Functional composite materials with mechanical elasticity that possess substantial dielectric permittivity (ε), magnetic permeability (mu;), low dielectric and magnetic loss are needed for the fabrication and miniaturization of flexible electronic devices including radio-frequency (RF) antennas, sensors and electronic identification chips. We report on the fabrication of flexible magneto-dielectric composites with low magnetic and dielectric loss, achieved by dispersion of high saturation magnetization, low coercivity air-stable iron (Fe) nanoparticles and iron/silver (Fe/Ag) core-shell nanoparticles in polydimethylsiloxane (PDMS) matrices. The magneto-dielectric composites made of air-stable Fe nanoparticles are adapted for RF communication devices with broad operation frequency, since they possess high mu; (4.4) and low ε (16.4), while maintaining low magnetic (0.28) and dielectric (0.053) loss values. Additionally, the Fe nanoparticle composites allow tensile elongation of 15% before breaking, which demonstrates the flexible nature of the material. The magneto-dielectric composites fabricated using Fe/Ag core-shell nanoparticles can combine high mu; (3.68) and ε (26) with low magnetic (0.28) and dielectric (0.1) loss, which makes them suitable for the RF communication device miniaturization. The Fe/Ag core-shell nanoparticle composites can also deform plastically under tensile elongation as high as 70%.

Optimization of one-pot synthetic procedure for the control of the morphological features of magnetic hybrid materials (HNM) based on spinel-metal-oxide nanoparticles (SMON) and carboxymethyl-cellulose (CMC)/cetiltrimethyl-ammonium-bromide (CTAB) coordination complexes, is reported. The influence of their synthesis parameters (precursor ratio, stirring rate, temperature and synthesis time) on the particle size distribution and the size and morphology of CMC/CTAB templates was investigated. The synthesized HNM were characterized by transmission electron microscopy and their related techniques, such as bright field and high angular annular dark field (HAADF-STEM) imaging, as well as selected area electron diffraction (SAED), using a FEI Titan G2 80-300 microscope. The interactions among SMON, CMC and CTAB were investigated by Fourier transform infrared spectroscopy (FTIR) in a Thermo Scientific Nicolet spectrometer. Magnetic response of the synthesized HNM was evaluated in a Quantum Design MPMS3. The experimental evidence suggests that, the formation of these HNM occurs due to the competition between CTAB molecules and SMON to occupy CMC intermolecular sites nearby to its carboxylate functional groups. Thus, the morphology and size of CMC/CTAB templates can be tuned varying the CTAB:SMON ratio. Moreover, it was found that the magnetic response of the HNM depends on the confinement degree of the SMON into the CMC/CTAB template. Hence, its magnetic characteristics can be adjusted controlling the size of the template, the quantity and distribution of the SMO nanoparticles within the template and its sizes.

Electron holography is a unique electron microscopy technique to visualize electromagnetic fields on the nanometer scale. We developed the electron holography system on the basis of a JEM-3000F electron microscope by installing a Lorentz lens and a magnetizing stage for detailed study of magnetic nanostructures of various functional materials [1]. Recently we have also utilized an HF-3300X electron microscope by which “split-illumination electron holography” can be carried out [2]. In the split-illumination method, additional biprisms are inserted into a condenser lens system. This method is especially effective for electron holography study of specimens with strong stray fields around them. This is because a reference wave can be obtained from a region far from the specimen, being free from the stray fields. Thus, electron holography analysis with high precision can be carried out, and this system is expected to widen the application of electron holography extensively.
One of the applications of electron holography study of functional materials is magnetic nanostructure analysis of Zn-doped Fe3O4 (FZO). It was reported that shape/size-controlled nanowires obtained from solid solutions based on Fe3O4, which was prepared by atomic force microscope lithography and pulsed laser deposition, show good functionalities, e.g., nonlinear I-V character and large magnetoresistance [3]. Dark field microscope images of FZO obtained by an energy-filtered electron microscope revealed the microstructure consisting of fine antiphase domains (APDs) whose size is less than 100nm. On the other hand, electron holography study combined with Lorentz microscopy clarified small magnetic domains whose size corresponds to that of APDs. The complex magnetic domain structure was interpreted by the change in the local spin order being induced by the modulation of Fe-O-Fe bonding in the antiphase boundary region. Eventually the good functionalities are discussed in terms of the small magnetic domains clarified by electron holography.
[1] D. Shindo and Y. Murakami, J. Phys. D 41, 183002 (2008).
[2] T. Tanigaki, Y. Inada, S. Aizawa, T. Suzuki, H. S. Park, T. Matsuda, A. Taniyama,
D. Shindo, A. Tonomura, Appl. Phys. Lett., 101, 043101 (2012).
[3] K. Goto, T. Kanki, T. Kawai, and H. Tanaka, Nano Lett., 10, 2772 (2010).

Tuning the ferromagnetism of LaCoO₃ by doping bulk samples with smaller Sr atoms or straining thin film sample have been previously shown experimentally. In this work, we will use atomic-resolution Z-contrast imaging, annular bright field (ABF) imaging and electron energy-loss spectroscopy (EELS) in the aberration-corrected JEOL JEM-ARM200CF in combination with in-situ cooling experiments to examine the magnetic and spin-state transitions in La_(1-x)Sr_xCoO₃ (x=0-0.3) at liquid nitrogen and room temperature. Using energy-loss magnetic circular dichroism method, we confirm the magnetic ordering transition at room temperature with increasing doping concentrations. A magnetic transition is observed in 5% doped sample at liquid temperature. EELS mapping is, also utilized to determine the presence of nanoscale islands of different spin states using both the Co L₃/L₂ ratio and Oxygen prepeak intensities. Additionally, with increasing doping concentration, a change in crystal structure is measured using ABF imaging, more specifically areas with different distortions of the CoO₆ octahedral within the same doping concentrations and change in distortion in different Sr doping concentrations.
Acknowledgment:
The authors acknowledge funding from the National Science Foundation [DMR-0846784] and DMR-0959470 for the acquisition of the UIC JEOL JEM-ARM200CF.

The use of magnetoresistive thin films based on layers with perpendicular magnetic anisotropy (PMA) is very interesting for many applications such as high density magnetic recording media, MRAM and magnetic logic. Such films can exhibit good thermal stability in nanoscale dimensions and it is easier to define an easy axis than the case for thin films with in-plane anisotropy. Basic magnetic characterization for these thin films includes measuring hysteresis loops by using magnetometry such as vibrating sample magnetometry (VSM) and superconductor quantum interference device (SQUID), or using extraordinary Hall Effect (EHE). However, these techniques can only measure average magnetization behavior. It is hard to image the magnetic domain reversal directly. Magneto-optical Kerr effect (MOKE) microscopy and magnetic force microscopy (MFM) are the two most used tools to image the magnetic domains of these thin films. The lateral resolution for MOKE is limited to micron-size, too low to image magnetic domains on the nanoscale. MFM has high resolution, and can easily image nanoscale magnetic domains. However, most MFM images are taken in zero magnetic field and cannot image magnetic domain structure during magnetization reversal.
In this work, we present non-contact MFM images taken in magnetic fields on Co/Pd-based pseudo spin valves. The multilayer sample has the following composition: Ta 5/Pd 5/(Co 0.2/Pd 1)8/Co 0.4/Cu 2.9/Co 0.4/(Pd 1/Co 0.4)3/Pd 5 (nm). All layers were grown on thermally oxidized Si wafers at 300 K by DC-magnetron sputtering in a shamrock sputtering tool. The initial magnetization curve and hysteresis loop with applied magnetic field perpendicular to the films was first measured by SQUID at 300 K. It clearly shows two step sharp switching, which indicate good PMA of our multilayers. The multilayers with thicker Co layers (0.4 nm) which lie above the Cu spacer layer have smaller coercivity. The non-contact MFM images were taken on a NanoScan hr-MFM in a constant average height mode. The height was regulated using the electrostatic force due to a DC tip-sample bias. The MFM images were taken in increasing magnetic fields, in order to elucidate the domain structure during the magnetization reversal. Starting at 0 mT, MFM images of the as-grown state reveal domains of ~1000 nm in size. These domains shrink with applied field following the initial curve until the top layer is saturated at a field of 42 mT. Taking magnetization ratios from each image, the series of MFM images matches the hysteresis loop as measured by SQUID with an offset of 10 mT which can be accounted for the additional field applied by the tip itself (the images were taken without removing the magnetic field). Once on the hysteresis loop, the domain structure was imaged at the reversal points and nicely matches the SQUID measurements. The ability to image domain reversals at such high resolution demonstrates a complementary tool for pseudo spin-valve optimization.

Perpendicular magnetic recording (PMR) media is a technology to record data on hard disk drives with large data storage density. PMR media can store any analog information, for example, text, image or sound by converting it to digital data which is composed of either 0 or 1 as a unit of bit. Each bit is then recorded on PMR media by changing its magnetization direction either out of or into the magnetic film. Thus far, the data storage density of PMR media has been improved by decreasing the size of a bit. However, the bit size has now reached its limit and cannot be further scaled down due to superparamagnetism in which bits are no longer stable and stored data is lost. This is due to the thermal energy which can randomly flip magnetization direction of a nanometer scale bit. In order to overcome this superparamagnetism issue, a thorough understanding of the relationship between magnetic and structural properties is required. However, magnetic imaging techniques such as magnetic force microscopy, resonant x-ray holography, or neutron scattering can only image the magnetic information of the material. Herein, we present a novel technique using transmission electron microscopy (TEM) to simultaneously obtain the magnetic information as well as the structural information of the PMR media in nanometer scale.
In this study, we use Cs-corrected Lorentz TEM to directly image the magnetic domains and magnetic grains concurrently. Fresnel Lorentz TEM (FLTEM) and electron holography are both used to observe the magnetic profile of the PMR media recorded with repeating bits less than 100nm in width. In conventional FLTEM, electrons from the gun are deflected by the Lorentz force of the magnetic field perpendicular to the electron path. In defocus, this deflection forms bright and dark contrasts at the location of magnetic domain walls. Interestingly, when magnetization direction of PMR media is parallel to that of electrons from the gun whole domain appears in the image with either bright or dark contrast in large defocus. With electron holography, PMR media is viewed from the side to reveal the magnetic profile extending outside the top surface of the magnetic layer. This technique also utilizes the deflection of the electron beam due to the magnetic field of the specimen which is imaged as interference fringes. This allows the extraction of a phase image. In addition to magnetic domain images of FLTEM and electron holography, the granular structure of the PMR media is imaged in Lorentz TEM in which the objective lens is off. This allows a magnetic-field-free environment for the magnetized specimen inside the TEM while imaging. Hence, correlation between magnetic domains and granular structures is revealed by comparing FLTEM and electron holography with Lorentz TEM images. This direct imaging method of PMR media allows better understanding of the bits in nanometer scale can be widely applied to study magnetic structural relationships in nanometer scale.

