#64258;y loops (of several cycles) were observed in the PFM amplitude signals of the BFO #64257;lm. In addition, the phase signal indicated a clear (180^o) switching behavior at the switching voltage of 4-5 V, providing unambiguous evidence for the occurrence of ferroelectricity in BFO #64257;lms integrated on Si (100). The main focus of this work was to investigate the magnetic properties of multiferroic BFO when it is in contact with the metallic hard ferromagnet SRO. BFO showed typical antiferromagnetic features as expected. The Curie temperature of SRO was found be ~ 175K, close to reported value of 170K. Interestingly, we have noticed that the coercive field of SRO was increased from 3982 Oe to 7309 Oe (1.8 times) when it is in contact with BFO. Could the multiferroic/ferroelectric nature of BFO cause such changes in magnetic properties of SRO? Or is it due to the classical antiferromagnetic-ferromagnetic interaction?. We have performed several control measurements to better understand this observation. We will present our experimental findings with in-detail discussion.
¹Interface Magnetism in Epitaxial BiFeO₃#8209;La_0.7Sr_0.3MnO₃ Heterostructures Integrated on Si(100), S. S. Rao, Nano Letters 13, 5814 (2013); ²Ferroelectric and magnetic properties of multiferroic BiFeO₃-La_0.7Sr_0.3MnO₃ heterostructures integrated with Si (100); Srinivasa Rao Singamaneni, J. Appl. Phys., 117, 17D908 (2015); ³Multifunctional heterostructures integrated on Si (100), Srinivasa Rao Singamaneni, Emerging Materials Research, 4, 141-199 (2015).

In the brain, learning is achieved through the ability of synapses to reconfigure the strength by which they connect two neurons. Artificial hardware with performances emulating those of biological systems require electronic nanosynapses endowed with such plasticity. Promising solid-state synapses are memristors, simple two-terminal nanodevices that can be finely tuned by voltage pulses [2]. Their conductance evolves according to a learning rule called spike-timing-dependent plasticity [3], conjectured to underlie unsupervised learning in our brains. Future neuromorphic architectures will be based on up to 10¹⁵ of such synapses [4]. This complexity requires a clear understanding of the physical mechanisms responsible for plasticity to predict the conductance evolution with robust models supported by experimental observations. However, most memristive solid-state synapses operate through concomitant structural and chemical changes that are difficult to characterise and simulate, while also challenging device integrity in the case of continuous learning [5]. Here we report on purely electronic ferroelectric synapses and show that spike-timing-dependent plasticity can be harnessed and tuned from intrinsically inhomogeneous ferroelectric polarisation switching. Through combined scanning probe imaging and electrical transport experiments, we demonstrate that conductance variations in such BiFeO₃-based ferroelectric memristors [6] can be accurately controlled and modelled by the nucleation-dominated electric-field switching of domains with different polarisations. Our results show that ferroelectric nano-synapses are able to learn in a reliable and predictable way, opening the way towards unsupervised learning in spiking neural networks.
This worked received support from ERC projects Nanobrain (#259068) and Femmes (#267579).
[1] D. O. Hebb, The Organization of Behavior, NewYork: Wiley and Sons, 1949.
[2] J. J. Yang, D. B. Strukov and D. R. Stewart, Nature Nanotech. 8, 13-24 (2012).
[3] S. Saïghi et al, Front. Neurosci. 9, 1 (2015) ; R. Ananthanarayanan et al, Proc. Conf. High Performance Comput. Netw. Storage Anal. (2009) DOI: 10.1145/1654059.1654124.
[4] D.B. Strukov and R.S. Williams, PNAS 106, 20155 (2009)
[5] Q. V. Le et al, Proc. IEEE Int. Conf. Acoust. Speech Signal Process. 8595 (2013)
[6] A. Chanthbouala et al, Nature Mater. 11, 860 (2012) ; H. Yamada et al, ACS Nano 7, 5385 (2013) ; S. Boyn et al, Appl. Phys. Lett. 104, 052909 (2014) ; S. Boyn et al, APL Mater. 3, 061101 (2015)

The central challenge in tunable magnetic microwave devices lies in finding an energy efficient way to perform wide range ferromagnetic resonance (FMR) voltage tuning in a reversible and reproducible manner, rather than with a current-driven electromagnet.¹ Multiferroic heterostructures, exhibiting a strong strain-mediated magnetoelectric (ME) coupling between distinct ferromagnetic and ferroelectric phases, have shown great promise for frequency agile microwave applications. In these materials, a single control parameter of in situ voltage-induced piezo-strain, arising from ferroelectrics, is used to shift FMR frequency in elastically-coupled ferromagnetic phases via magnetoelastic effects.^{2, 3} Therefore, devices based upon such materials are, in principle, light-weight, fast, and energy efficient, overcoming some of the intrinsic limitations in conventional microwave components, while providing new functionality. However, in most prototype ME microwave devices, tuning of FMR frequency has been achieved through the use of a linear piezo response.^{4, 5} Upon removing the electric field, the FMR decays to the initial state. While these devices point towards a unique pathway for enhancing FMR tunability, reversible and non-volatile tuning of FMR using strain has remained relatively unexplored, and this is indispensable from a device application point of view. In this work, we will demonstrate electrically non-volatile tuning of FMR in magnetoelectric composites using electric polarization switching, ferroelastic domain switching, ferrolectric phase transition.⁶ These results point to opportunities for electrical tuning of strain sensitive properties in all materials and provide a framework for realizing reconfigurable, frequency agile, non-volatile and energy efficient electronics and microwave devices.
Reference:
[1] G. Srinivasan, Annual Review of Materials Research, Vol 40, Vol. 40, 2010, 153.
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[6] M. Liu, B. M. Howe, L. Grazulis, K. Mahalingam, T. Nan, N. X. Sun, G. J. Brown, Adv. Mater. 2013, 25, 28.

There has recently been much interest to multiferroic magnetoelectric composites based on relaxor ferroelectric single crystals as potential candidates for devices such as magnetic field sensors, energy harvesters, or transducers. Large magnetoelectric coupling coefficient is prerequisite for superior device performance in a broad range of frequencies and functioning conditions. In magnetoelectric heterostructures based on ternary relaxors Pb(In_1/2Nb_1/2)O₃-Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (PIN-PMN-PT) crystal better operational range and temperature stability as compared to binary relaxors can be achieved. Giant linear converse magnetoelectric coupling up to 2 x 10^-6 s m^-1 were observed in heterostructural composites with multilayered FeCo/Ag deposited on (011) PIN-PMN-PT crystals. Further enhancement of magnetoelectric coupling is demonstrated by utilizing inter-ferroelctric rhombohedral - orthorhombic phase transitions in PIN-PMN-PT Mechanical clamping was a precondition to utilize this inter-ferroelectric transition mode to bring the crystal to a point just below its transformation threshold when very small perturbations at the input will cause large swings at the output generating a sharp uniaxial increase in strain (~0.5 %) and polarization change, giving rise to nonlinear effects. This phenomenon was demonstrated in magnetolectric hybrid composite using Fe₈₁Ga₁₉ and PIN-PMN-PT single crystal. High magnetoelectric coupling non-resonant coefficient order of 40 V/cm Oe was achieved in broad range of frequencies. Details of these results and their implications will be presented.

The multiferroic BiFeO₃/La_0.7Sr_0.3MnO₃ (BFO/LSMO) heterostructure in which the magnetization of ferromagnetic LSMO can be altered by applying electric fields across ferroelectric/antiferromagnetic BFO has attracted much attention during the last decade. The mechanism with which the coupling occurs at the interface is believed to be due to the exchange bias effect. We characterized the interface of the heterostructures with different stack sequences of LSMO/BFO and BFO/LSMO using TEM revealing sharp and rough interfaces, respectively. The magnetometry and X-ray resonant magnetic reflectometry measurements (XRMR) show neither exchange coupling nor traces of ferromagnetic Fe atoms at the sharp interface between LSMO and BFO, respectively, in agreement with recent literature [Bertinshaw et al., Phys. Rev. B 90, 041113(R) (2014)]. Instead, the heterostructures with rough and chemically intermixed interfaces exhibit the coupling and ferromagnetic Fe atoms were detected via XRMR measurements. In addition, we find an exponentially decaying exchange bias induced coercive field change, indicating a spin glass state at such rough interfaces. The authors acknowledge the financial support by EU&’s 7th Framework Programme IFOX (Grant No. NMP3-LA-2010 246102) and DAAD (SpinNet).

Manipulating magnetism with electric fields rather than magnetic fields or large currents in materials is a goal for future low-energy consumption magnetoelectric (ME) devices such as electric-writing magnetic-reading memories and E-field tunable microwave devices. Antiferromagnetic (AFM) spintronic, which has no stray magnetic field and the relative insensitivity to external magnetic fields, is especially favorable for ultrafast and ultrahigh-density spintronics. Intermetallic FeRh undergoes a distinct magnetostructural transition, that is, a first-order AFM to ferromagnetic (FM) transition near room temperature. Recently, great efforts have been made to manipulate the AFM spins or mediate the transition temperature for FeRh-based memory resistors. In this talk, we will investigate the distinctive magnetization temperature hysteresis of epitaxial FeRh films grown on different substates. It is shown that a kind of magnetic glass state exist during the disorder influenced FM-AFM transition, and the first-order transition and magnetic properties could be elastically controlled via substract selection or electric field in magnetic/ferroelectric heterostructures.

The direct and efficient coupling between the electric signals and the elastic, thermal, optical and magnetic signals in ferroelectric based electroactive materials makes them attractive for exploring a broad range of cross-coupling phenomena which have great promise for new device technologies. This talk will present the recent advances at Penn State in developing electrocaloric materials which may provide alternative cooling technology to the century old vapor compression cycle (VCC) based cooling which employs strong greenhouse gases as the refrigerants. Electrocaloric effect (ECE), which is the temperature and/or entropy change of dielectric materials caused by the polarization change induced by electric fields, is attractive to realize efficient cooling devices. However, the relatively small ECE observed in dielectrics in the last century makes it unimpressive for any practical applications. Recent studies in our group show that by working with dielectrics of weak polar-correlation, giant electrocalroic effect can be achieved over a broad temperature range. This talk will also discuss considerations on and present recent works in using nanocomposites to further enhancing the ECE beyond the pure relaxor polymers, on the giant ECE in a class of dielectric liquid, and in ferroelectric ceramics near the invariant critical point. The works related to developing the chip-scale EC cooling devices, exploiting the newly discovered large ECE in ferroelectric materials and featuring high cooling power density and high efficiency, will also be presented.

Metamagnetic FeRh is a potential candidate in intelligently designed artificial multiferroics for it&’s close to room temperature phase transition from AFM to FM ordering. Recently it has been demonstrated that the ordering temperature in FeRh films can be controlled using a small piezo-electric substrate induced strain when grown on BaTiO₃ (BTO) (001) substrates¹. The realization of such a structure opens the door to many new spintronics based devices that, rather than simply reorienting spins in a ferromagnet, harnesses control of the materials intrinsic magnetic ordering using only an applied electric field.
Though these new findings are quite exciting, the physical nature of the strain mediated phase transition in such a heterostructure is still unclear. Recent studies have employed methods such as XPEEM², and electrical transport^3,4 to probe the mechanisms of the strain, however these methods only reveal effects occurring at the very surface of the films, while the strain effects should be strongest closer to the interface. Furthermore, the overall strain induced magnetization effects have only been measured by magnetometry methods which measure the magnetization of the entire film without revealing specific depth-dependent details of the system. Using polarized neutron reflectometry (PNR) we can map both the density variations and absolute magnetization of the film as a function of depth⁵; and in doing so, we have revealed some exceptional qualities.
In this study a unique growth method involving a post growth in-situ rapid thermal anneal has been employed to produce FeRh films with dramatically reduced transition temperatures and a large magneto-thermal hysteresis. Remarkably, these films show giant magnetization switching (measured to be as high has ~25%) when subjected to the interfacial strain changes induced at two structural phase transitions of the BTO 001 single crystal substrate. These magnetization changes are the largest seen to date to be controllably induced in the FeRh system. Using PNR we have unraveled the intricacies of this strain induced switching and explored the effects of temperature and strain on the stabilities of the FM and AFM phases. These results show further the promise of a strain mediated handle for the control of magnetic ordering in FeRh films, and reveal the stability balance between the temperature and strain based control mechanisms in such BTO - FeRh heterostructures.
[1] Cherifi, R. O. et al. Nat. Mater.31, 345-351 (2014)
[2] Phillips, L. C. et. al. Sci. Rep.5, 10026 (2015)
[3] Marti, X. et at. Nat. Mater.13, 367-374 (2014)
[4] Lee, Y. et al. Nat. Commun.6:5959 doi: 10.1038/ncomms6959 (2015)
[5] Bennett, S. P. et al. Sci. Rep.5, 9142 (2015)
* This work was supported by the Scientific User Facilities Division, and grant DE FG02 87ER-45332, DMS funded by the Department of Energy&’s Office of Basic Energy Science

Vertically aligned epitaxial two phase multiferroic nanocomposites consisting of ferri- or ferromagnetic pillars in a ferroelectric matrix have been widely studied for their potential application in memory and logic devices. These composites show strong magnetoelectric coupling through strain at the interface between the magnetoelastic pillar phase and the piezoelectric matrix phase. The vertical orientation of the interface avoids clamping from the substrate and allows for larger strain transfer between the phases, compared to layered composites. One way to achieve an increased magnetoelectric effect in such composites is by increasing the change in strain in the ferroelectric phase. Studies on single phase bismuth ferrite (BiFeO₃, BFO) under different epitaxial strains have revealed the existence of a morphotropic phase boundary between rhombohedral (BFO-R, with c/a ~1) and tetragonally distorted BFO (BFO-T, with c/a ~1.3) at which an increased electromechanical effect is observed when one phase transforms to the other.
In this work, CoFe₂O₄ (CFO)/BFO nanocomposites were grown in which the structure of BFO is changed between BFO-T, BFO-R or mixed phase. CFO/BFO nanocomposites were grown using pulsed laser deposition on different crystal substrates that impose different epitaxial strain on the BFO, thereby controlling its structure. Buffer layers used during the growth process also strongly influence the structure of BFO grown as well as the magnetic anisotropy of the CFO. Single crystal substrates LaAlO₃ (100) (a = 3.79 Å), (LaAlO₃)_0.3(SrAl_0.5Ta_0.5O₃)_0.7 (100) (a= 3.87 Å) and SrTiO₃ (100) (a= 3.905 Å), coated with a thin buffer layer of La_0.7Sr_0.3MnO₃ (a = 3.85 Å) to form a bottom electrode, yielded BFO-T, mixed phase and BFO-R respectively. However, a SrRuO₃ underlayer produced BFO-R on all substrates. The presence of these phases was confirmed using X-ray diffraction. The films are epitaxial at the interfaces which results in an out-of-plane strain that is compressive in the CFO pillars and tensile in the BFO matrix. The presence of BFO-T modifies the typical columnar microstructure usually observed for the CFO phase in BFO-CFO nanocomposites. The hysteresis curves show that the magnetic anisotropy correlates with the out-of-plane strain in CFO due to its strong magnetoelastic response. Local probe piezoresponse force microscopy studies are used to confirm the ferroelectric nature of BFO and to determine the key differences between ferroelectric switching properties of nanocomposites made with BFO-R, mixed phase and BFO-T.

This presentation is an overview of the research on multiferroic materials for future tunable RF devices that is performed at the Air Force Research Laboratory. Several different oxide heterostructures are being explored ranging from bi-layers to oxide superlattices. The primary deposition technique utilized is pulsed laser deposition. For property measurements and deposition optimization a wide variety of characterization techniques are employed, such as high resolution x-ray diffraction and reciprocal space mapping, cross-sectional transmission electron microscopy, piezoforce microscopy and ferromagnetic resonance. Some of the novel results from our deposition and characterization studies will be highlighted.

Magnetoelectric (ME) sensors, built with cantilevers carrying magnetostrictive and piezoelectric layers, are preferably operated in the bending resonant mode. The magnetic field to be measured is converted to an electric charge or voltage greatly taking advantage from the resonance amplification. Regarding the signal to noise ratio, the intrinsic noise sources of the sensors are comparatively weak. Noise is mainly introduced extrinsically by undesired acoustic couplings, by 1/f- and other noise sources from the read-out electronics and the AD-converter, as well as by distorting magnetic fields from the environment. The latter can be reduced by magnetically shielded chambers. Aiming at the measurement of biomagnetic signals in the frequency range between about 0.1Hz to 200Hz, it becomes difficult to utilize the mentioned resonance amplification, because of sensor size limitations, acceptable settling time and increasing 1/f-noise, accompanied by a significant loss in sensitivity. In order to overcome this problem and still be able use resonant sensors, frequency conversion approaches of the desired signals to higher frequencies may be applied [1, 2], utilizing nonlinear properties of either the magnetostrictive or the piezoelectric materials involved, thus avoiding 1/f-noise and allowing reduced sizes of the sensors. In this contribution, special features of various modifications of frequency conversion techniques will be presented and discussed, including quadratic and cubic magnetic frequency conversion as well as electrically induced frequency conversion by nonlinear piezoelectric or magnetomechanic properties, greatly enhancing the signal to noise ratio as compared to direct detection schemes. In addition, sensitivity enhancement by acoustic noise suppression using a tuning-fork arrangement of a ME-sensor will be demonstrated [3].
[1] R. Jahns, R. Knöchel, E. Quandt, German Patent Application DE102011008866A1, 18.1.2011
[2] R. Jahns, H. Greve, E. Woltermann, E. Quandt, R. Knöchel, Sensors and Actuators A 183, 16 (2012).
[3] R. Jahns, R.Knöchel, H. Runkowske, European Patent Application EP000002811314A1, 06.06.2013

The coupling between the multiferroic and ferromagnet at interface is critical in the design of magnetoelectric devices. Among different candidates, the all-oxide system BeFeO₃/manganite has been demonstrated to be very promising due to the observation of electric field controllable exchange bias at low temperature. The effort to optimize this functionality is concentrated on two aspects: enhancing the magnitude of exchange bias and increasing the blocking temperature. Exchange bias originates from the interface coupling. Moreover the active region of electric control in this system is also near the interface. Therefore a systematic study of interface design is highly desirable to understand the phenomena as well as to optimize the performance. Here we present the results on tuning the carrier concentration, atomic size and termination at the interface by controlling the growth in atomic scale to account for the delicate coupling between spin, charge and lattice. Our results illustrates the origin of the effect as well as pathways to achieve better performance.

Multiferroic materials have potential for use in the detection of magnetic fields, microwave applications, and advanced electronics. Previous work has shown that nickel ferrite has potential as the magnetostrictive layer in heterogeneous multiferroic multilayers and can be co-fired with PZT, but that a Pt layer is required to minimize Pb diffusion into the NFO. Thus, alternative piezoelectric materials are desired. Due to its recent discovery of ferroelectricity in hafnia, using hafnia as the electrostrictive layer in a multiferroic system could mitigate the concerns of using lead while still allowing for strong magnetoelectric coupling. Here we report on the hafnia/nickel ferrite multilayers synthesized via a chemical solution deposition route. X-ray reflection, area x-ray diffraction, AFM, and SEM will show the thickness of the films, the phases present, the roughness of the film, and the resulting microstructure, respectively. Magnetization measurements and electrical polarization measurements are also be presented.

Microelectromechanical systems (MEMS) can provide platforms for a variety of multifunctionalities including sensing, actuation, and energy conversion, storage and harvesting. We have developed miniaturized arrays of multiferroic cantilever devices on a MEMS platform where various energy scales (Zeeman, magnetic anisotropy, mechanical) are comparable in magnitude, and the mechanical degree of freedom emerges as a key tunable device parameter directly coupled to multiferroic properties. Our heterostructured cantilever arrays are fabricated on 4” wafers buffered with oxide/nitride/oxide (ONO) multilayers, which are used to fine-tune the stress in the cantilever. A room-temperature sputtered layer of magnetostrictive FeGa (0.5 micron thick) on top of a sol-gel deposited piezoelectric (Pb(Zr,Ti)O₃) layer (0.5 micron thick) with a Pt/Ti adhesion under-layer over the ONO layer serve as the core of our multiferroic heterostructure. Driving them in the non-linear regime, enhanced mechanical coupling at the tunable resonant frequency can be used to perform versatile read/write memory operations. In such devices, electric field and/or magnetic field can be used as input. Our -based multiferroic devices are scalable with high fabrication yields, and have stable write/read operations at room temperature which have been tested for > 10⁴ cycles. We also describe the use of parametric amplification to substantially increase the magnetoelectric (ME) coefficient of the multiferroic cantilevers. While the cantilever is driven by AC magnetic field at the resonant frequency, by using a pump voltage signal to modulate the spring constant of the system at double the resonant frequency, we are able to significantly increase the gain as the pump signal approaches the threshold value. Using this method, the ME coefficient has been amplified from 33 V/(cm Oe) to 2,000,000V/(cm Oe). We discuss the implication of the enhanced ME signals for various applications. This work was carried out in collaboration with Tiberiu-Dan Onuta, Yi Wang, Chris Long, and Samuel Lofland, and was funded by DARPA and NSF.