Functional magnetic materials have garnered interest due to the potential applications in the emerging field of spintronics. More specifically magnetic vortices have been widely studied due to the quasi-particle/topological nature and are described as having two degrees of freedom, polarity and chirality. While there has been several studies exploring the fascinating behavior of vortices under perturbations using electrical measurements, scanning probe microscopy, and synchrotron based x-ray techniques, transmission electron microscopy provides unparalleled high resolution magnetic and structural information. This allows for a detailed analysis on how the local structure of different materials affects the translation motion of a vortex core under perturbation. Here, we present a direct imaging study of magnetically soft and hard mesoscopic discs of Permalloy and Cobalt under external field perturbations from equilibrium through annihilation and nucleation. The high resolution magnetic imaging of sub 6nm in Lorentz mode affords detail below critical domain wall features of less than 20nm where the soliton-regime breaks down into transverse Bloch walls far from equilibrium. By holding the ferromagnetic system in an external field we can capture the magnetic configuration far from equilibrium states near annihilation and during the nucleation process. Integrating these experiment with micromagnetic simulations we will describe how differently magnetic vortices can behave under perturbations, which is important when attempting to use them in applications such as logic, memories, and antennas.
Work supported by the DOE-BES-MSE,under Contract number DE-AC02-98CH10886 .

Nanoparticle magnets are a novel class of materials, in which a lattice of magnetic ions is replaced by a meta-lattice of magnetic nanoparticles. Inter-particle exchange interactions are absent, while dipolar interactions dominate. As a result, nanoparticle magnets behave differently from conventional magnets and their properties may be controlled and tuned by selecting the spacing and compositions of the constituent nanoparticles. We have studied dipolar interactions in self-assembled Cobalt nanoparticle magnets using off-axis electron holography in the transmission electron microscope. The technique enables the orientation and magnitude of the magnetic moment of each nanoparticle in an array to be determined and correlated with the structural properties of the meta-lattice, including the degree of order of the particles and their size distribution. Our study reveals that dipolar interactions are sufficiently strong to support long-range ferromagnetic order, even when the lattice of nanoparticles is highly disordered. This observation supports the possibility of creating amorphous dipolar magnets, in contrast to the expectation that a disordered dipolar system necessarily implies spin-glass behavior.
Chain-like assemblies of 15 nm ε-Co particles were prepared with an oleic acid coating on a carbon substrate (with no external magnetic field applied). For chains that are wider than 1 particle across, the particles are typically assembled into triangular (close-packed) lattices, although square lattice arrangements are also occasionally seen. We used off-axis electron holography to map the projected magnetic fields of several elongated nanoparticle assemblies non-invasively with a nominal spatial resolution of 6.3 nm. The data set was acquired at remanence after applying an off-plane field of 2 T to the specimen, and reveals the magnetic moment topography of each chain directly.
In order to quantify dipolar ferromagnetic order, we estimated the magnitude and orientation of the magnetic moment of each individual particle. The measurements were correlated with the geometrical arrangements of the particles. For each pair of particles, we measured the spatial separation r between their centers and the angular difference Δtheta; between their moments, from which magnetic and lattice order parameters were determined. Our results show that short-range magnetic order with small domains dominates the initial states, with the local magnetic order (ferromagnetic or antiferromagnetic) often depending on the particle lattice (triangular or square, respectively).
In contrast, at remanence after saturation, overall dipolar ferromagnetic order persists even in case of a non-triangular lattice.
We interpret our results as supporting the existence of amorphous dipolar ferromagnets: i.e., dipolar ferromagnetism in elongated nanoparticle assemblies even in the absence of underlying crystallinity.

Thermal transport driven by the quantized spins of electrons opens up a new realm of engineering possibilites centered around magnetic materials and processes. In addition, the ability to control these thermal processes via magnetic fields opens the door to controlling thermal properties on the nanoscale. To address this, we report on the magnetically-controlled thermal conductivity of a magnetic tunnel junction (MTJ). We synthesize the magnetic tunnel junction through deposition of a layered 7 nm Ru/3 nm CFA/2.3 nm MgO/5 nm CFA/15 nm InMn/30 nm Ru on a MgO substrate. This structure has shown nearly a factor of 4 change in electrical resistivity in the presence of a magnetic field. We measure the thermal conductivity of the MTJ using time domain thermoreflectance (TDTR); an optical pump-probe technique that has been well established to provide thermal conductivity measurements of thin films. We show that the thermal conductivity of the MTJ stack-structure can be controlled via the field strength and orientation using a rare-earth permanent magnet. The thermal conductivity exhibits a four-fold increase between its off and on states, which is attributed to the change in magnetoresistance of the MTJ due to spin-current channels.

We have used a combination of aberration-corrected Lorentz TEM and magnetic force microscopy (MFM) to carry out in-situ magnetizing experiments and explore the magnetization reversal behavior of a range of magnetic nanostructures. In all cases we are interested in the way in which the magnetic nanostructures interact, either through layering different materials within a single nanostructure or via interactions between separate nanostructures in an array. Examples will be presented for artificial spin ice arrays, which are nanoscale geometrically-engineered systems that display magnetic spin frustration. Square spin-ice lattices with island size 290 × 130 nm were fabricated using electron-beam lithography from 20 nm thick Py films deposited by sputtering on electron-transparent silicon nitride membranes. For closely-spaced lattices the process proceeds by a cascade of islands that reverse one at a time along the lattice diagonal as an applied magnetic field is reduced. As reversal occurs, so the magnetic structure at the nodes at which four islands meet changes. Of particular note are the node structures at either end of the line of reversed islands: these carry a local net magnetization and can be regarded as magnetic monopole defects. Examples will also be presented for patterned NiFe disks (single layer, trilayer and exchange-biased) in which interlayer interactions control both the magnetic structure that forms and also the way in which magnetization reversal occurs.
Work carried out at Argonne National Laboratory, a US DOE Science Lab operated under contract DE-AC02-06CH11357 by UChicago Argonne, LLC. We acknowledge use of the Center for Nanoscale Materials at ANL, and MDG acknowledges DOE-BES for partial support (DE-FG02-01ER45893).

There is great excitement in developing magnetic nanostructures for energy-efficient non-volatile memory and logic devices which rely on the movement of domain walls in nanostructured thin films. Patterning of thin films into 100 nm or less structures is essential for these applications to have switching energies and data densities competitive to that of field effect transistors. Additionally, low edge roughness is required for reproducibility of the magnetic switching characteristics, since edge roughness in the nanostructures can act as domain wall traps.
We report on patterning sub-100 nm ferromagnetic wires with very low edge roughness using a removable bilayer poly(methyl methacrylate) (PMMA) and hydrogen silsesquioxane (HSQ) resist mask. All patterning was done on silicon substrates with a native oxide. 20 nm-40 nm of polycrystalline Co60Fe20B20 was deposited using UHV DC magnetron sputter deposition. 2% PMMA in Anisole (a positive electron beam resist) and 2% HSQ in methyl isobutyl ketone (a negative electron beam resist) were spun on the CoFeB. The HSQ was exposed using a Raith 150 electron beam lithography tool at 10 kV electron energy and 400 mu;C/cm2 dose. After development, an O2 reactive ion etch (RIE) was used to remove the PMMA except under the HSQ, resulting in a bilayer removable mask. The RIE power and time are specified to the wire width. The CoFeB was ion milled using Ar ion etching at base pressure 2e-7 Torr with 10 mA beam current. After etching the pattern, the PMMA/HSQ mask was removed by NMP along with sonication. Patterned 30 nm, 50 nm, 75 nm and 100 nm wide lines were made using this method. SEM imaging gives a low average edge roughness, less than 4% of the wire width. The magnetic properties were measured using magnetic force microscopy to identify domain structures. This demonstrates the practicality of making sub-100 nm width wires using this method, that liftoff of the resist mask after etching is possible, and that these wire widths have low edge roughness to reduce pinning of domain walls in the magnetic structures.

Topological insulators have attracted great interest because of their interesting properties such as possible spin current on edge states, and been considered to be a promising candidate in future applications for spintronic devices. As three-dimensional topological insulators, Bi_1-xSb_x, Bi₂Se₃, and Sb₂Te₃ compounds have been well known. In this study, we theoretically investigate the electronic band structure of Sb₂Te, which is known as a semimetal, aiming to search topologically insulating phases controlled by external strain. Our calculations are based on the density functional theory within the local density approximation using the all-electron FLAPW method including the spin-orbit interactions. At first we study the structural stability of layer stacking sequence in Sb₂Te. Within the experimentally reported space group P-31m, that includes inversion symmetry, there are four crystallographically possible layer stackings. Calculated results show that the lowest-energy structure of Sb₂Te has two Sb₂ bilayers between Sb₂Te₃ quintuple layers, and calculated band structure is semimetallic with the band overlap of about 0.15eV. We calculate the Z₂ topological invariant for Sb₂Te by checking the parity at time-reversal invariant momenta. Calculated Z₂ invariant (1;000) indicates that Sb₂Te can be a strong topological insulator if the semimetallic band overlap is lifted. To search a topologically insulating phase for Sb₂Te, we study strain effects on the band structure by controlling lattice constants. Our calculation shows that an in-plane strain expanding in the layer direction can induce a band gap, and as result a strain-induced topologically insulating phase for Sb₂Te is actually realized. We generally discuss effects of strain and layer structure on the electronic structure of Sb₂Te, focusing on role of the spin-orbit interaction.