The ΔE effect magnetic field sensor exploits the frequency shift of a high frequency electro mechanical resonator, due to the change in Young&’s modulus of a magnetostrictive material in a magnetic field. The sensor allows broadband magnetic field measurements at low frequencies down to the DC range, is robust against microphony effects and mechanical noise, and provides full device integrability. Since our first publication of the sensor concept, in 2011 [B. Gojdka et al., Appl. Phys. Lett. 99, 223502 (2011); Nature 480, 155 (2011)] various improvements and modifications of the original set-up have led to an increase in sensitivity by almost five orders of magnitude down to 35 pTHz^-1/2 at 20 Hz. The different approaches will be reviewed. Our current sensor versions are fully integrated microelectromechanical devices and incorporate a piezoelectric AlN layer on top of a poly-Si cantilever and a magnetostrictive FeCoBSi bottom layer. The AlN layer serves two functions: It drives the resonator, and it is used for electrical read out.

Magnetoelectric (ME) sensors have found a broad range of applications for many decades. Substantial research efforts in the past decade have resulted in composite materials with giant magnetoelectric response. Among various composite configurations investigated, the multilayerd structures, comprising of alternating magnetostrictive alloy layer and piezoelectric layer, have shown large ME coupling coefficients at room temperature and have potential to be used as high sensitivity magnetic sensors, electrical current sensors, and other devices.
A magnetoelectric laminate heterostructure consisting of two shear-mode latitudinally magnetized magnetostrictive Tb_0.3Dy_0.7Fe_1.92 (Terfenol-D) alloy plate, a shear-mode piezoelectric polyvinylidene fluoride (PVDF) layers (sensor operated at shear piezoelectric and shear magnetostrictive modes (S-S mode)), and a mechanical clamping alumina substrate has been demonstrated that has a noticeably improved ME effect (5.54 times) relative to previous laminate configurations of these two materials. The ME laminates of Terfenol-D/PVDF/Terfenol-D were experimentally studied and the performance was compared with the prediction from the theoretical analysis. A giant ME coefficient of 7.93 V/(cm#65294;Oe) at low frequencies under an optimal dc magnetic bias of ~2300 Oe was found. The improved ME coefficient derives from S-S mode heterostructure design, which allows both the Terfenol-D and PVDF to operate in shear mode that has maximum magnetoelectrical coupling coefficient.

Multiferroic magnetoelectric materials which exhibit simultaneous ferroelectric and ferromagnetic behavior and control the magnetic order parameter via electric field in a switchable manner and vice versa have drawn significant interest in recent years because of their intriguing physical origin and great potential for multifunctional applications. Multiferroic composites/ heterostructures, showing magnetostrictive and piezoelectric (ferroelectric) properties, are very attractive as compared to single-phase magnetoelectric materials due to their combined robust electric and magnetic polarizations at room temperature with effective magneto-electric coefficients. In the present work we have chosen Pb(Fe_0.5Nb_0.5)O₃ (PFN)/ Ni_0.65Zn_0.35Fe₂O₄ (NZFO) heterostructures to achieve enhanced multiferroic properties and magneto electric coupling at room temperature. PFN single layer and PFN/NZFO multilayer thin films were grown at 600^o C by pulsed laser deposition (PLD) using a KrF excimer laser (lambda;=248 nm) on LaNiO3 buffered (001) oriented (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT) substrates under an oxygen pressure of 20 mTorr followed by annealing at 700^o C for 30 min in oxygen at a pressure of 300 Torr. The thickness of LNO was 60 nm for all films, while the thicknesses of PFN/NZFO multilayers are found to be ~ 200 nm. The highly c-axis oriented growth containing only (00l) diffraction peaks of PFN and NZFO films along with in plane epitaxial relationship were confirmed by high resolution X-ray diffraction measurements. From the atomic force micrographs it was observed that all the films were densely packed, smooth, free from microcrack and particulates with uniform grain-size distributions. The existence of ferroelctricity and switching of polarization are confirmed from the band excitation Piezoresponse Force Microscopy (PFM). PFM images showed clear polarization switching above ± 5 V. Enhanced ferroelectric, magnetic and magnetoelectric properties observed in these heterostructures. Detailed studies on dielectric, ferroelectric, magnetic, magnetoelectric properties and PFM studies of these above mentioned thin films will be discussed in the meeting.

Multiferroic materials with simultaneous ferroelectric and magnetic order have drawn significant interest and provide a new fertile ground for fundamental research due to their multifunctionality. BiFeO₃ (BFO) is the only known single-phase multiferroic material, but is not suitable for practical applications due to its semiconducting behavior. Thus, solid solutions of BFO (with PbTiO3, Pb(ZrTi)O3, BaTiO3 etc.) were studied, where B-site of the perovskite is shared by both Fe and Ti ions. However, in solid solutions, one of the compositions is already magnetically ordered! It is difficult to prove that from where the magnetic contribution is coming. In order to better understand the effect of combination of Fe/Ti ions at B-site, it is desirable to introduced magnetic ions in pure ferroelectric matrix.
In the present work, thin films of Fe substituted BaZr_0.05Fe_xTi_(1-3x/4)O₃, with x = 0 and 0.015, were grown on Pt/TiO₂/Si/SiO₂ substrate using pulsed laser deposition (PLD) at a substrate temperature of 700°C in Oxygen gas pressure of 30 mTorr inside the Vacuum System (Excel Instruments, India). Both the targets were ablated using KrF (lambda; = 298 nm) laser with an energy density of ~ 2 J/cm². X-ray diffraction revealed the formation of single phase polycrystalline films. Thickness of the films was estimated using optical profilometer and was found ~ 100 nm. AFM showed that the roughness of the films was less than 20 nm. XPS studies showed that all the ions of the composition are in their desired electronic states. Room temperature ferroelectric/piezoeelcric nature of the films studied using P-E loop measurement as well as piezoforce microscopy (PFM). A well saturated P-E loop is observed for both the films. Room temperature M-H loop was measured using SQUID. Ferromagnetic loop with M_r ~ 1.1x10^-3 emu/cm³, is observed for Fe substituted film, whereas the pure BZT film was found diamagnetic.

Knowledge of the electronic band structure of materials is a cornerstone of modern technology. In functional dielectric and multiferroic oxides, traditionally seen and used as insulating materials, electronic structures have been much less explored than in semiconductors, even in the model and thoroughly studied multiferroic BiFeO₃. However, today, they gain importance with the growing interest for interactions of ferroic materials with light, namely in the view of photovoltaic or photoelectric properties and become crucial for the understanding and tuning of photo-induced effects such as above-band gap photovoltages, anomalous photovoltaic effects, photo-conductive domain walls etc. On the other hand, in ferroelectric and multiferroic oxides the investigation of band-to-band transitions is difficult to address experimentally because the absorption onset is broad, especially when compared to the generally sharp transitions in classical semiconductors. The appearance of Urbach tails, notably at higher temperatures, complicates the quantitative analysis and the discrimination of direct and indirect transitions. Other classical techniques are rapidly limited by thermal effects, charging of the insulating samples (ARPES) or require synchrotron sources (resonant inelastic X-ray scattering).
In BiFeO₃, according to absorption studies on single crystals and thin films, the optical band gap lies at ambient conditions in the visible range at approximately 2.7 eV. It is also experimentally established that the optical band gap of BiFeO₃ shrinks with increasing temperature down to 1.3 eV at 925#9702;C where it then closes abruptly as a consequence of an insulator-to-metal phase transition concomitant with a structural transition to the so-called γ phase. But the direct and indirect band-to-band transitions are difficult to separate; furthermore, it is not yet understood how this gradual shrinking of the optical band gap relates to the electronic structure.
Here, we show how resonant Raman spectroscopy allows probing electronic levels of the model multiferroic BiFeO₃ (BFO). Resonant Raman scattering enables to separate direct from indirect band-to-band transitions. Using twelve different excitation wavelengths ranging from the deep blue (442 nm = 2.8 eV) to the infrared (785 nm = 1.6 eV), we show that both the first- and second-order Raman signatures of the crystals differ significantly for different laser wavelengths. Careful analysis of the Raman scattering intensities allows the discussion of oxygen electronic defect levels and both direct and indirect band-gaps. Notably, temperature-dependent experiments provide the first experimental indication that the reported strong variations of the optical band-gap in BFO originates from a decreasing indirect electronic gap. More generally, our study suggests that Raman scattering at various wavelengths offers a well-adapted probe for the investigation of electronic excitations in multiferroic functional oxides.

Using a physically-motivated form for the energy as a function of magnetic ordering and lattice distortions around the high symmetry reference structure, we present a systematic method for determining the ground state and low-energy structures of transition-metal ABO₃ compounds from first principles. The structural information obtained through this method forms the foundation for the first-principles structural determination of the structure of perovskite oxide superlattices. The method is demonstrated for SrMnO₃, which has a nontrivial phase sequence with varying epitaxial strain that has been of recent interest both in first-principles and experimental investigations.

Piezoresponse Force Microscopy (PFM) has emerged as a powerful tool for experimental investigations of ferroelectric materials. In the imaging mode, PFM allows visualization of static domain structures with nanometer spatial resolution. Application of a sufficiently large voltage through a conductive scanning probe microscopy (SPM) tip can induce local polarization switching and can be extended for creation of tailored domain structures and ferroelectric data storage. Finally, acquisition of the piezoresponse signals during polarization reversal allows measurement of local hysteresis loops, which can be used for characterization of the switching process in the nanoscale area in the vicinity of the tip. The broad application of PFM for probing domain structures and polarization reversal in ferroelectrics demands deep understanding of the basic mechanisms involved. PFN (Pb(Fe_0.5Nb_0.5)O₃) is a well-known multiferroic material with high dielectric constant, very good ferroelectric properties, and low dielectric loss value at room temperature. PFN thin films were grown by optimized pulsed laser deposition (PLD). The thicknesses of PFN thin films are found to be ~ 70 nm. The highly c-axis oriented growth containing only (00l) diffraction peaks of PFN films along with in plane epitaxial relationship were confirmed by high resolution X-ray diffraction measurements. PFN thin films possess well saturated ferroelectric hysteresis and weak ferromagnetism at room temperature. The existence of ferroelectricity at nanoscale is confirmed by band excitation PFM. The local polarization reversal by an electric field produced by a conductive SPM tip as a function of the relative humidity and temperature in an SPM chamber has been studied. The decrease of piezoresponse is observed with increase of relative humidity and temperature. The observed phenomena are attributed to the existence of a water meniscus in the vicinity of the tip-surface contact, reducing the effective applied potential with increasing humidity. The ferroelectric phase transition is also probed by the temperature dependence of piezoresponse studies. In addition to the temperature dependence of piezoresponse studies the phase transition is also confirmed by temperature dependent dielectric spectra. Detailed studies on effect of humidity and temperature on coercive field, imprint, switchable polarization and nucleation bias of PFN will be discussed in the meeting.

Structural distortions of ABO₃ perovskites to the parent simple cubic Pm3m structure can be classified into: i) tiltings and rotations of the BO₆ oxygen octahedra, ii) stretching of oxygen octahedra (Jahn-Teller-type distortions) and iii) cation displacements. Here we demonstrate that each of these distortions can be affected by the strain induced through ex situ He implantation in epitaxial perovskite thin films.
I) SrRuO₃: SRO films are grown under a small compressive biaxial in-plane strain on SrTiO₃ substrates. A thorough X-ray diffraction study confirms that the as-grown films have the bulk-like orthorhombic crystal structure with a small monoclinic distortion that is related to oxygen octahedral rotation pattern described by the Glazer notation a^minus;a^-c⁺. The expansion of the out-of-plane lattice induced by He implantation is shown to induce rotations of the oxygen octahedra and stabilize the a⁰a⁰c^minus; tetragonal phase of SRO.
II) La_1-xSr_xMnO₃: LSMO films are grown on SrTiO₃ substrates und biaxial tensile in-plane strain. He implantation results in an out-of-plane lattice expansion bringing the unit cell closer to the cubic state. The Jahn-Teller distortion is reduced. The orbital occupations are drastically varied due to the reduction of the e_g level energy splitting, which is most significantly reflected in a decrease of the metal-insulator transition and enhancement of the insulating character [1].
iii) BiFeO₃: BFO films are grown on LAO substrates in the pseudo-tetragonal monoclinic M_C phase. He implantation leads to a rotation of polarization vector into the out-of-plane direction that is accompanied with a structural transition to the pure tetragonal phase that is otherwise only observed at elevated temperatures. These results demonstrate that He implantation can be used to tailor cation displacements in perovskites.
Supported by the US DOE Office of Basic Energy Sciences, Materials Sciences and Engineering Division and under US DOE grant DE-SC0002136.
[1] H. Guo et al., “Controlling the length of a single axis in a complex oxide lattice via helium implantation and extraction” Phys. Rev. Lett. , accepted

LiNbO₃ is a ferroelectric crystal associated with the R3c crystal structure which lacks an inversion symmetry, leading to the presence of a spontaneous tunable non-zero electric polarization, which reverses across a 180⁰ domain boundary. This leads to applications of LiNbO₃ as surface acoustic wave sensors, optical parametric oscillators and quasi-phase matched crystals for second harmonic generation, all of which depend on domain engineering of LiNbO₃ crystals. In this work we have utilized aberration-corrected Scanning/Transmission Electron Microscopy (S/TEM) imaging and electron energy loss spectroscopy to probe the atomic and electronic structure of LiNbO₃ domains. First principles calculations are combined with our experimental observations to further uncover the relationship between atomic displacements and the local electronic structure of the crystal.

Magnetoelectric (ME) 2-2 layered composites based on piezoelectric and magnetostrictive constituents were shown to have the ability for low magnetic AC field sensing. The highest ME voltage response can be achieved at a working point at mechanical resonance and in a magnetic bias field parallel to the long cantilever axis and along the hard axis of the ferromagnetic layers [1]. Here, we report on the use of these composites, consisting of a 2 µm thick AlN layer as the piezoelectric and an exchange biased multilayer as the magnetostrictive component. The two functional ME components are sputter deposited on opposite sides of silicon cantilevers with typical dimensions of 2.2 mm x 25.2 mm. The magnetostrictive multilayer consists of multiple repetitions of Ta/Cu/MnIr/Fe_70.2Co_7.8Si₁₂B₁₀ layers, whereby the total thickness of FeCoSiB layers is varied from 1 µm to 4 µm. After magnetron sputter deposition exchange bias is aligned by heat treatment with the application of an external magnetic saturation field.
In mechanical resonance ME coefficients of up to α_ME = 3.9 kV/cm#8729;Oe for 4 µm total thickness of the ferromagnetic layers are measured for AC magnetic fields with frequencies of about 860 Hz. In a magnetically shielded environment the best corresponding limit of detection (LOD) of such ME sensors is 1.5 pT/radic;Hz. Exchange bias, formerly introduced to set the maximum slope of the magnetostriction curve into zero external bias field [2], leads to nearly single domain magnetic structures, where the magnetostrictive response is taken place by rotational magnetization processes [3]. In that case magnetic noise contributions are significantly reduced during magnetic reversal, which is beneficial for the frequency conversion technique based on high amplitude magnetic field modulation [4]. This technique is essential for measuring biomagnetic fields in the frequency range from 1 to 100 Hz, where direct field measurements suffer from the 1/f noise and the missing resonance effect enhancement. Using this approach a LOD of 180 pT/radic;Hz for a multilayer stack with 1 µm total thickness of the ferromagnetic layers could be achieved for 10 Hz signals [5].
Funding by the German Science foundation (DFG PAK 902) is gratefully acknowledged.
References:
[1] H. Greve et al., Appl. Phys. Lett. 96, 182501 (2010).
[2] E. Lage et al., Nature Materials 11, 523-529 (2012).
[3] E. Lage et al., Appl. Phys. Lett. 104, 132405 (2014).
[4] R. Jahns et al., Sensors and Actuators A 183 16-21 (2012).
[5] V. Roebisch et al., J. Appl. Phys. 117, 17B513 (2015).

The technological interest of multiferroic materials arises from their intrinsic multifunctionality. The simultaneous magnetic and ferroelectric ordering combined with a strong magnetoelectric coupling might give rise to the external control of magnetism through electric fields and, viceversa, the control of the ferroelectric domains by magnetic bias. Promising intrinsic multiferroics are some manganites with the cubic ABO₃ perovskite structure. For instance, previous works have shown that the antiferromagnetic paraelectric SrMnO₃ could become ferroelectric upon artificial expansion of the unit cell, either by epitaxial strain [1,2] or partial substitution of Sr with Ba [3]. In this case, the induced lattice distortion would induce ferroelectricity by the off-centering of the magnetic cation Mn⁴⁺.
In this work we exploit the potential of atomic resolution imaging by aberration-corrected Scanning Transmission Electron Microscopy (STEM) to demonstrate the formation of polar domains in pseudocubic (Sr_1-xBa_x)MnO₃, 0 le; x le; 0.4, (SBMO) epitaxial thin films. These have been grown by pulsed laser deposition from bulk polycrystalline hexagonal phase targets onto (001)-oriented perovskite substrates. Strain analysis of High Angle Annular Dark Field (HAADF) images confirms the epitaxial coherent growth and the pseudocubic structure of the films and the presence of a strain gradient towards the surface that might be coupled with the polar state of the film. Simultaneous imaging by HAADF and Annular Bright Field (ABF) techniques reveals significant displacements of the Mn atomic positions in addition to a strong shift of the oxygen atoms, leading to distorted MnO₆ octahedra. These displacements have been quantitatively mapped with picometer resolution by the precise structural analysis of ABF images [110]. Displacement maps of large areas reveal extended domain walls due to the progressive spatial variation of the O-Mn relative displacements in SBMO.
References
[1] J. H. Lee and K. M. Rabe, Phys. Rev. Lett. 104, 207204 (2010)
[2] C. Becher, L. Maurel et al., Nature Nanotech. 10.1038/nnano.2015.108 (2015)
[2] H. Sakai et al., Phys. Rev. Lett. 107, 137601 (2011)

Conductive transition metal oxides such as La_0.5Sr_0.5TiO₃ (LSTO) are an important class of materials because of their potential for use as electrodes in all-oxide based devices. Conduction in LSTO is mainly governed by the addition of electrons to the Ti conduction band, but the magnitude of conductivity is dependent on film thickness and is strongly coupled to structural distortions at the atomic scale. This behavior is similar to that seen in recent research on nickelates, where it is demonstrated how interfacial coupling at polar interfaces can be used to tune atomic scale structure. We show how similar interfacial coupling at the polar LSTO-STO and LSTO-vacuum interface controls the metal-insulator transition of thin LSTO films. We use a combination of experiment and first principles theory to identify the key structural differences in LSTO thin films grown using molecular beam epitaxy and pulsed laser deposition. We find polar structural relaxations at both the LSTO-STO interface and the LSTO-vacuum interface as well as cation intermixing at the LSTO-STO interface. For film thicknesses less than 5 unit cells, these effects dominate the electronic transport, and the films are insulating. As the film becomes thicker, the structural distortions become less important to transport, and a high conductivity is recovered.