The meticulous ac susceptibility measurement and high-resolution X-ray diffraction indagation commingled with isothermal magnetic entropy measurements from 1 K to 300 K are employed on intermetallic pseudo-binary enormous magnetocaloric lanthanide compounds Er₅Si_3.5Ge_0.5 to adumbrate aboratively its hypostatic magnetic properties, crystalline configurations, phase transform silhouette, and lapidarian optimal mechanical idiosyncrasy, which applied to enamoredly crank out the innocuously environmentalistic, extirpating compressor, high efficient, and oecumenically retrenching-energy magnetic refrigerator applying at aeronautik voyage, sputnik, and industry automatics at our experiments. It is praiseworthy that the real part of ac susceptibility of specimen chi;_ac' exhibits a precipitous pinnacle at 28.0 K with the hyperplasia of the temperature when we inflicted an ac magnetic field of 5 Oe and frequency of 1000 Hz onto the magnetic specimen. The depicted inverse chi;_ac&’ function curve dependent on the temperature flaunts an approximatively linear modality notwithstanding that there appears a poignant abysm inflexion situating at 27.86 K and the 1/chi;_ac' function curve commences to proliferate with the decrease of the temperature below this abyss point. The ace-high fitted coefficients of the linearity equation A and B corresponding to this function curve are 379.95 and 19.1765, respectively. Furthermore, the Endsville fitted magnetic moment of Er atom from our experiment is 9.02 mu;_B and it is immaculatedly congruent with the theoretical value of 9.58 mu;_B. By the same token, the imaginary part of ac susceptibility chi;_ac&’&’ of the magnetic specimen manifests a spiculate aiguille locating at approximate 23 K, despite that its amplitude is just one-tenth of the real part of ac susceptibilitychi;_ac&’. Succeedingly we plume-lined the crystalline configuration of magnetic specimen exploiting the advanced X-ray diffraction technique utilizing a Mo target, with the maximum operating voltage and current of 60 kV and 200 mA, respectively, scanning from 8 to 50 degrees at the room temperature. It is worthy to mention that the most vehement aiguille appears at 15.7 degree with the intensity of 1404. The second and third intensive pinnacles situate at 14.7 and 17.4 degrees respectively, and their intensities are 1091 and 932, respectively. Our X-ray diffraction pattern trenchantly reveals that this highly-oriented colossal magnetocaloric lanthanide crystal possesses the monoclinic structure with the Shubnikov space group P112₁/a at room temperature. The exquisitely numerically fitted lattice parameters a b, c, and γ from our elaborate X-ray diffraction pattern are achieved as 7.3751#8491;, 14.415 #8491;, 7.5760 #8491;, and 92.947 degree, respectively. Legitimately, It is explicit that at 28 K the magnetic lanthanide compound undergoes the first-order magnetoelastic transition from a high-T monoclinic-paramagnetic-P112₁/a to a low-T orthorhombic-ferromagnetic-Pnma configuration.

The meticulous dc and ac susceptibility measurements from 1 K to 300 K are exploited on intermetallic pseudo-binary enormous magnetocaloric lanthanide compounds Dy₅Si_3.1Ge_0.9 to adumbrate aboratively its hypostatic magnetic properties, crystalline configurations, phase transform silhouette, and optimal mechanical proclivity, which applied to enamoredly crank out the innocuously environmentalistic, extirpating compressor, high efficient, and oecumenically retrenching-energy magnetic refrigerator at our experiments. It is explicit that dc susceptibility chi;_dc of specimen with diversified applied magnetic field amplitude H of 0.1, 1, and 10 kOe decrease vehemently with the prolification of H. Praiseworthy is that three curves exhibit one or two dramatically poignant aiguilles situating at the elysium of 1 to 120 K. The maximum piton values situate at 4 and 74, 72, and 22 K, respectively, corresponding to various dc magnetic amplitudes of 0.1, 1, and 10 kOe, respectively. Subsequently we plotted the delineation of inverse dc susceptibility 1/chi;_dc dependent on the temperature. Noteworthy is that the curve flaunts an approximately linear relation with a variegate slope at T=73 and 100 K when H=0.1 KOe. The ace-high fitted coefficents of linear equation y = A+Bx are A1 = -120.32, B1 = 2.126, A2 = 1222.34, B2 = 15.137 corresponding to 73 Kle;Tle;100 K and Tge;100 K, respectively. The optimized fitted magnetic moment of Dy atom is 10.11 u_B, rippingly congruous with the theoretical value of 10.65 u_B. Furthermore, it is trenchant that the inverse dc susceptibility 1/chi;_dc at H = 1 KOe displays three segments of linear modality with miscellaneous slopes at T = 5, 79, and 102 K, respectively. The topgallant fitted coefficents of linear equation y = A+Bx are A1= -649.03, B1= 9.233, A2 = -1036.49, B2 = 14.106 corresponding to 79 Kle;Tle;102 K and Tge;102 K, respectively. The ace-high fitted magnetic moment of Dy atom is 10.47 u_B, consummately congruous with the theoretical value of 10.65 u_B. At H = 1kOe, the 1/chi;_dc flaunts a more saponaceous linearity behavior with disparate slopes at T = 82 K. The Endsville fitted coefficents of linear equation y = A+Bx are A1= -993.956, B1=13.905 when Tge;100 K. The best fitted magnetic moment of Dy atom is 10.54 u_B, consummately congruous with the theoretical value of 10.65 u_B. The exquisite measurement of real part of ac susceptibility chi;_ac &’ versus to the temperature exhibits two precipitous pinnacles situating at 81 K and 102.3 K, respectively. Furthermore, the imaginary part of ac susceptibility chi;_ac&’&’ versus temperature flaunts one abyss at T = 81 K and an aiguille at T = 102 K, respectively, immaculately consilient with the results of chi;_ac &’ instance. Synoptically, the Dy₅Si_3.1Ge_0.9 compound undergoes the first-order magnetoelastic transition from a high-T monoclinic-paramagnetic-P112₁/a to a low-T orthorhombic-ferromagnetic-Pnma configurations at T= 102 K and a second magnetic transition occurs at approximately 82 K and 5 K, respectively.

Magnetic nanoparticles such as transition metal ferrites have been widely used in sensors and magnetic resonance imaging. The large magnetostriction coefficient and high Curie temperature of CoFe2O4 make it excellent candidate for creating ferromagnetic order at the nanoscale, and provide a pathway to fabricate nanocomposites. Barium titanate (BaTiO3), on the other hand, is well known for its excellent dielectric behavior and ferroelectric properties. In this work, a series of particulate, 0-3 type, nanocomposite films composed of ferromagnetic cobalt ferrite and ferroelectric barium titanate nanocrystals are prepared by various methods. The dielectric, ferroelectric and magnetic properties of such films are systematically investigated. Aiming at establishing and enhancing magnetoelectric coupling effect between the two phases, surface modification of the nanoparticles and various film fabrication methods will be performed. The structure and properties of the nanocomposite films will be characterized using XRD, TEM, STEM, SEM, MPMS and LCR meter.

Ferrite materials are of enormous technological importance, as these materials are used as recording media for hard disks, magnetic storage, microwave, sensing devices etc. In order to meet the current technological demands, further improvement in the magnetic properties of ferrites is essential. Improvement in magnetic and dielectric properties has been reported when ferrites are used in two component composite system. Pure phase exchange coupled nanocomposites of hard-soft magnetic oxides, (1-x)SrFe₁₂O₁₉ - x. Wt.% La_0.7Sr_0.3MnO₃ were prepared via autocombution method. The structural and morphological characterizations were performed via x-ray diffraction (XRD) and transmission electron microscopy (TEM). Magnetic properties of the nanocomposites were assessed as a function of soft-phase content in the nanocomposite. XRD analysis shows presence of pure phase components in the nanocomposites. TEM images show presence of needle shape Sr-Ferrite particles in close contact with La_0.7Sr_0.3MnO₃ particles. Room temperature smooth hysteresis loops of the nancomposites clearly demonstrates efficacy of auto-combustion method in producing well exchange-coupled nanocomposites. Furthermore, a linear increase in Mr/Ms with the increase in the soft-phase content up to x=40% was observed. This indicates the presence of enhanced exchange coupling between hard and soft phases of the nanocomposite. The highest Mr/Ms ratio of 0.592 was obtained for nanocomposite containing 40 Wt.% of the soft-phase. Concomitantly enhancement in coercieve field (Hc) was also observed with the increase in the soft-phase content of the nanocomposite reaching to a value of 6.59 kOe which is 80% higher than that of SrFe₁₂O₁₉ (Hc~3.633 kOe) and 1156% higher than that of La_0.7Sr_0.3MnO₃ (Hc ~523 Oe). Thus observed magnetic parameters, Mr/Ms and Hc, of the nanocomposites are far superior to the corresponding values of the individual components of the nanocomposite. The adopted synthesis method is low cost, rapid, and results in pure nanocomposite powder. This simple method seems a promising way to tailor and enhance the magnetic properties oxide based hard-soft magnetic nanocomposites.

Systems with a single or several coupled electron spins are commonly described with the quantum approach while ferromagnetic domains with millions of coupled spins are classical systems. Large spin clusters and superparamagnetic nanoparticles contain hundreds of coupled electron spins, and are on the boundary between classical and quantum behavior. Electron magnetic resonance (EMR) observed in ultra-fine iron oxide nanoparticles (~ 5 nm size) reveals several features which are typical for paramagnetic spins and absent in macroscopic systems, including multiple quantum transitions observed at Hr/ m, where m = 2, 3, 4 and Hr is the field of the main resonance. In order to better understand the transition from quantum to classical behavior and magnetization dynamics at the nanoscale, we study the evolution of the EMR signal with increase of the particle size in suspensions of magnetite nanoparticles. We also test the effects of nanoparticle mutual arrangements and use of a different kind of magnetic material, such as mumetal and permalloy. The experimental data are compared with numerical simulations based on the quantum and classical approach.

In recent years, self-assembled nanocomposite thin films such as BaTiO₃-CoFe₂O₄, BiFeO₃-CoFe₂O₄ (BFO-CFO) and BiFeO₃-NiFe₂O₄ in which a ferrimagnetic spinel phase grows epitaxially as pillars within an immiscible ferroelectric perovskite phase have attracted great interest as new multiferroic materials. However, to date these composites have been exclusively grown on single crystal oxide substrates which limits their utility in microelectronic devices. In this study we describe the integration of a spinel-perovskite epitaxial nanocomposite thin film on a Si substrate by using a Sr(Ti,Fe)O₃ (STF)/CeO₂ (Ceria)/YSZ (yttrium-stabilized zirconia) thin film as a buffer layer, and the tuning of its magnetic properties. We have found previously that thin films of STF with x = 0.1 ~ 0.5 exhibit room-temperature magnetism and a strong out-of-plane anisotropy when grown epitaxially on ceria/YSZ-buffered (001) Si substrates.[1-2]
From the x-ray diffraction, SEM analysis and by removing the BFO with HCl etching, it was observed that the CFO nano-pillars formed as epitaxial pillars within a BFO matrix on STF thin film. The magnetic hysteresis loop of the nanocomposite on STF thin film shows a sum of the STF thin film hysteresis and that of the CFO pillars. The STF was magnetic even though the BFO-CFO was fabricated in 5 mTorr of oxygen, despite prior evidence that STF films grown in oxygen had very low saturation magnetization.[1] The nanocomposite on STF shows a strong out-of-plane anisotropy as a result of both the shape anisotropy of the pillars and the dominant magnetoelastic anisotropy of the CFO and STF films which are under out-of-plane compression as a result of epitaxy with the BFO matrix and CeO₂ buffer layer respectively. Composition modulation of the CFO by layering it with other spinels will also be described. Results from modulated composites grown on STF/ceria/YSZ/Si enable both control of magnetic properties and integration on Si substrate of perovskite and spinel nanocomposites.
References
1. D. H. Kim, L. Bi, P. Jiang, G. F. Dionne, and C. A. Ross, Phys. Rev. B, 84, 014416 (2011).
2. D. H. Kim, L. Bi, N. M. Aimon, P. Jiang, G. F. Dionne, and C. A. Ross, ACS Combinatorial Science, 14, 179 (2012).