Relaxor ferroelectric solid solutions display compositional and charge disorder thought to lie at the heart of their peculiar properties, and exhibit giant electrostrictive and electromechanical response that sees them as ideal materials in a wide array of applications spanning from actuators to transducers and sensors. Yet, disorder at the mesoscale in the relaxor ferroelectrics remains poorly understood, but are particularly amenable to scanning probe microscopy (SPM) study. Here, we utilize band-excitation piezoresponse force microscopy and spectroscopy (BE-PFM) to study one of the most widely used relaxor ferroelectrics, (1-x)Pb(Mg_1/3Nb_2/3)O₃-xPbTiO₃ (PMN-xPT), near morphotropic phase boundary (MPB) composition x=0.28 By applying DC pulses of increasing magnitude and alternating polarities and measuring the piezoresponse as a function of time, the polarization dynamics are mapped across the surface and deconstructed using independent component analysis (ICA). The ICA analysis implicates spatially varying fractions of field-induced monoclinic or tetragonal phases as drivers of the mesoscopic disorder. Hysteresis loop acquisition shows clusters with specific mound-like features, and displays softening before large increases of response are observed, consistent with a field-induced phase transition. Phase-field modeling of an MPB system with high levels of random-field disorder suggests a gradual polarization rotation in the probed volume of the tip, consistent with the experimental findings. In tandem with thermodynamic modeling, these BE-PFM techniques offer tremendous opportunities to spatially map disorder in these systems, while big and deep data approaches are shown to be critical to deciphering the possible mechanisms at play when confronted with multidimensional datasets ushering in the new age of “big data SPM”.
This research was supported by the US Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), Materials Sciences and Engineering Division (RKV, SVK, LQC under Award No. DE-FG02-07ER46417(Chen)). This research was conducted at and partially supported by (MBO, SJ) the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. NBG gratefully acknowledges funding from the US National Science Foundation under grant number DMR-1255379.

The objective of this research is to investigate the magneto-dielectric properties and morphology of flexible magneto-dielectric polymer composites consisting of multi-ferroic nanoparticles that possess high permeability (µ), high permittivity (ε) and minimal dielectric loss (tan δ) at the frequency range 1MHz-1GHz. The main challenge to develop such polymer composites is the synthesis of nanoscale magnetic fillers displaying high saturation magnetization (M_s), limited coercivity and their homogeneous dispersion in a polymeric matrix. Iron oxide based nanoparticles such as magnetite and maghemite are considered promising magnetic fillers for magneto-dielectric polymer-nanoparticle composites, because of their ease of synthesis, high bulk saturation magnetization (M_s) and, size dependent coercivity. It is possible to enhance the limited magnetic properties of magnetite nanoparticles in order to attain values similar to the bulk oxides by synthesizing nanostructures consisting of oriented aggregates of iron oxide nanocrystals. These “collectively assembled” nanostructures consist of nanocrystals with common crystallographic orientations directly combined together to form nanoclusters with enhanced overall magnetization. Here, we report the fabrication and characterization of stretchable magneto-dielectric composites by dispersing collectively assembled iron oxide nanostructures with high saturation magnetization and low coercivity in polydimethylsiloxane (PDMS) elastomer matrices. Collectively assembled iron oxide nanostructures with magnetite rich composition have demonstrated saturation magnetization values of 88 emu/g with zero coercivity. Iron oxide nanostructures with maghemite rich composition exhibit a saturation magnetization of 74 emu/g, and a coercivity of 30 Oe. Magneto-dielectric composites prepared with magnetite rich iron oxide composites have a permeability of 2.1, and a magnetic loss of 0.13 at 1 GHz. The composites assembled using maghemite rich iron oxide nanostructures have demonstrated a permeability of 2.3 with a magnetic loss of 0.12 at an apparent magnetic resonance frequency of 420 MHz. Additionally, both types of magneto-dielectric polymer-nanoparticle composites exhibit 165% tensile elongation at break, matching the elongation at break condition for the PDMS elastomer matrix.

The possibility was established of monitoring both ferroelectric and ferromagnetic phase transition in the BiFeO₃ multiferroics using photoinduced SHG. As photoinducing beam we applied a UV laser 337 nm with 7 ns laser pulses. The probing laser beams were from a 1540 nm Er:glass laser. It was demonstrated that the SHG is more sensitive to the occurrence of the multiferroic phase transitions than traditional electric and magnetic measurements. The proposed method makes it possible to study the samples in poster, film and bulk forms. The method is characterized by very quick characterization. Possible errors in the taking of measurements are discussed. The regime of the phototreatment was one-two- and three beam illumination. It is shown that the thermal effects are not significant here. The increase of temperature does not exceed 0.2 K. The problems of light scattering are eliminated by using a special time-resolved signal discrimination. SHG was detected for both static and time kinetics. A correlation was found between the time delay and the light scattering. The possibilities of enhancing this method&’s sensitivity are discussed. These include additional dc-electric and magnetic poling. All the discussion is additionally performed using the band structure DFT calculations.

Materials that couple strong ferroelectric and ferromagnetic order hold tremendous promise for next-generation memory devices. Meticulous engineering of the local chemistry and lattice distortions, strain and tilt patterns at the atomic-scale has produced a number of novel ferroelectric and multiferroic materials. Topological defects—such as domain walls that occur at the nano- or micron length scales—can further stabilize emergent phenomena in these materials due to reduced dimensionality and distinct chemistry. In order to harness these topological features, we must not only understand the electronic and spin states generated at these sites but also be able to control their positions during materials synthesis. Here we directly engineer ferroelectricity such that it enhances magnetism at all length scales: at the atomic-scale we impose sub-Angstrom lattice distortions to drive a nominally paraelectric material into a ferroelectric ground state and at the nanoscale place the domain walls with monolayer precision. This constructs a strongly magnetically ordered-ferroelectric with a transition occurring near room-temperature.
In these (LuFeO₃)_m(LuFe₂O₄)₁ superlattices, the frustrated ferrimagnet LuFe₂O₄ is epitaxially clamped by LuFeO₃. Statistical analysis of scanning transmission electron microscopy (STEM) images shows that the room-temperature improper ferroelectricity characteristic of LuFeO₃, manifest as a sub-Angstrom displacement of the lutetium atoms, propagates through the structure for m ge; 2. Increasing the number of LuFeO₃ layers in the superlattice enhances the ferroelectric distortions up to m~9. Density functional theory calculations indicate that these distortions reduce the LuFe₂O₄ spin frustration, which in turn boosts the magnetic transition temperature from ~240 K in the bulk compound to ~280 K for the (LuFeO₃)₉(LuFe₂O₄)₁ superlattice. Additionally, we design the domain architecture to position the charged ferroelectric domain walls at the LuFe₂O₄ layer. Charge is transferred to the domain wall, alleviating the otherwise electrostatically unstable polarization arrangement, further increasing the magnetic moment. Coupling the precise engineering of the sub-Angstrom ferroelectric distortions and nanoscale domain patterns enables us to synthesize a strong magnetically-ordered ferroelectric with the highest transition temperatures.

Reversible control of physical properties, including charge carrier transport, superconductivity, phase transitions and magnetism, is a key requirement for utilization of transition metal oxides in device applications. A number of recent studies have shown that ionic-liquid gating allows to reach carrier concentrations sufficient for modulation of physical properties of certain materials, including inorganic oxides and organic semiconductors. In this presentation, we investigate the effect of ionic-liquid gating on charge carrier transport and magnetic properties of a popular oxide material, SrRuO₃.^[1] By application of a small gate voltage (within 3 V) to an ion-gel, we observed the reversible and continuous modulation of metal-insulator crossover temperature and the onset of magnetoresistance of ion-gel gated SrRuO₃ thin films. Additional measurements, including the responce of SrRuO₃ to the gate sweeps at different rates, the effect of oxygen atmosphere and the temperature dependence of resistivity, suggest that the dominant mechanism of field effect in ionic-liquid gated SrRuO₃ FETs is an electric field induced migration of oxygen vacancies in the SrRuO₃ lattice (creation and annihilation of oxygen vacancies).^[1] We believe that our findings uncover one of the fundamenatl mechanisms of electrolyte gating effects in oxides and further contribute to the development of oxide electronics.
References:
[1] H. T. Yi, B. Gao, W. Xie, S.-W. Cheong, and V. Podzorov, Sci. Rep. 4, 6604 (2014).

Oxygen octahedral tilts (OOTs) are ubiquitous in perovskites structures, and engineering of OOT in multiferroic materials has potential applications such as information storage, due to its strong coupling with polarization and magnetization. For example, recently it is observed that OOT can induce spontaneous polarization and magnetization in layered perovskites above room temperature. In this case, the ferroelectric and ferromagnetic domain wall properties are strongly correlated with the structural OOT domain walls. Here we propose a rotational compatibility condition to identify low-energy domain walls in perovskites with OOT instability. It is derived from the strong domain wall energy anisotropy arising from the rigidity and corner-sharing feature of the octahedral network. Based on Ginzburg-Landau-Devonshire theory, we analyze quantitatively the domain walls in SrTiO₃ and explain successfully the unusual ferroelectric domain wall width and energy in multiferroic BiFeO₃.

To date, several types of ferroelectric domain walls showed dc conduction. Because of intrinsic nanoscale size and field-controlled topology, electronically conducting domain walls can turn into elementary building blocks for future electronic devices. Elucidation of the mechanism and subsequent control over domain wall conduction has so far been impeded by a large contact resistance at the metal-ferroelectric interface. In all reported cases of single domain wall conductance, electronic current through the domain wall could only be measured at relatively large dc bias. Current-voltage characteristics are typically highly rectifuing, with detectable current only at one bias polarity. Although the contact resistance can be overcome with sufficiently large applied bias, the variability in the contacts and large electric fields at or above the threshold values for domain wall motion largerly rule out quantitative analysis.
The problem of contacts can potentially be overcome using high-frequency AC voltage to probe domain walls. Here we will present our successful measurements of microwave conductance at 180^o domain walls in lead zirconate titanate, using microwave microscopy operating at 3 GHz. The domain walls are detected by microwaves even when they are undetectable by dc voltage. AC conducting domain walls can be repeatably reconfigured and have extraordinary stability in time and temperature. Quantitative modeling reveals that the conductance of domain walls is comparable to doped silicon. These results set the stage for a new generation of local experiments on conducting domain walls, and furthers the prospects of their application in fast electronic devices.
This research was sponsored by the Division of Materials Sciences and Engineering, Office of Science, Basic Energy Sciences, U. S. Department of Energy (A. T., S. V. K., P. M.). Microwave measurements were conducted at CNMS, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U. S. Department of Energy. P. Y. was financially supported by the National Basic Research Program of China (grant 2015CB921700) and National Natural Science Foundation of China (grant 11274194).
1. A.Tselev, P. Yu, S. V. Kalinin & P. Maksymovych, “Microwave ac conductivity of ferroelectric domain walls”, submitted 2015.

Magnetoelectric multiferroics have the potential to be applied in a wide variety of technologies, such as magnetic field sensors, probes, transducers, electrically-switched magnetic tunnel junctions, and multi-state memory devices. Renewed interest in magnetoelectric multiferroics, particularly those with room temperature effects, is made possible by advances in materials processing, characterization, and first-principles calculations. Substituted perovskite oxides, such as transition metal (M = Fe, Co) substituted Sr(Ti, M)O₃, have exhibited room-temperature ferromagnetism when deposited under reducing conditions leading to a significant oxygen deficiency. These studies and others determined that both strain and oxygen vacancy concentration are capable of affecting the ferroic properties.
In this study we characterize the microstructure, magnetic properties, and ferroelectricity of oxygen-deficient Co-substituted SrTiO₃ (STCo) thin films grown on (100) SrTiO₃ to better understand the effect of oxygen vacancy concentration and microstructure on the strain state and ferroic properties of this single-phase perovskite oxide thin film. The films were grown by pulsed laser deposition under high vacuum (1 mu;Torr), and varied in thickness from 47 nm to over 200 nm, with 30% Co on the Ti sites. The thinnest samples showed a cube-on-cube epitaxy in which the STCo had a (100) orientation with in-plane compression and a tetragonally distorted unit cell. Above a critical thickness the films formed a double epitaxial structure in which crystals of (110) orientation were embedded in the (100) matrix, both growing epitaxially to the substrate and having the same composition. This is the first report of double epitaxy in STCo, though it has been previously reported in Fe-substituted SrTiO₃. X-ray diffraction data including pole figures and transmission electron microscopy indicate that the double epitaxy is associated with strain relaxation in the film. We describe the microstructure and its effect on the magnetic and ferroelectric properties of the films. The thinner films had saturation magnetization up to 52 emu cm^-3 but this decreased for the thicker films to 4 emu cm^-3. XPS excluded the presence of metallic Co and indicated the Co was a mixture of 2+ and 3+ valence states. Ferroelectric properties were also measured at room temperature after lithographically patterning contacts on the top surface of the films, showing small remanent polarization in ferroelectric loops and PUND tests. Understanding microstructure and its effect on magnetism and ferroelectricity greatly impacts the ability to realize single-phase room temperature magnetoelectric multiferroics.

Material systems with negative compressibility in a particular order parameter can be used to achieve intrinsic gain in the variable conjugate to the order parameter. In the case of ferroelectrics, this gives rise to negative capacitance and voltage gain. This concept was initially proposed to combat fundamental limits on power dissipation in CMOS electronics imposed by the thermal distribution of electrons in a semiconductor channel [1-5], and since then it has been experimentally demonstrated [6,7]. In this study, we explore the effects of coupling polarization to other variables, such as strain, mass, and magnetization. In particular, we see how modifying the Landau-Khalatnikov equation can introduce an intrinsic timescale into ferroelectric phenomena and therefore give rise to resonance in multiferroic systems.
In this talk, we will show that associating a magnetic flux with a change in polarization is equivalent to artificially increasing the ionic mass associated with polarization reversal, which forces one to restore the neglected kinetic term in the Landau free energy. By placing a discrete inductance in series with a ferroelectric thin film to reduce the required circuit bandwidth, we examine the small-signal characteristics, including the modulation of resonant properties, at different points in the switching process. Furthermore, we propose methods of altering mechanical boundary conditions to control the resonance parameters. Importantly, we show that negative capacitance coupled to “mass-like” kinetic phenomena gives rise to feedback-free oscillation, potentially within a single device.
References:
1. Salahuddin, S., Datta & S. “Use of negative capacitance to provide voltage amplification for low power nanoscale devices.” Nano Lett. 8, 405 (2008).
2. Khan, Asif Islam, Yu, Pu, Trassin, Morgan, Lee, Michelle J., You, Long, Salahuddin, Sayeef. The effects of strain relaxation on the dielectric properties of epitaxial ferroelectric Pb (Zr0. 2Ti0. 8) TiO3 thin films. Appl. Phys. Lett. 105 (2), 022903 (2014).
3. Zhirnov, V. V. & Cavin, R. K. “Negative capacitance to the rescue.” Nature Nanotechnology 3, 77 (2008).
4. Theis, T. N. & Solomon, P. M. “It&’s time to reinvent the transistor!” Science 327, 1600-1601 (2010).
5. Theis, T. N. & Solomon, P. M. “In quest of the next switch: prospects for greatly reduced power dissipation in a successor to the silicon field-effect transistor.” Proc. IEEE 98, 2005-2014 (2010).
6. Khan, Asif Islam, Korok Chatterjee, Brian Wang, Steven Drapcho, Long You, Claudy Serrao, Saidur Rahman Bakaul, Ramamoorthy Ramesh, and Sayeef Salahuddin. "Negative capacitance in a ferroelectric capacitor." Nature Materials 14, 182 (2015).
7. Catalan, Gustau, David Jiménez, and Alexei Gruverman. "Ferroelectrics: Negative capacitance detected." Nature Materials 14, 137 (2015).

The substantial progress of discovering multiferroic compounds with magnetically induced ferroelectricity and exploring the underlying physics in the past decade has been one of the major driving forces for subsequent efforts in fabrication of thin films and relevant devices physics. Due to the high degree of magnetic frustration and strong spin-orbital/spin-lattice couplings in these multiferroic compounds such as manganites, the lattice distortion and interfacial effects coupled into the thin film structures impose additional degrees of freedom in controlling every aspect of multiferroicity, which certainly deserves for extensive investigations. In this talk, we focus on the preparations and characterizations on multiferroic manganite DyMnO₃ and GdMnO₃ thin films epitaxially deposited on SrTiO₃ substrates. The in-plane twinned microstructure as the core characteristic in these thin films is revealed, which is demonstrated to impose substantial influences on the magnetism, ferroelectric polarization, and magnetic control of the polarization reversal etc. In particular, we show that a proper domain engineering of DyMnO₃ thin films allows a continuous polarization reversal driven by in-plane rotating magnetic field.

Materials such as BiFeO₃ have motivated a generation of scientists to dream of the exciting applications that could be realized by the further development of multiferroics and magnetoelectrics. But, despite tireless efforts in the community to understand, manipulate, and, ultimately, utilize the innate properties of what has become the most widely studied multiferroic today, countless questions as to the fundamental nature of BiFeO₃ remain. In fact, few materials can offer the paradigmatic combination of vast potential and excitement with a frustrating lack of practical utility and overall complexity that BiFeO₃ has provided to date. Ultimately, such complex materials remain ensconced in our minds because they challenge our ability to the exert control we desire.
In this spirit, this presentation will revisit our fundamental understanding of BiFeO₃ with special attention to new insights about the role of domain walls and strain in the evolution of magnetic properties. We have demonstrated, for the first time, pathways to achieve ordered arrays of 180° stripe nanodomains in (110)-oriented BiFeO₃ films grown on GdScO₃ (010)_O substrates. Nanoscale ferroelectric 180° stripe domains with {11-2} domain walls are observed in films < 32 nm thick to compensate for large depolarization fields. Increasing film thickness results in a domain structure crossover to 71° ferroelastic domains determined by the elastic energy. These 180° domain walls (which are typically cylindrical or meandering in nature due to a lack of strong anisotropy associated with the energy of such walls) are found to be highly-ordered and straight which indicates that an additional anisotropy term must be present. Additional studies of Co_0.9Fe_0.1/BiFeO₃ heterostructures reveal exchange bias and exchange enhancement in heterostructures based-on BiFeO₃ with 180° domain walls and an absence of exchange bias in heterostructures based-on BiFeO₃ with 71° domain walls; suggesting that the 180° domain walls could be the possible source for pinned uncompensated spins that give rise to exchange bias. In turn, using our ability to produce BiFeO₃ films with ordered arrays of only 71°, 109°, or 180° domain walls, we return to explore detailed X-ray magnetic circular dichroism studies of samples controlled to have each type of domain wall variant. This work finds that that films with 109° and 180° domain walls have larger magnetization than those with primarily 71° domain walls. Likewise, we leverage our ability to grow films on a range of different substrates and X-ray magnetic linear dichroism studies to reassess the evolution of the preferred antiferromagnetic axis with epitaxial strain. These studies find that compressive and tensile strain lead to a down-selection of {112}- or {110}-type preferred axes, respectively. All told, we will aim to provide a comprehensive update to the nature of magnetism in BiFeO₃ and a reassessment of what we know about this material.

The observation of pronounced magnetoelectric coupling in spin-spiral systems triggered tremendous interest in so-called multiferroics and their potential application for information processing. The aspired functionality ultimately relies on spin-charge interactions on the level of domains and domain walls. After more than a decade of intense research on spin-spiral multiferroics, however, little is known about the domain physics. This even applies to fundamental key aspects such as the equilibrium distribution of domains and their response to external fields. Here, we present the field-induced domain dynamics of TbMnO₃ in the multiferroic ground state and across a first-order spin-flop transition. In spite of the discontinuous nature of this transition, the reorientation of the order parameters is deterministic and preserves the multiferroic domain pattern. This is explained by Landau-Lifshitz-Gilbert simulations which reveal that the observed behavior is intrinsic to the spin-spiral system. Our findings thus prove the scalability of macroscopic magnetoelectric properties onto the level of domains. In particular, the achievement of deterministic control of charge and transport properties at the domain boundaries paves the way for novel domain wall-based nanoelectronics. [Science 348, 6239, 1112-1115 (2015)]