The remarkable interplay of chirality and magnetism in
helical single-wall nanotubes of iron (FeSWNTs) is investigated using fully
unconstrained spin-density-functional calculations. Spin-spiral waves exist
and noncollinear helimagnetism appears only for the specific chirality of
(6,3) and (5,3) FeSWNTs, whereas collinear ferromagnetism persists in
other chiral FeSWNTs as unfolded monolayers, that is, chirality selectively
involves the unusual helimagnetic phase transition (chiral selectivity). The
emergence of quantum helimagnetism plays a variety of significant roles in
(i) the stabilization of the chiral FeSWNTs as a long-lived “magic”
structure in both freestanding and tip-suspended conditions, (ii)
interference with quantum ballistic conductance by interband repulsion,
and (iii) the involvement of chiral conductivity in which electric currents
pass helically through the FeSWNTs. These chiral characteristics are a
novel addition to the intriguing rich diversity of chirality-driven physics and phenomena.

The possible origin of dilute ferromagnetism and associated magnetoelectric coupling in deficient PbTiO3 are studied using first-principles calculations based on the screened hybrid Hartree-Fock density functional, which has successfully reproduced the band gap of PbTiO3 and the localized/delocalized defect electronic states that predominates vacancy-driven properties. We found oxygen vacancies are likely to form at surfaces and grain boundaries. Such oxygen vacancies clustered at surfaces or grain boundaries are revealed to induce ferromagnetism, whereas oxygen vacancies inside a PbTiO3 single crystal are just non-magnetic. We also demonstrate that magnetism of the oxygen vacancies at surfaces and grain boundaries shows remarkable polarization-dependence: The emerged ferromagnetism undergoes the intriguing rich transitions of the ferromagnetic-antiferromagnetic-nonmagnetic phases depending on the spontaneous polarization directions switched by external electric fields. This signifies that the magnetoelectric effect can exist in deficient PbTiO3, which is, in addition, expected to be a nonlinear coupling due to the rapid phase transition in conjunction with the local polarization switching. Our findings can provide fundamental insights for designing magnetoelectric multiferroic materials in conventional nonmagnetic ferroelectrics.

Magnetoelectric Multiferroic materials showing coupled magnetic and electrical order parameters through strain have drawn increasing interests due to their unique physical properties and widespread technological applications in magnetic/ferroelectric data storage media, spin based and magnetocapacitive device, magnetic sensors, and nonvolatile memories, etc and also rich in fundamental physics. Here, we report Raman spectroscopic studies, and magneto-dielectric properties of a multiferroic composite ceramics, (1-x) Pb(Fe_0.5Nb_0.5)O₃ - x Ni_0.65Zn_0.35Fe₂O₄ ( x =0.2). Pb(Fe_0.5Nb_0.5)O₃ (PFN) is known to be a single phase multiferroic material having good ferroelectric properties, weak magnetic properties and shows very small magnetoelectric coupling around 151 K. Ni_0.65Zn_0.35Fe₂O₄ (NZFO) compound shows good magnetic properties around room temperature with good magnetostrictive properties. The X-ray diffraction patterns disclosed the presence of both PFN and NZFO binary phases in theses composites without any secondary phases. Detailed analysis of Raman spectroscopic studies revealed that apart from the presence of the zone centre Raman active modes of the parent compounds, some new peaks are observed in the low (around 30 to 50 cm^-1) frequency region. The electric field controlled peak position of the low frequency Raman modes suggests these modes are of magnon in origin. The capacitance and tangent loss in this sample at room temperature decreases considerably with increasing magnetic field whereas the impedance and phase increase systematically with increasing magnetic field. The electric field control of magnetic properties at room temperature in the sample shows a considerable and systematic change of coercive field (Hc), remanent magnetization (Mr) and saturation magnetization (Ms).Our above mentioned results suggest that this composite shows very good magneto-dielectric coupling at room temperature which can be useful for potential multifunctional device applications.

Three and four alternate layers of Bi_3.4La_0.6Ti₃O₁₂/CoFe₂O₄ were synthesized by chemical solution method and deposited by spin coating on Pt (Pt/TiO₂/SiO₂/Si) substrate. The films were characterized by X-ray diffraction, SEM, dielectric response, leakage current, and ferroelectric and magnetic responses. X-ray diffraction revealed composite-like multilayer structure. SEM showed polycrystalline films grain sizes ~ 100 nm. Both composite structures show exponential decrease in dielectric constant with increasing frequency from 10³ Hz to 10⁶ Hz as typical in dielectrics. Low leakage currents (10^-7 to 10^-6 A) is observed with applied voltage from 5 to 8 V followed by the breakdown and space charge limited current. The composite films were characterized for ferroelectric and ferromagnetic responses. The ferroelectric polarization Pr = 30 µC/cm² and magnetization Mr = 150 emu/cm³ were observed for the composite film structure. One can compare Pr = 85 µC/cm² for individual Bi_3.4La_0.6Ti₃O₁₂ and Mr = 230 emu/cm³ for CoFe₂O₄ film on Pt substrate. Since there is no clustering in film deposition, the observed co-existence of ferromagnetic and ferroelectric coupling is the intrinsic property of the composite

The Ni/NiO nanoparticles studied in this work exhibit sub-nanostructured features, with crystallites with average diameters below 6 nm for Ni and 2 nm for NiO. These nanoparticles were obtained by mechanical alloying followed by Al leaching and passivation. The particles thus obtained have a core-shell structure with a Ni core and a NiO shell. This production process induces porosity in the particles. The powder samples were characterized by XRD, SEM-EDS, TEM, and ferromagnetic resonance (FMR) at 300 K. The FMR experiments provide information about the core and shell effects of magnetic nanoparticles. The Ni-NiO FMR spectrum showed that microwave absorption is associated with two atomic sites that differ structurally and magnetically: one with g core = 3.2855 at H = 2055G and one with g shell = 2.1605 at H = 3125 G. Using the values of g obtained by FMR spectroscopy, it was possible to determine the ratios between the orbital magnetic and spin moments, which were (µL/µS)core = 0.64 and (µL/µS)shell = 0.08. Additionally, the relationship between the orbital magnetic moments, µL-shell = 0.027 µL-core and µs-shell = 0.324 µs-core, were determined, as were the relaxation times (T2) for each region.

Single phase Multiferroic materials have attracted much attention due to their two or more ferroics order parameters (ferroelectric, ferromagnetic, ferroelastic and ferrotoroidic) and their coupling, which are the potential candidates for actuators, switches, magnetic field sensors, and new types of microelectronic memory devices based on magnetic order switching by electric field and vice versa. BiFeO₃ (BFO) is a known room temperature multiferroic material which exhibits ferroelectricity and antiferromagnetic ordering at room temperature with high leakage current. To reduce the leakage current and improve electrical, magnetic, and multiferroic properties, we introduced small fraction of Nd and Mn at A and B-site of BFO. X-ray diffraction (XRD) patterns revealed the formation of perovskite (Bi_0.95Nd_0.05)(Fe_0.95Mn_0.03)O₃ (BNFM) single phase with small (2%) trace of impurity, however Raman spectra suggest pure phase for local nanostructure. Temperature and frequency dependent dielectric studies confirmed the ferroelectric-paraelectric phase transition (Tc) at 630 K with dielectric constant 290 at 1 kHz and relaxor type behavior, this result was confirmed by temperature dependent Raman studies. The complex impedance spectroscopic technique was used to investigate the role of grain and grain boundary effects in the electrical response of the system. Reduced leakage current was obtained, it also showed negative temperature coefficient of resistance (NTCR) behavior with temperature. The above material shows non-saturating magnetic hysteresis till 7 T, with remnant magnetization ~ 0.03 emu/g and coercive field of 0.4 T. Zero field cool (ZFC) and field cool (FC) data at various bias magnetic fields suggest the spins are frustrated. Two competing forces , namely anti/ferromagnetism and superparamagnetism displayed two prominent magnetic phase transitions at low temperatures, the detailed observation will be discussed. High magnetoelectric ME coupling (120 mu;V/cm.Oe) was observed at zero bias magnetic field which reduces with increase in the applied dc magnetic field.

To downsize and integrate magnetic and microwave components it is needed to make thin films of complicated stoichiometries magnetic materials. Here, we report the epitaxial growth of high quality Ba₂Co₂Fe₁₂O₂₂ thin films at the atomic scale by using alternating target laser ablation method, in which three targets of BaFe₄O₇, CoFe₂O₄ and Fe₂O₃ were used for sequential deposition to localize the ions in the proper crystal structure. Hexagonal Ba₂Co₂Fe₁₂O₂₂ thin film was grown on sapphire (0001). A KrF excimer laser with a wavelength of 248nm, energy of 400mJ/pulse and repetition rate of 10Hz was used and the distance between the target and the substrate was 5cm. The substrate was heated to 910°C in an oxygen pressure of 300mTorr. The four step deposition routine consisted of 12 laser pulse shots on the Fe₂O₃, 16 shots on the CoFe₂O₄, 12 shots on the Fe₂O₃ and finally 8 shots on BaFe₄O₇ targets. The growth under the above conditions was approximately 43.8 #8491;/cycle. A post annealing process at 1050C for 30min in air reinforced single phase of the film and improved the magnetic properties. The composition and structure of these films were determined by X-ray diffractometer (XRD) and scanning electron microscope (SEM) which show that the c- axis alignment falls within 0.625° of the normal to the film plane and the films are reasonably well ordered. The magnetic properties were determined by vibrating sample magnetometer (VSM) and ferromagnetic resonance (FMR). The saturation magnetization was measured to be 2520G, the magnetic uniaxial anisotropy field was estimated to be 13.2kOe and g=1.52. The magnetic and structural properties are in agreement with bulk parameters.

Magnetoelectric effect at room temperature can open up new opportunities in miniaturizing microwave devices. Here, we report an M-type magnetoelectric (ME) thin film which is grown on the oxide conductive layer of ITO by pulsed laser deposition. A single target of magnetoelectric material SrCo₂Ti₂Fe₈O₁₉ is prepared by conventional solid state method. The ITO/ SrCo₂Ti₂Fe₈O₁₉ multilayer structure is deposited on a sapphire (0001) substrate using a KrF eximer laser with a wavelength of 248 nm, energy of 400 mJ/pulse and 10 Hz repetition rate. During ITO deposition the substrate was heated up to 400°C in an oxygen atmosphere with pressure of 10mTorr for 10 min, which resulted in 400 nm thickness of ITO layer and immediately after that the substrate temperature raised to 600°C in 200 mTorr of oxygen pressure and the laser was set to impinge on SrCo₂Ti₂Fe₈O₁₉ target for 50 min which resulted in an amorphous film structure. After deposition, the films were annealed in oxygen atmosphere in a tube furnace at 1050°C for 40 min. The ferrite film thickness was measured to be 0.7 µm. These thin films were characterized by vibrating sample magnetometer (VSM), ferromagnetic resonance (FMR), X-ray diffractometer (XRD), scanning electron microscope (SEM) and energy-dispersive spectroscopy (EDS). The g-factor was calculated to be 2.66, the saturation magnetization (4πM_s) was measured to be 1250 G and the FMR linewidth was 1000 Oe. The magnetoelectric measurements were performed by measuring the changes in magnetization with the application of a DC voltage which shows that the required voltage in order to observe the effect was substantially smaller than the voltage required for bulk materials with same composition. The deduced ME coupling coefficient, α, for SrCo₂Ti₂Fe₈O₁₉ thin film was 6.07×10^-9 sm^-1.