The FeS alloy is widely recognized as one of the most abundant in planet outer cores and Earth deposits.^1,2 At ambient conditions, the stoichiometric FeS is found in the so-called “Troilite” structure as an antiferromagnetic semiconductor (energy gap ~0.04 eV).³ The Troilite phase appears through a displacive phase transition at 413 K from the metallic high-symmetry NiAs-type structure.⁴ In particular, the lattice distorts from a centro-symmetric to a non-centro-symmetric space-group, but it does not induce a net polarization, contrarily to experimental findings (P~0.1 µC/cm²).⁵ The ferroelectric phase, indeed, seems to be related to a further structural transition occurring at lower temperature (at about 400 K) between P-62c to P31c space-group. Here, we explore from first principles studies the phase transitions of FeS and we analyze its unexplored multiferroic properties. Interestingly, we didn&’t find a ferroelectric ground state but a magnetoelectric phase instead in which each spin channel is electrically polarized in opposite directions. We will show that this spin channel polarization is related to the magnetoelectric monopoles⁶, which makes FeS very attractive for spintronic applications.
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After the rebirth of BiFeO₃ as strong magnetoelectric (ME) Multiferroic (MF) candidate, there has been a large surge in the search of novel single phase materials with higher order of magneto-electric coupling. This was partly due to rich, complicated and interesting physics involved in this subject and partly for the development of low power nanoelectronics logic and memory devices based on the electric control of magnetization and magnetically switched polarization. The future low power consumption nonvolatile random access memory (NVRAM) elements based on magnetoelectric materials would permit the combination of fast (possibly sub-nanosecond) electrically WRITE operation with nondestructive magnetic READ operation. We have recently synthesized a few novel single phase ceramic materials, such as (PbZr_0.52Ti_0.48O₃)_1-x(PbFe_0.5Ta_0.5O₃)_x [PZTFTx; 0.2 < x < 0.4]. Its bulk ME coupling properties are comparable to well known Cr₂O₃. We also made single crystalline laminas for some of the above compositions (PZTFTx=0.3 & 0.4) using focus ion beam and subjected to domain study under magnetic and electric fields. These single crystalline laminas exhibited exceptionally high ME coupling ~ 10^-7 s/m under applied external magnetic field and also showed switching behavior of ferroelectric domains under application of high magnetic fields. These systems show biquadratic magnetoelectric coupling for in-plane application of magnetic field, however, out of plane applied magnetic field favor the linear ME coupling.
We have also developed high quality PZTFT/LSMO multilayers and superlattice nanostructures. X-ray diffraction patterns and Raman spectra revealed the formation of multilayer structure and superlattice structures with various thicknesses and periodicity, respectively. The piezoforce microscopy studies illustrated ferroelectric switching reversal for these superlattice nanostructures.
We will discuss about the enhancement of tunneling electroresistance (TER) in presence of magnetic field for Pt/PZT (7nm)/LSMO ferroelectric tunnel barriers. The significant variation in TER was observed when heterostructure junction was exposed to in-plane magnetic field and its values changed from 57 (at 0 G) to 110 under 10 kG magnetic field. Ferroelectric polarization reversal and application of magnetic field changed lattice strain, chemical bonding and charge modulation near PZT/LSMO interface which in turn affects the charge carrier density and transmission probability. A robust resistive switching for data retention was achieved over long period of time. These results may be useful to design high performance magnetic control of nanoscale ferroelectric tunnel devices.

In order to reduce required voltages to generate practical E fields to induce measurable ME effects in single phase hexaferrite films¹ at room temperature we have developed a new technique to apply E fields in the film plane. The technique involves the deposition of N parallel metallic strips separated by a distance d so that there are N-1 capacitors connected in series over the surface of the film. The voltage is applied equally across each capacitor and, therefore, the E field is uniform in the film plane. The aim of these studies was to use ME films for magnetic field and electric field sensors compatible with other planar technologies as well as to perform fundamental measurements. Films are prepared by the PLD deposition technique.
Specifically, the following family of M-type hexaferrites materials were investigated _, where x varied from 1.2 to 3.5. The parameter α was measured for the cases (a) the induced magnetization was perpendicular, , and (b) parallel, , to the applied electric field. A maximum value of 3.05×10^-8 was measured for x=2.2. This optimum value may be related to the maximum packing density of cobalt ions in octahedral sites and still maintain charge neutrality. Also, reached a maximum value of 1.5×10^-8 , but for x =1.9.
A thin film of SrCo2Ti2Fe8O19 on sapphire substrate was utilized to explore the technique from 1 KHz to 10 MHZ. In the first technique, we refer to it as a “direct” ME method, an ac magnetic field, H, generated via a solenoid coil, induced an electric field, E, or polarization, P. These induced E fields are measured via a multi-capacitor detection scheme. The second technique referred to as the “converse” ME method, an ac voltage or electric field, E, is applied via multi-capacitors and the induced magnetization, M, was detected with the use of a solenoid coil. For H-field sensing we obtained sensitivity of 4×10^-4 Vm^-1/G and for E-field sensing the sensitivity was 10^-3 G/Vm^-1 In the direct experiments we measured the ME coupling α = 6x10^-9 sec/m. In the converse experiments inductance tunability of up to 6% was achieved for tunable inductor applications. Sensitivity can be easily improved with narrowing of electrodes spacing making these techniques very compatible with any other planar IC technology and may be utilized for measurements of fundamental ME coupling parameters.
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6. C. Vittoria, S. Somu, and A. Widom, Phys. Rev. B 89(13), 134413 (2014).

Covalent bonding plays an important role in proper ferroelectric perovskites (ABO₃), which generally have a d⁰-ion (Ti, Zr, etc.) at the B site, or Bi/Pb at the A site [1]. Since magnetism in perovskites is usually due to unpaired spins of the transition metals, having a d⁰ ion at the B site makes proper ferroelectric ferromagnets exceedingly rare. Exceptions can be found among materials with a non-d⁰ ion at the B-site if Bi or Pb occupies the A-site, since these two ions exhibit strong covalent bonding or 6s-lone pairs that stabilize a distorted crystal structure, suggesting that it may be possible to design new types of ferroelectric ferromagnets by artificial crystal design. One way to obtain a multiferroic material is to apply strain on a magnetic crystal, as happens in EuTiO₃ films and bulk (Ba,Sr)MnO₃ crystals [2]. In this work, we explore the effects of epitaxial strain on ferromagnetic double perovskite La₂NiMnO₆ thin films. We show that by stretching the rhombohedral La₂NiMnO₆ lattice along the [111]_cubic direction, the normally paraelectric material develops A-site driven ferroelectricity without losing the ferromagnetic order.
Strained La₂NiMnO₆ thin films were grown on LSAT, SrTiO₃ and Nb:SrTiO₃(001) (Nb:0.05wt%) substrates by pulsed laser deposition [3]. X-ray diffraction scans showed that the La₂NiMnO₆ films were epitaxially grown along the c-axis, and that the B-site Ni and Mn ions were ordered along the [111] _cubic direction. Magnetic characterization by SQUID gave a saturation magnetization of 2 mu;_B/B-site with a Curie temperature of 280 K. The presence of spontaneous dielectric polarization in strained La₂NiMnO₆ films was shown by observing ferroelectric hysteresis loops, by pyroelectric current measurement, and by scanning nonlinear dielectric microscope domain imaging [4]. Density-functional theory (DFT) calculation results indicated that the tilting of MnO₆ and NiO₆ octahedra is inhibited by the threefold crystal symmetry of the strained rhombohedral La₂NiMnO₆ lattice. Electron localization mapping showed that covalent bonding with oxygen and 6s orbital lone-pair formation are negligible in this material.
[1] N. A. Hill, J. Phys. Chem. B 104, 6694 (2000). [2] D. G. Schlom, et al, MRS Bull. 39, 118 (2014). [3] M. Kitamura et al. Appl. Phys. Lett. 94, 132506 (2009) [4] R. Takahashi et al. Phys. Rev. B, 91, 134107 (2015)

Hafnium oxide (HfO₂) is one of a few metal oxides that is thermodynamically stable on silicon and silicon oxide. It is been used as high-k dielectric material for gate applications in complementary metal oxide semiconductor (CMOS) devices. There has been renewed interest in HfO₂ due to the recent discovery of ferroelectricity and antiferroelectricity in doped HfO₂. Typical ferroelectrics - such as strontium bismuth tantalate (SBT) and lead zirconium titanate (PZT) - contain elements that easily react with silicon and silicon oxide at elevated temperatures; therefore, such ferroelectrics are not suited for device applications. Meanwhile, ferroelectric HfO₂ offers promise regarding integration with silicon.The stable phase of HfO₂ at room temperature is monoclinic, but HfO₂ can be stabilized in the tetragonal, orthorhombic or even cubic phase by suitable doping. We stabilized Y-doped HfO₂ thin films using pulsed laser deposition. The strain state can be controlled using various perovskite substrates and controlled growth conditions. We report on Y-doped HfO2 domain structures from piezo-response force microscopy (PFM) and structural parameters via X-ray reciprocal space maps (RSM). We hope this work spurs further interest in strain-tuned ferroelectricity in doped HfO₂.

Magnetic field sensors based on 2-2 magnetoelectric (ME) composites, which consist of a magnetostrictive layer coupled with a piezoelectric layer, open up opportunities for the sensing of low magnetic field amplitudes. From a magnetic point of view the ME response is directly connected to the local susceptibility with varying magnetic field and thereby to the magnetic domain and especially the magnetic domain wall structure. On the other hand, magnetic domain activity is a well-known noise source in various magnetic field sensing applications. Sudden magnetization changes from domain nucleation and domain wall motion will impact the sensor performance. Additionally, the characteristics of magnetic domains act as a perfect mirror for magnetic anisotropy distribution, effects of internal film or device stress related to the use of magnetostrictive materials, or coupling and demagnetization effects in patterned magnetic structures. Overall, due to the formation of magnetic domains in the piezomagnetic phase, the local change of magnetization can be very complicated.
The role and relevance of magnetic domains and domain walls for the ME response will be discussed in detail. With the variation of shape, thickness and layering in the piezomagnetic phase, different kind of domains and domain walls are forming that significantly influence the ME response in terms of sensitivity, working point position, and level of magnetic noise. The influence of different Néel and Bloch-type domain walls on the magnetic field response of ME sensors will be shown. Moreover, the local magnetization behavior of the magnetostrictive phase of ME sensors is compared directly to the hysteretic ME response using advanced magneto-optical imaging techniques. Using second order magneto-optical effects, a direct correlation of local magnetostrictive response to the exhibited sensor signal is derived, proving a direct correlation of the domain effects to the sensor performance. With different magnetic history, especially for very soft-magnetic piezomagnetic phases, alterations in maximum ME sensitivity of up to 50% for the same sensor are revealed. Also the bias field dependence for obtaining maximum ME signal differs strongly with magnetic field history. Exhibited changes in the magnetic domain structure are one-to-one reflected in the ME response.
Local magnetoelastic effects highly dictate the overall magnetic domain formation, even in extended magnetic structures. Controlling and tailoring magnetic domain formation processes is the foundation for reversible high amplitude and low noise ME behavior.
Support by the DFG through the Collaborative Research Centre SFB 855, MC9/15-1, and the DFG Heisenberg Programme (MC9/9-1, MC9/9-2) is acknowledged.
J. McCord, J. Phys. D: Appl. Phys., accepted as Topological Review (2015)
V. Röbisch et al., JAP 117, 17B513 (2015)
E. Lage et al., APL 104, 132405 (2014)
N. O. Urs et al., APL 105, 202406 (2014)
T. von Hofe et al., APL 103, 142410 (2013)