Synthesis and self-assembly of ferromagnetic/ferrimagnetic nanoparticles are important for magnetic data storage application in hard disk drive and magnetic tape. In order to get high density data storage, uniform size and shape of the nano-building blocks are needed. As an important ferrimagnetic material, cobalt ferrite (CoFe2O4) shows very high magnetic anisotropy and coercivity, which is a desirable system for this application. Here I will discuss the solution-based synthesis of uniform cobalt ferrite nanocubes. The magnetic properties can be easily tuned by controlling the size of the cube and composition of Co in cobalt ferrite. Using the self-assembly process, thin film from the uniform nanocubes is fabricated as the magnetic recording medium for the data storage demonstration.

Magnetically hard FePt nanoparticles have been attractive in the past few years because of their potential application in high density storage media and permanent magnets. In this work, a new green chemical strategy for the direct synthesis of L1₀ FePt alloy nanoparticles is reported. The starting material is a polycrystalline molecular complex (Fe(H₂O)₆PtC₁₆), in which Fe and Pt atoms are arranged on alternating planes, like in the L1₀ structure. The starting compound was milled with crystalline NaCl to form nanocrystals. Then the mixture was annealed under reduction atmosphere (5 % H2 and 95% Ar) at 400°C for 2h with a heating rate of 5°C/min. After the reduction, the mixture was washed with water to remove the NaCl and L1₀ FePt nanoparticles were obtained without using organic solvents or metal additives. Transmission electron microscopy (TEM) images revealed that this method is able to produce L1₀ nanoparticles with different average size varying from 13.9 nm to 5.4 nm depending on the (Fe(H₂O)₆PtC₁₆)/NaCl ratio. The X-Ray Diffraction (XRD) pattern showed the presence of the characteristic peaks of the fct phase. The hysteresis loop, measured both at room temperature and 50 K, shows a high coercivity of 7.6 kOe and 11.2 kOe, respectively as expected for the high anisotropy L1₀ phase. This method has the following advantages with respect to other wet chemical synthesis methods of L1₀ FePt nanoparticles; it works at temperatures much lower than those reported in the literature and without metal additives, it does not use organic solvents and it leads to a highly ordered L1₀ phase.

It was widely recognized that the rare earth metals are critical components for many new technologies because of their extraordinary magnetic properties; for example, heavy rare earth elements of lanthanide series, such as Terbium (Tb) and Dysprosium (Dy), exhibit huge magnetic moments of 9.05 mu;B (Bohr magnetrons) and 10mu;B; compared with the conventional values of 0.6 for Ni and 2.2 for Fe. Meanwhile, the curie temperatures of rare earth-iron compounds increase with increasing rare earth concentration: the Curie temperature of amorphous TbFe2 is 388K while Terbium&’s Curie point is about 220K. Below the Curie temperature, the crystal lattice is elongated or compressed in the direction of magnetization if an external magnetic field is applied; asymmetrical lattice spacing is generated subsequently and this strain is determined by the orientation of the magnetic domains. The strain generated will get to its maximum value once all the magnetic moments become aligned with the applied field, thus the saturation magnetostriction has been reached and little strain change will be provided even more. TbFe2 could have a saturation field as high as 23KOe with a magnetostriction of 2520ppm while Tb0.3Dy0.7Fe1.92 has a saturaton field of 3.1KOe with a magnetostriction of 1800ppm (with load), both of these Rare earth-Fex components have been studied by our group and coupled with Piezo ceramic layer to form magnetoelectric sensor which is sensitive with tiny magnetic field while also has a large saturation field compared with commercialized magnetic sensors.
The Rare earth-Fex (TbFe2 or Tb0.3Dy0.7Fe1.92) layer will generate AC modulated strain if an AC magnetic field is applied, the strain will be coupled to the laminated Piezo ceramic layer (Pb(Zr, Ti)O3) and thus an AC electric charge signal will be generated because of the piezoelectricity of Pb(Zr, Ti)O3. A custom made charge amplifier is adopted for signal collection and both magnetoelectric coefficient and sensitivity were studied; the Tb0.3Dy0.7Fe1.92/ Pb(Zr, Ti)O3 sensor system presents an output as high as 84.7 mV under a charge amplifier gain of 2000pC/V while the TbFe2/ Pb(Zr, Ti)O3 sensor system has a high saturation field of 4.3KOe, this is the highest saturation point ever reported for Rare earth-Fex/Piezo coupling sensors.
A gradiometer structure is then formed with two Rare earth-Fex/Piezo coupling sensors with a baseline of 5.30 cm and used for tiny magnetic field gradient measurement. A differential output of 4.83mV is observed under a 0.0135Oe magnetic field gradient with a common mode magnetic AC signal of 1.233Oe: The common mode signal was cancelled while the gradient signal still could be measured. A noise density of 1.9*10-9 Tesla/ rt Hz at 10 Hz is also observed which proves that the gradiometer structure based Rare earth-Fex (RFex) magnetoelectric sensor system could be used for nano Tesla magnetic field gradient measurement within high common mode noise environment.

RE-TM alloys, like GdFe100Co alloys are example of the multi-sublattice magnets, where an important role is played by the exchange of angular momentum between non-equivalent sublattices. GdFeCo alloys are ferrimagnets, where the Fe and Gd sublattices are coupled antiferromagnetically, while the Co magnetic moments are parallel to those of iron.
In such systems, the time scale of the magnetization dynamics becomes dependent on the exchange interaction and the balance of the angular momentum between the sublattices [1]
The inequivalency of the magnetic sublattices, on the one hand, and a fine balance of their angular momenta on the other, lead to a very interesting dynamic behavior. This becomes especially obvious at short time scales, when even the exchange coupling could be overmastered by an efficient energy and angular-momentum exchange with the electronic system, leading to a transient ferromagnet-like state at time scales below a few picoseconds. This state is followed by an inter-sublattice relaxation of the angular momentum, leading to a very deterministic switching of the magnetization driven solely by ultrafast laser-induced heating [2].
Because there are certain controversies about interpretation of the experimental results [e.g. 2,3], the following results of the theoretical analysis will be presented:
10 - analysis of the magnetization dynamic based on the three thermodynamic subsystem: the spin system, the electronic system and the lattice. Analysis of the thermodynamic approximation, taking into account short time scale, will be presented; 20 - relaxation rates between these reservoirs, associated with the characteristic energies of the interactions that mediate the coupling between these reservoirs will be analyzed; 30 - influence of the helicity-dependent absorption in the RE-TM magnetic layer, caused by the circular magnetic dichroism on magnetization dynamics, especially laser induced femtomagnetism.
[1] C.D.Stanciu et al., Phys. Rev. B, 73, 220402(R); [2] A. Kirilyuk et al., Rep. Prog. Phys., 76, 026501; [3] D.Popova et al., Phys. Rev., B 84, 214421.

Recently, we reported the intrinsic DNA magnetism induced by a helical charge transport along the helical π-ways of dsDNAs. In the context, we used electron magnetic resonance (EMR) and superconducting quantum interference device (SQUID) magnetometer in order to detect the magnetic phenomena. In the EMR spectroscopy, we observed the extremely broad EMR peak of g>10 together with the relatively narrow one of g~2. The g>10 and g~2 were assigned as a cyclotron resonance (CR) and Zeeman magnetic transition, respectively. Here, the CR was suggested to be guided by the helical π-way of single dsDNA, which reminiscent of molecular solenoid. If such dsDNA solenoids were coherently interfered through an inductive coupling, and thus extended in the perpendicular direction to be a bundle of dsDNA solenoids in the well ordered regions of fibril structure of A-dsDNAs, a circular loop current might be formed in normal plane of the helical axis, resulting in the s-shaped magnetization (M)-magnetic field (H) curves at the magnetic field ranges of ±5000 G in SQUID measurements.
The scenario mentioned above is only possible when the helical π-ways are preserved well from molecular conformational to morphological levels. This implies that there is no CR peak in an A-dsDNAs with an amorphous phase alone. Therefore, whether the CR is or not is strongly depend on the presence or absence of the well ordered regions.
Here, we prepared various salmon DNAs under different concentrations of Na+ cation. And then, EMR (electron magntic resonance) and SQUID (superconducting quantum interference device) magnetizaton measurements were performed for them at room temperature. Especially, TEM (tunnelling electron microscope) analysis was imployed to characterize DNA's morphology. Interestingly, the DNA's magnetism has one-to-one correspondence to morphological variations.

There exist a number of compounds which consist of spin clusters with the icosahedral symmetry such as spin dodecahedra and spin icosahedra. However, spin-glass behaviours had been commonly observed in these compounds without an exception, and only recently antiferromagnetic long-range orders were found in Cd-based binary systems [1]. In this presentation, we report the first ferromagnetism at low temperatures in solids consisting of spin icosahedra, i.e., Au-based ternary compounds, which are described as bcc packing of rare-earth icosahedra. We will discuss the characteristic features of the ferromagnetism observed in the unique compounds. An attempt to increase the Curie temperature will be also addressed.
[1] Tamura et al., Phys. Rev. B 82, (2010) 220201(R).

MnBi considered as a replacement rare earth magnetic material due to its strong magnetization and coercive powers, but also its ability to retain its magnetization at elevated temperatures while related compositions suffer large diminishment. Extensive experimental studies on MnBi have shown that it is a strong ferromagnetic compound with a 2.0 T coercive force and an 4.6 MGOe energy product at 400K for the NiAs hexagonal phase[1, 2]. The magnetic anisotropies of MnBi and MnSb are similar in terms of spin-alignment, but the difference in the transition temperatures for spin alignment is large. As MnBi has a positive temperature coefficient for coercivity, a valuable trait for applications where magnetic power needs to be maintained at elevated temperatures. To investigate the origin of this temperature dependence, we have performed a series of first principles electronic structure calculation on the thermo magnetic properties of MnBi and MnSb within the local density approximation and plane wave basis using the ABINIT program system[3]. The total magnetization and local magnetic moment in easy direction (c-axis) of these two compounds have been calculated as a function of electronic temperature from 30K to 315K. Finite temperature effects on the electronic structure were examined by cold smearing and Fermi-Dirac population methods. To discern potential structural roles, three crystal forms common to the MnX series were investigated for compositions, MnBi and MnSb, the room temperature hexagonal NiAs structure, an elevated temperature orthorhombic MnP structure, and tetragonal zincblende structure. All three forms have been observed for MnAs by earlier investigators[4]. Our first principles calculation has shown good agreement with experimental structural and thermomagnetic results for MnBi and MnSb. Comparing results from the NiAs, MnP and zincblende-type structures, our results clearly show a structural role in the thermomagnetic behavior of these compounds, with marked temperature sensitivity for MnSb. Using thermal expansion effects derived from experimental measurement [5, 6], total magnetization of MnBi shows a small increase with larger cell volumes. Our results imply the possibility that the positive thermomagnetic property of MnBi is due to structural role and remnant even with easy-axis aligned spin moment where the magneto anisotropic spin reorientation does not occur.
References:
1. Ravindran, P., et al., Physical Review B, 1999. 59(24): p. 15680-15693.
2. Yang, J.B., et al., 2002. 14(25): p. 6509-6519.
3. Payne, M.C., et al., Reviews of Modern Physics, 1992. 64(4): p. 1045-1097.
4. Suzuki, T. and H. Ido, Journal of the Physical Society of Japan, 1982. 51(10): p. 3149-3156.
5. Roberts, B.W., Physical Review, 1956. 104(3): p. 607-616.
6. Willis, B.T.M. and H.P. Rooksby, Proceedings of the Physical Society of London Section B, 1954. 67(412): p. 290-296

With rapid growth of nanotechnology applications in medicine the ability to guide nanoparticle targeting and control nanoparticle assembly becomes an issue of great importance. Magnetic nanoparticles coated with gold or silver featuring a unique combination of biocompatibility and magnetic properties show a great potential to use a magnetic field to guide the assembling/disassembling process with a high precision We have synthesized by a thermally-activated processing technique magnetic nanoparticles (NP) with ferrite core doped with Mn and Zn, e.g., MnZn Ferrite (MZF). The MZF NPs, upon coating with Ag or Au shell (MZF/Au and MZF/Ag nanoparticles), exhibit novel functionalities. DC magnetization and ac susceptibility tests were performed using SQUID magnetometry. The Curie-Weiss paramagnetism was observed at higher temperature for MZF/Ag nanoparticles, whereas the magnetization was larger for MZF/Au nanoparticles. This difference was probably caused by interactions between Au or Ag shell and MZF core, which had alternated the nature of the magnetic exchange within the core/shell structure. X-ray absorption measurements will be employed to examine the local chemical environment around Fe, Mn, Zn atoms inside the magnetic core, around Au and Ag atoms at the core/shell interface, as well as in the shell of magnetic nanoparticles to study the atomic-scale structures responsible for induced spin polarization at core/shell interface in magnetic nanoparticles coated with noble metals.

Exchange spring magnets, formed by the exchange coupled nanograins of hard and soft magnetic phases, leads to improve magnetic properties. The soft phase, rich in transition metal, enhances saturation magnetization whereas the hard phase provides the required magnetic anisotropy and stabilizes the exchange coupled soft phase from demagnetization. In order to increase the maximum energy product, (BH)max and to lower the cost due to reduction of rare earth content, YCo5 can be taken as a potential candidate for the hard phase of nanoconposite.
Magnetic hard-soft composite of YCo_5-x% FeNi (x = 5, 10, 15) were prepared by high energy ball milling and subsequent vacuum annealing. Before the synthesis of these nanocomposites, the hard phase YCo₅ was prepared by arc melting. The structural properties of these composite YCo_5-x% FeNi (x = 5, 10, 15) were investigated by X-ray diffraction and magnetic measurements were done by Vibration sample magnetometer in the presence of 12KOe applied magnetic field. After 45m of milling, the powders were highly amorphous with specified amount of FeNi. Subsequent high vacuum annealing in the temperature range from 550°C - 750°C head to the reformation of crystalline sample with high magnetization around 104 emu/gm for 15% FeNi. Due to the annealing the sample were changed into magnetically soft phase showing low remanance and coercivity. The composite samples are seen to have low magnetocrystalline anisotropy along the basal plane. Among all the annealing temperature which is used to anneal the sample, 650°C is seen to be suitable to obtain good magnetic properties of the YCo_5-x% FeNi composites.

The interactions through the interface in nanoscale have been one of the target of primary research due to the tremendous fascinating physical properties. However, the researches on 0-dimentional transition metal oxides now are all focusing on core-shell nanoparticles/nanocrystals, which always show an irregular arrangement and are hard to control the structures well. In this study, monodispersed epitaxial core-shell oxide nanocrystals with one covering the other have been successfully created by utilizing the instable characteristics of bismuth-based complex ternary oxides. This new structure can possesses both the virtues of traditional core-shell nanostructures and the epitaxial supported nanostructures, showing the control of facet and interface of core-shell nanocrystals.
Here, we fabricated nanocrystals combined with rock-salt structure of antiferromagnetic CoO and spinel structure of ferrimagnetic Fe3O4, where we could manipulate one through the other easily. Our results show that the magnetic properties such as the magnetic anisotropy, the coercivity, and the magnetization are tunable and can be precisely controlled by the size, thickness, orientation, interface and the role of core or shell at room temperature. In addition, a large exchange bias and the direction- dependence of exchange coupling have been observed due to the strong magnetic interaction between the core and shell magnetic nanocrystals.
This approach can be expanded into all sorts of bismuth-containing oxide and then demonstrates different epitaxial core-shell metal oxide nanocomposite easily. Based on the novel structure and their possibility of convenient control of physical characteristics, this study provides us a new opportunity to understand the fundamental properties of nanoscopic metal oxides and the potential to design more desirable functional devices in the future.

We present measurements and micromagnetic simulations demonstrating spin wave mediated magnetic vortex core reversal with excitation and reversal times of less than 100 ps.
Essential progress in the understanding of nonlinear vortex dynamics was achieved when low-field vortex core reversal was found by (sub-GHz) excitation of the vortex gyromode with 4 ns excitation time [1]. The switching scheme involved, based on the creation and subsequent annihilation of a vortex-antivortex (VA) pair as suggested in [1] and investigated experimentally later [2], has been proven to be universal and in particular independent of the type of excitation, e.g., pulsed magnetic fields or spin transfer torque.
Vortex structures possess azimuthal spin wave modes which, compared to the gyromode, show much higher eigenfrequencies in the multi-GHz range. We demonstrated [3,4] by both, experiments and micromagnetic simulations, that unidirectional vortex core reversal also can be achieved by exciting these azimuthal spin wave modes with rotating GHz magnetic field bursts. VA pair creation and annihilation is also involved in this process [3,4].
Recently we demonstrated that spin wave mediated vortex core reversal can be realized with switching times below 100 ps. For that purpose we excited Permalloy discs in the vortex state, 0.5 mu;m in diameter and 40 nm thick, with orthogonal magnetic field pulses in x and y direction. Experimental and simulated phase diagrams will be shown. The magnetic vortex core could be successfully switched experimentally and by micromagnetic simulations with excitation times down to 70 ps. An additional delay of 20 - 30 ps for vortex core reversal will be discussed, caused by the time needed for transferring the homogeneously distributed excitation energy to the center of the vortex structure [4,5]. Even considering this phenomenon, vortex core reversal times below 100 ps have already been achieved.
Experiments have been performed by time-resolved scanning transmission X-ray microscopy at the MAXYMUS endstation at BESSY II, Berlin, which combines a time resolution of about 40 ps with a lateral resolution of about 25 nm. A sophisticated data acquisition enables operation in the standard multi-bunch mode of the synchrotron and thus allows us to take advantage of the about a magnitude higher ring current. In addition, a significant enhancement in the signal-to-noise ratio and a virtual elimination of low-frequency fluctuations and beam instabilities is obtained by a ‘lock-in&’ like technique and by performing a complete time-scan at one pixel before moving to the next one.
[1] B. Van Waeyenberge et al., Nature 444, 461 (2006)
[2] A. Vansteenkiste et al., Nature Physics 5, 332 (2009)
[3] M. Kammerer et al., Nature Communications 2, 279 (2011).
[4] M. Kammerer et al., Phys. Rev. B 86, 134426 (2012)
[5] M. Kammerer et al., Appl. Phys. Letters 102, 012404 (2013)

Skyrmions are winding vector fields with a characteristic spherical topology. These configurations are found in a wide range of research areas, such as high energy physics, Bose Einstein condensates, and spin systems. Skyrmionic spin structures may arise due to various interaction energies. One example for such a winding spin structure is the magnetic bubble, which is stabilized by dipolar interactions. Recent theoretical investigations predict a GHz gyrotropic motion of such a topological configuration after excitation in a restoring potential, analogous to vortex gyration. However, in contrast to vortices, bubbles are predicted to exhibit inertial effects, manifesting in a second gyrotropic mode of reverse chirality and in additional degrees of freedom. Here we demonstrate the presence of an inertial mass in a magnetic bubble by imaging its gyrotropic trajectory using pump-probe x-ray holography. We find that the inertial mass is very large compared to other magnetic systems, which we attribute to the non-local energy reservoir of the bubble's breathing mode. The breathing mode is a unique feature of the geometrically confined skyrmionic spin structure, which is a direct consequence of its topology, and thus lends itself to describe the inertia of Skyrmions in terms of a topological mass. As such, the presence of strong inertia is also expected for other skyrmionic spin structures, including chiral Skyrmions.