This talk explores how nanoscale structure can be used to tune the properties of ferromagnetic, piezoelectric, and magnetoelectric materials. We consider both intrinsic and composite magnetoelectrics, asking how a combination of small size and mechanical flexibility can alter materials properties. We will first consider nanoporous bismuth ferrite (BFO) produced through wet chemical polymer templating routes. Porosity provides mechanical flexibility to this nanocrystalline oxide network, resulting in dramatically enhanced changes in magnetization upon electrical biasing in nanoporous materials, but not in dense thin films made using the same chemistries. Polymer templating can also be applied to piezoelectric materials such as lead zirconate titanate (PZT) or oxide magnets like cobalt ferrite (CFO) to produce porous versions of these materials. In some cases, fully epitaxial porous materials can even be generated. These porous materials can serve as building blocks for multiferroic composite materials where the nanoscale structure can again be used to tune the magnetoelectric coupling. Finally, nanocrystal building blocks provide a simple method to tune magnetic domain sizes in nanoscale magnetoelectric composites. Both strain based and voltage controlled changes in magnetization are possible. For example, strain can be used to convert nanocrystals from a superparamagnetic state to a ferromagnetic state and voltage can be used to change the direction of the direction of easy axis. In all cases, the goal is to use controlled nanoscale architecture as another handle to tune properties in multiferroic materials.

Driven by the approaching limits of transistor scaling, recent interest has surged in exploring the memristor, a two-terminal device with a pinched current-voltage hysteresis loop. The most widely studied types of memristors are based on the dynamics of defects such as oxygen vacancies or metal ions. High On/Off ratios (>10³), required by applications, are often achieved, but the defect-motion-based switching mechanism poses intrinsic challenges in the control of the materials, affecting the yield and reliability of the devices. Memristors based on switching mechanisms other than defect dynamics have been demonstrated, but achieving high On/Off remains challenging. Recently, several papers reported memristive behavior from electric-field controlled magnetism in magnetic tunnel junctions (MTJs) [1] and magnetic-metal/ferroelectric junctions [2-4], but the On/Off ratio is typically smaller than 10.
Here we report the discovery of memristive behavior with high On/Off ratio in transition-metal-oxide (TMO) interfaces. Such interfaces have been found to exhibit many other unusual properties. We demonstrate memristive switching in a La_0.7Ca_0.3MnO₃/PrBa₂Cu₃O₇ bilayer with an On/Off ratio greater than 10³ without external magnetic field. We attribute this phenomenon to a new type of interfacial magnetoelectricity. It is known that at the interface of ferromagnetic (FM) and non-ferromagnetic TMOs, there exists a “magnetic dead layer” (MDL) where the first layer of the FM TMO can be antiferromagnetically (AFM) coupled to the bulk of FM TMO [5]. Using results from first-principles calculations, we show that an external electric field induces subtle displacements of the interfacial Mn ions at the La_0.7Ca_0.3MnO₃/PrBa₂Cu₃O₇ interface, which switches on/off the interfacial MDL, resulting in memristive behavior for spin-polarized electron transport across the bilayer. The interfacial nature of the switching entails low energy cost, about of a tenth of atto Joule for write/erase a “bit”. Our results indicate new opportunities for manganite/cuprate systems and other transition-metal-oxide junctions in memristive applications.
Reference:
[1] W. G. Wang, M. Li, S. Hageman, and C. L. Chien, Nature Mater.11, 64 (2012).
[2] Y.-H. Chu, et al. Nature Mater.7, 478 (2008).
[3] V. Garcia, et al. Science327, 5969 (2010).
[4] S. Valencia, et al. Nature Mater.10, 753 (2011).
[5] W. Luo, S. J. Pennycook, and S. T. Pantelides, Phys. Rev. Lett.101, 247204 (2008).
Acknowledgement
This work was supported by NSF grant DMR-1207241, by NSF XSEDE grant TG-DMR130121, by DOE grant DE-FG02-09ER46554, by NERSC under DOE contract DE-AC02-05CH11231, by the McMinn Endowment at Vanderbilt University, by Spanish MICINN grants MAT2011-27470-C02 and Consolider Ingenio 2010-CSD2009-00013, by CAM through grant S2014/MAT-PHAMA II, and by the EPSRC.

Intrinsically the energy it takes to manipulate a magnet is very small compared to most electronic devices. As a result, magnetic devices, in principle, could be much more energy efficient. However, in reality, a significant energy needs to be applied to generate the driving force that will eventually control the magnet, such as magnetic field or spin polarized current. One way to reduce this ‘parasitic&’ energy dissipation and approach the intrinsic energy efficiency of the magnets is to use voltage for controlling magnetization. In this talk, we shall discuss our recent work with natural and synthetic multiferroic heterostructures where such control of magnetization and subsequent significant reduction in dissipated energy density has been demonstrated.

Multiferroic materials attract tremendous scientific and technological interests due to their ability to exhibit a magnetoelectric (ME) effect which enables the control of the magnetization/polarization with an applied electric/magnetic field respectively. Compared with the bulk multiferroics, nanocomposite multiferroics attract more research interests. The self-assembled epitaxial BiFeO₃-CoFe₂O₄ (BFO-CFO) nanostructured composite thin films, which contain nanopillars of one phase embedded inside the matrix of the other phase, could present different multiferroic properties by depositing on different oriented SrTiO₃ (001), (110) and (111) substrates by pulsed laser deposition (PLD) method.
Here, we have utilized self-assembled BFO nanopillars in a BFO-CFO two phase layer on STO as a seed layer on which to deposit a secondary top BiFeO₃ layer by PLD. The growth mechanism of this secondary BFO layer has been investigated, and its multiferroic properties studied. It has been found from cross section SEM studies that the top BFO layer preferentially grows from the bottom BFO seeds and its grain size could be controlled by these seeds. The ferroelectric, ferromagnetic and multiferroic properties of this new nanostructure has also been studied.
Furthermore, based on that above mentioned new heterostructures, we adjusted the experimental conditions and then post-deposited one more BFO layer on the top of that heterostructures. Then, a new structure with second phase CFO nanoparticles embedded in a primary BFO matrix phase has been obtained. We have confirmed good multiferroic properties by several kinds of characterization methods. Moreover, we demonstrated, by focusing on switching characteristics of the piezoresponse, that the heterostructure showed magnetic field dependence of piezoelectricity due to the improved coupling enabled by good connectivity amongst the piezoelectric and magnetostrictive phases. We have also provided statistical study for the characteristics and then absolutely confirmed that the yielding notable ME effects result from a better coupling between the ferroelectric and ferromagnetic phases of our newly designed special architecture. The improved connectivity amongst the constituent phases of the composite heterostructure has been examined by the state-of-the-art electron microscopy. Such new ME nanocomposite heterostructures and their combination of properties hold substantial promise for multifunctional applications.

Ferroic domain walls with polar discontinuity exhibit diverse transport characteristics ranging from highly conducting to strongly insulating states and bear great application potential as nano-sized strip conductors or capacitors in next-generation devices. During the last years, great progress has been made in creating domain walls with enhanced electronic conduction properties, but tuning their functionality towards a technologically feasible working range remains a major challenge.
Here, we present a readily accessible approach for optimizing electronic domain-wall properties by the intentional implantation of ionic defects. In our multiferroic model system Er_1-xCa_xMnO₃ we modify the conductance at charged domain walls by partially replacing trivalent Er³⁺ for divalent Ca²⁺. A moderate doping level of 1% is found to enhance domain-wall currents by almost two orders of magnitude and simultaneously reduces the effective domain-wall width by about 50%. Our results thus reveal a promising pathway for controlling the nanoscale physics at charged domain walls. In particular, the research highlights a useful crosslink between classical semiconductor physics and modern domain-wall engineering, bringing us an important step closer to future domain-wall-based nanoelectronics.

Complex oxides with perovskite structure attract a lot of interest due to their superior catalytic activity and sensory properties.¹ Perovskite surfaces also provide fertile ground for the discovery of novel electronic and magnetic phenomena. In this work, we combine scanning transmission electron microscopy imaging and electron energy loss spectroscopy (EELS) with density functional theory (DFT) based calculations to study the surface of a (LaFeO₃)_m/(SrFeO₃)_n heterostructure grown on SrTiO₃ substrate using molecular beam epitaxy. Using EELS, we observe a reduction in the oxidation state of Fe from Fe³⁺ in the bulk to Fe²⁺ at the surface over a length of ~5 unit cells. Simultaneously acquired annular bright field and dark field images allow us to map the associated changes in their structure, such as cation displacements and changes in oxygen polyhedral tilts. DFT calculations taking into account the STEM results highlight the possibility of unique electronic and magnetic properties based on the level of surface reduction. Specifically, the calculations show that by reducing the surface layer of a LaFeO₃ film such that the surface is terminated with FeO₄ tetrahedra instead of the FeO₆ octahedra as present in the bulk, it is possible to stabilize an exotic phase where the surface layer displays a half-metallic ferromagnetic behavior, while the bulk remains antiferromagnetic and insulating, similar to the class of topological insulators. Further calculations enabled us to establish the lowest vacancy concentration which is sufficient to transform electronic and magnetic properties of the surface from an insulating state to a ferromagnetic and metallic state to be 0.375 (fully tetrahedral layer corresponds to 0.5).
Acknowledgements: This research was supported by the Materials Sciences and Engineering Division, Office of Basic Energy Sciences (BES), U.S. Department of Energy (DOE), via user proposals to ORNL&’s Center for Nanophase Materials Sciences, which is supported by the Scientific User Facilities Division, DOE BES, and by Korea Basic Science Institute grant to Y.M.K (T34429). This research used resources of the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the U.S. DOE under Contract No.DE-AC02-05CH11231.