Over the last decade magnetism research focused on a fundamental understanding and controlling spins on a nanoscale. Recently, it has been recognized, that the next step beyond the nanoscale will be governed by mesoscale phenomena, since those are supposed to add complexity and functionality, which are essential parameters to meet future challenges in terms of speed, size and energy efficiency of spin driven devices.
Magnetic soft X-ray microscopy is a unique analytical technique combining X-ray magnetic circular dichroism (X-MCD) as element specific magnetic contrast mechanism with high spatial and temporal resolution. Utilizing the inherent time structure of current synchrotron sources fast magnetization dynamics in ferromagnetic elements can be performed with a stroboscopic pump-probe scheme with 70ps time resolution. At next generation light sources fsec spin dynamics can be addressed.
In this talk I will review recent achievements and outline future opportunities. We have found that the symmetry breaking DM interaction causes a stochastic character in the nucleation process of magnetic vortices [4]. Time resolved studies of dipolar coupled magnetic vortices indicates the path for an efficient energy transfer mechanism, which can be used for novel magnetic logic elements [5]. Tailoring the geometry of magnetic nanodisks allows to control the switching of circularity independently from the polarity of the vortex core [6]. First attempts to image 3dim magnetic domain structures in rolled-up Ni nanotubes are very promising [7].
Future capabilities at next generation light sources, e.g. X-ray free electron lasers will allow to take snapshot images of spin structures and their ultrafast dynamics down to fundamental length and time scales.
The collaboration with Mi-Young Im, Weilun Chao, Sang-Koog Kim, Guido Meier, Vojtech Uhlir and Denis Makaro is highly appreciated. This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, of the U.S. Department of Energy under Contract No. DE-AC02-05-CH11231.
[1] BESAC report: From Quanta to the Continuum: Opportunities for Mesoscale Science (2012), http://science.energy.gov/~/media/bes/pdf/reports/files/OFMS_rpt.pdf
[2] P. Fischer, Materials Science & Engineeering R72 81-95 (2011)
[3] W. Chao, et al. Optics Express 20(9) 9777 (2012)
[4] M.-Y. Im, P. Fischer, Y. Keisuke, T. Sato, S. Kasai, Y. Nakatani, T. Ono, Nature Communications 3 983 (2012)
[5] H. Jung, K.-S. Lee, D.-E. Jeong, Y.-S. Choi, Y.-S. Yu, D.-S. Han, A. Vogel, L. Bocklage, G. Meier, M.-Y. Im, P. Fischer, S.-K. Kim, NPG - Scientific Reports 1 59 (2011)
[6] V. Uhlir, M. Urbánek, L. Hladík, J. Spousta, M.-Y. Im, P. Fischer, N. Eibagi, J. J. Kan, E. E. Fullerton and T. Scaron;ikola, Nature Nanotechnology 8 341-346 (2013)
[7] R. Streubel. D. Makarov, D. Karnaushenko, L. Han, O. G. Schmidt, J. Lee, S.-K. Kim, R. Schäfer, M.-Y. Im, P. Fischer (2013) submitted

The controlled displacement of magnetic domain walls in ferromagnetic nanostructures is a key prerequisite for memory, logic and devices based on switching by domain wall motion [1]. In domain wall logic devices driven by circular fields [2] the propagation of the domain wall is determined by the field strength and the frequency of the rotating field, giving an extra degree of freedom to control the domain wall motion. The effect of transverse and tangential fields on the domain wall spin structure leads to wall displacement but also to domain wall oscillations and transformations, at high domain wall velocities (Walker breakdown) as predicted numerically [3].
We present the first direct experimental visualization of the precessional motion of domain walls in permalloy nanorings controlled by circular fields with varying speed and strength [4]. Employing scanning transmission x-ray microscopy (STXM) we image the propagation of a pair of domain walls in a stroboscopic measurement scheme and we determine the velocity, spin structure and phase as a function of position and excitation. We image directly dynamic domain wall transformations and find that the domain wall velocity varies strongly during the periodic transformation processes between vortex and transverse walls. The propagation and domain wall spin structures are highly reproducible, showing that we can observe the Walker breakdown, which is a deterministic process and we find good agreement with theoretical predictions.
In addition, we also observe a time delay between tangential field component and the velocity evolution, due to domain wall inertia. This direct imaging technique allows us to pinpoint the origin of the domain wall inertia to the domain wall spin structure changes (see [5]). Beyond measuring the depinning process we see here for viscous wall motion how the Zeeman energy is not only transferred into displacement but also into exchange energy as the wall is deformed. In particular the domain wall velocity is strongly enhanced as the vortex domain wall undergoes a transformation to a transverse domain wall, which is directly linked to the energy change as the vortex core is expelled.
Finally, we systematically study the propagation of domain walls for various field strength and field rotation frequencies. We find for low domain wall velocities that the motion is dominated by pinning events, directly visualizing previous theoretical prediction of kinetic and static domain wall pinning.
[1] M. Hayashi, et al., Nat. Phys. 3, 21 - 25 (2007).
[2] D. A. Allwood et al., Science 309, 1688 (2005).
[3] A. Thiaville et al., in Spin Dynamics in Confined Magnetic Structures II, Springer (2006).
[4] A. Bisig et al., Nature Comm. (in press 2013).
[5] J. Rhensius et al., Phys. Rev. Lett. 104, 67201 (2010).

Strain is of fundamental importance in controlling microstructure, switching dynamics and order parameter coupling at ferroic and multiferroic phase transitions, whether the driving mechanisms are structural, ferroelectric, electronic or magnetic. Equilibrium values of spontaneous strains coupled to structural order parameters typically fall in the range ~0.1 - 5 % but the associated changes in elastic properties which they give rise to are 10&’s of %. For magnetic transitions, the strains can be sufficiently small that they may not easily be detected by conventional diffraction techniques. In this context, Resonant Ultrasound Spectroscopy (RUS) provides a particularly sensitive method of investigating the strength and dynamics of strain coupling through observations of elastic and anelastic behaviour in the frequency range ~0.1 - 2 MHz. As part of a wider study of magnetoelastic coupling phenomena in phases such as BiFeO3, KMnF3, (Pr,Ca)MnO3, (La,Ca)MnO3, FexO and PrAlO3, it has been found that the influence of elastic contributions due to other instabilities or ferroelastic twin walls can be much greater than that of the magnetic ordering. In order to investigate the limiting case of a “pure” antiferromagnetic ordering system, i.e. without other instabilities, CoF2 was selected as a model system. RUS data collected from a polycrystalline sample in the interval 10-290 K show distinct softening of the shear modulus and a peak in acoustic dissipation as T → TN (~39 K) from above and below. Softening of the shear modulus at T > TN can be fit with an expression usually used for Vogel-Fulcher freezing dynamics, perhaps implying that there is a small activation energy barrier for reorientation of spins in dynamically ordered domains within the paramagnetic phase. The softening, ΔC, scales with a tail in the strain, e, as ΔC prop; e^2. The pattern of softening at T < TN conforms closely to expectations from Landau theory for a transition with approximately tricritical character, linear/quadratic coupling between non-symmetry breaking strains and the driving order parameter, and rapid relaxation of the order parameter in response to an applied (dynamic) stress. A distinct dispersion with respect to frequency for both the softening and attenuation in the stability field of the antiferromagnetic structure has been used to determine values for relaxation times. These fall in the range 10^-8 - 10^-9 s and are believed to be due to spin-lattice coupling; they are weakly temperature dependent, increasing as temperature increases towards the Néel point. Such relaxation behaviour is not universal, however, as magnetic ordering in multiferroic YMnO3 gives rise only to elastic stiffening in proportion to the square of the magnetic order parameter. It seems likely that a second order parameter, whether ferroelectric or ferroelastic in origin, might have a profound effect on the spin-lattice relaxation dynamics.

Perovskite nickelates (chemical formula R-NiO₃, with R=Rare Earth) display a textbook example of bandwidth controlled Metal to Insulator (MI) transition and a rather unconventional AFM ground state, with the Ni spins ordered along the [111] direction[1]. These compounds have recently been studied in the form of epitaxial thin films, which allows their phase diagram to be controlled using strain and thickness induced effects[2]. Also, electric field effect and pump-probe light experiments have shown that nickelates display interesting functional properties[3,4].
Here we use synchrotron radiation to investigate the electronic configuration and the phase diagram of high quality epitaxial thin films grown by RF off-axis magnetron sputtering. We focused on the members of the family with R=Sm and R=Nd, grown on both [001] and [111] oriented substrates. We first performed Resonant Inelastic X-ray Scattering measurements. By tuning linearly polarized light at the Ni L-edge we studied the energy and temperature dependence of d-d and charge transfer excitations in the two systems. The data indicate an electronic configuration where the localized Ni d-orbitals are hybridized with the neighboring O p band. The resulting band diagram reveals the opening of a charge transfer gap as the temperature is lowered. Second, through Resonant x-ray diffraction we studied the evolution of antiferromagnetism in the two systems as a function of temperature, energy and light polarization. As a main result, a phase diagram specific for each of the films is obtained that shows how the bulk picture is modified in thin films.
[1] M. Medarde, J. Phys.: Condensed Matter 9, 1679-1707, (1997)
[2] R. Scherwitzl et al.: Phys. Rev. Lett. 106, 246403, (2011)
[3] R. Scherwitzl et al.: Adv. Mater. 22, 5517-5520, (2010)
[4] A. Caviglia et al.: Phys. Rev. Lett. 108, 136801, (2012)

Here we show that the structural and dynamical properties of iron oxide nanoparticles can be dramatically changed by incorporated rare earths in the synthesis. The particle size, crystallinity, surface spin structure, damping are all important in determining the utility of MNPs for MRI contrast and hyperthermia. We have focused on the incorporation of Tb into MNPs in an effort to increase the Gilbert damping in a manner similar to that demonstrated in metallic systems. Tb has a large orbital moment which effectively couples spin degrees of freedom to the lattice. A few percent of Tb has been shown to increase the Gilbert damping constant by a factor of 50. A surprising result in this study is that the addition of Tb to the synthesis process decreased the damping constant, rather than increased it; however there is a large unsaturable component at low temperatures.
Tb is incorporated into the synthesis in organic solution. We find that doping the particles causes a dramatic structural transformation from typical iron oxide spheres into cubes with an approximate diameter of 5 nm. X-ray diffraction (XRD) analysis shows that a change in the lattice spacing is observed by incorporating Tb in the synthesis process. XAS and XMCD show a subsitutional doping at the iron octahedral sites. Single particle energy-dispersive x-ray spectroscopy (EDS) confirms a doping concentration of approximately 4-6%. Finally, the particles are coated with SiO2 to make them compatible with biological systems.

Due to the combination properties of high magnetic susceptibility and low hysteresis loss, superparamagnetic nanocomposites are promising materials in applications such as transformer cores requiring high magnetization with low loss. Fe nanoparticles with size around 20 nm have been shown to exhibit optimized superparamagnetic properties. Previous synthesis approach based on thermal decomposition of Fe(CO)5 in octadecene with oleylamine as surfactant results in Fe nanoparticles with size up to 15 nm. The superparamagnetic properties of these nanoparticles are confirmed with both AC and DC magnetic measurements. Here, we use microwave heating to synthesize Fe nanoparticles in the presence of polymers, and characterize the nanoparticle size using dynamic light scattering (DLS) and transmission electron microscope (TEM). Another approach is to synthesize the Fe nanoparticles with suitable ligands for further cross-linking between neighboring nanoparticles. The structural, chemical, and magnetic properties of these magnetic nanocomposites will be characterized by TEM, FTIR and AC magnetometry. Since inexpensive precursor Fe(CO)5, low reaction temperature (< 200°C) and short time (le; 30 min) are used in these experiments, which produce good magnetic properties, we believe that microwave-assisted synthesis of superparamagnetic Fe nanocomposite is a promising approach for high volume applications.

Linear response theory (LRT) is typically used to model the hysteretic heating of single domain magnetic nanoparticles (SDMNPs) by alternating magnetic fields, but recent work suggests that LRT is strictly valid only when the applied field amplitude is much less than the zero temperature coercive field. In contrast, dynamic hysteresis loop simulations of coherent reversal in SDMNPs predict saturation behavior at high fields, enhanced heating of large SDMNPs driven by fields near or above their zero temperature coercive field, and convergence with LRT at low fields. The predictive successes and failures of these two models are investigated experimentally, with precise calorimetric measurements on Fe₃O₄ and MnFe₂O₄ SDMNP samples of varying diameter over a range of field amplitudes and frequencies.
Insight gained from this comparison of theory and experiment is used to predict and demonstrate the novel technique of bimodal magnetic heating, in which subpopulations of a bimodal mixture of SDMNPs can be independently heated by applying appropriate driving conditions. Potential applications and extensions of this technique are discussed, emphasizing minimally invasive biological signaling strategies for therapy and research.

Vertically aligned self-assembled epitaxial nanocomposites consisting of CoFe₂O₄ (CFO) ferrimagnetic spinel pillars embedded in a BiFeO₃ (BFO) ferroelectric perovskite matrix exhibit an enhanced strain-mediated magnetoelectric coupling due to the reduced clamping constraints imposed by the substrate on the vertical CFO/BFO interfaces. This is a desirable property for transducer, sensor or memory applications, but the randomness in the location, size and shape of the nanostructures limits their applicability. In this work, a general templating method is demonstated, which enables the controlled growth of CFO, MgFe₂O₄ (MFO) and NiFe₂O₄ (NFO) pillars in predefined locations within a BFO matrix. It relies on the selective nucleation of the spinel phase in few-nanometer deep pits and trenches etched into the perovskite substrate. Because this method is process and material agnostic, it offers great flexibility in the range of achievable designs.
The presentation will first detail the fabrication method, from the creation of 1-10 nm deep topographic features in the substrates, using Focused Ion Beam nanomachining as well as self-assembled triblock terpolymer and electron beam lithographies, to the growth of 50 - 200 nm thick nanocomposites by combinatorial Pulsed Laser Deposition. Then, examples of achievable structures will be presented, including square lattices of pillars with periods between 50 nm and 100 nm, sets of parallel lines with 75 nm to 100 nm spacings, and aperiodic structures, using several different spinels (CFO, MFO and NFO). Finally, structural (electron microscopy), magnetic and electric (scanning probe microscopy) characterization results will be presented, revealing the magnetoelectric properties of the templated nanocomposites as well as the switching field distribution and magnetostatic interactions of the pillars. In the case of CFO, the switching field was around 5 kOe, on par with untemplated BFO/CFO nanocomposites.

Magnetic nanomaterials have garnered increased attention because of the promise to tailor material properties based on the engineered size and shape of the constituent components. Nanostructured magnets offer the chance for new ranges of material performance necessary for technological advancement in commercial products. Additionally, nanomaterial research is beginning to show substitutions of rare-earth element alternatives that are less-expensive and environmentally sustainable for common magnet applications. As with ‘bulk scale&’ magnets, the employment of alloys in nanoparticle magnets opens the field of possibilities for tuning material properties. The wet- synthesis and material properties are presented of ternary alloy nanoparticles (eg. Fe49%Co49%V2%) with nanometers-thick silica shells, at a scale for nanocomposite formation and device fabrication. Analyses indicate the ternary alloy is formed directly in the synthesis. This represents an advantageous flexibility for complex magnetic nanoparticles formation, and is superior to top-down methods like melt-spinning. The size and composition of such Fe49%Co49%V2%-alloy nanoparticles can be manipulated directly during their formation through simple alterations to the synthetic parameters. Nanoparticles of sizes from less-than 10nm to greater-than 140nm have been produced. Magnetic characterization, as a function of nanoparticle size, shows how the nanomaterials produced range from ferromagnetic to superparamagnetic, by size-induced changes to the blocking temperature. These adjustments in synthetic conditions to control nanoparticle size showed no evidence of impacting the ternary-alloy formation. Magnetic saturation values exceeding 100 emu/g were demonstrated here for such nanoparticles. The conjunction of high magnetic saturation and tunable coercivity (importantly without modification of the outer silica shell) lends these alloy nanomaterials to a ‘generic building block&’ motif in the formation of more complex monolithic nanocomposites for applications including alternatives to rare-earth magnets and high-efficiency power electronics components. Although individual nanoparticles (and even clusters of nanoparticles) are of scientific interest, our application of these complex nanoparticles required the formation of structurally robust nanocomposites for device application that retained their appealing/interesting nanoparticle properties. Consolidations of core/shell magnetic nanoparticles to produce complex, sintered, monolithic nanocomposites were formed. The individual feature sizes of these monolithic nanocomposites where successfully kept to the necessary nanometer scale. Monolithic nanocomposites from 8mm, up to 40mm in diameter are shown. Structural and magnetic measurements of these nanocomposites are reported. Machining these nanocomposites and use in devices as both inductor cores and the critical soft-phase in an exchange-coupled permanent magnet are shared.

Topological insulators (TI) are a new state of quantum matter that possess metal-like surface states and behave as insulators in bulk. Bi2Te3, Bi2Se3 are some common examples of TIs. TI surface states are a manifestation of the strong spin-orbit coupling in these materials. Strong spin-orbit coupling which leads to band inversion in bulk is the necessary requirement for creation of TI states. Apart from spin-orbit coupling, electric field and lattice strain can also be used to invert the band structure. III-V nitrides in wurtzite phase possess a very strong internal electric field. This electric field due to intrinsic spontaneous and piezoelectric polarization (in strained nitrides) is sufficient to invert the band-ordering of a narrow-gap material such as wurtzite InN. In this work, it is demonstrated that a TI state can exist when a thin-film of InN is sandwiched between GaN layers. The combined piezoelectric and spontaneous polarization field in InN inverts the band-ordering at the Γ point.
For a certain quantum well thickness the inversion of bands happen at a threshold value of the polarization field. This suggests that tuning the internal polarization field can allow the control of band inversion. Since the creation of TI states requires inversion which in turn is polarization-dependent, this work investigates devices with varying internal fields. Internal fields are controlled by selecting a facet orientation of the quantum well layer. It is important to note that various orientations of the quantum well layer will either reduce the spontaneous or piezoelectric polarization. The choice of the facet is therefore adopted by determining the dominant polarization mechanism.
Ab-initio calculations were performed to obtain the conduction band effective mass at Γ and the direct band-gap for bulk InN. These values were inserted in to an 8-band k.p Hamiltonian including strain and the internal polarization field.
The work utilizes the spin-polarized surface states of the GaN/InN topological insulator. The surface states are degenerate at Γ. At a finite k-vector, the Rashba induced spin-splitting on the surface of this heterostructure is computed for the two spin states. The splitting under a first-order approximation is independent of k-vector and corresponds to the internal electric field&’s contribution to the Rashba coefficient α. Therefore, a pyro-electric control of spin-polarization coupled to a choice of facet orientation is accomplished.
Finally, the interplay of mechanisms that control the lifetime of spin-polarization and compute spin-relaxation times is used to design a resonant spin-lifetime transistor with InN/GaN. The lifetime of the spin-polarized states under certain conditions of growth is observed to cancel the Rashba and Dresselhaus splitting to suppress Dyakonov-Perel spin-relaxation mechanism.

Spin gapless semiconductors (SGS) are materials predicted to have a density of states displaying both half-metallic and zero-gap semiconducting properties. These materials are being investigated for spintronic devices due to their unique magnetic properties and high magnetic transition temperature (400-800K). Calculations predict several SGS compounds,^1,2 including Mn₂CoAl, Ti₂CoSi, V₃Al, and Ti₂MnAl. Recent research has been limited to bulk SGS compounds, and the magnetic and electrical properties of SGS thin films have not been investigated.
Mn₂CoAl thin films were grown by MBE on GaAs (100) substrates at 200 ^oC. The as-grown thin films were epitaxial with the substrate as confirmed by RHEED patterns and X-ray diffraction, which resulted in a tetragonal distortion. Annealing studies showed that the films lose their epitaxial registration and approach a cubic structure at higher annealing temperatures. At an annealing temperature of 325 ^oC the structure is cubic with a=c=5.80 Å. The magnetic and magnetotransport properties were investigated. The temperature-dependent resistivity shows metallic-like behavior below 200 K, but turns over into a semiconducting-like negative slope as temperature is increased above 200 K. The magnetoresistance is negative and the Hall resistivity is found to scale with ρ_xx² for all temperatures and magnetic fields, expected for an intrinsic anomalous Hall effect. Furthermore, the total Hall conductivity scales with the magnetization, indicating an anonymously small Hall voltage arising from a 2-carrier system. The connection of the electrical properties with the spin-gapless properties is discussed. Synthesizing thin film SGS paves the way for future devices based on such materials.
¹ S. Skaftouros, K.Ozdogan, E. Sasioglu, and I. Galanakis, App. Phys. Lett. 102, 022402 (2013).
² Siham Ouardi, Gerhard H. Fecher, and Claudia Felser, and Jurgen Kubler, Phys. Rev. Lett. 110, 100401 (2013).

Although high-temperature superconducting cuprates have been discovered for more than 25 years, high-field applications are still based on low-temperature superconductors (LTS), such as Nb₃Sn. Recently, we have demonstrated that the iron chalcogenide superconductors have a superior high-field performance over LTS at 4.2 K. Indeed, iron-based superconductors are of great interest for both basic science and applications, because they exhibit low critical current anisotropies with very high upper critical field slopes near T_c. In this presentation, I will discuss recent progress aimed at understanding the relationships between superconductivity, critical current, and nano-scaled structure defects in iron-based superconductors, with emphasis on superconducting films and coated conductors of iron chalcogenides. With a CeO₂ buffer, critical current densities (J_c) over 10⁶ A/cm² were observed in FeSe_0.5Te_0.5 films grown on single-crystalline and flexible metal substrates. These films are capable of carrying J_c exceeding 10⁵ A/cm² under 30 T magnetic fields, suggesting nano-scaled defects of the size comparable to the superconducting coherence length play important roles as effective pinning centers. Furthermore, we found that these films have significantly higher T_c (>20K) as compared to bulk samples (bulk T_c ~15 K) for the entire doping regime of FeSe_1-xTe_x. Structural analysis revealed that these films generally have significantly smaller c-axis and a-axis lattice constant than the bulk value, suggesting that the crystal structure changes have a dominating impact on the superconducting transition in iron-based superconductors.

We will be reporting on a recent experimental discovery [1] of a new antiferromagnetic material, having important fundamental and practical implications for nanomagnetism and nanotechnology. Using spin-polarized scanning tunneling microscopy (SP-STM) we observed room-temperature antiferromagnetic spin ordering in thin epitaxial films of c-FeSi on Si(111). Although some earlier GMR experiments [2] clearly indicated such a possibility, antiferromagnetism in this material has never been directly observed or predicted in theory. Using Fe-terminated STM tip, we found unusually high (75%) spin polarization of tunneling current. We also found atomically narrow spin-domain-boundaries, indicating that c-FeSi can be used for atomic scale (~12 atoms per bit) magnetic memory storage [3]. Our data analysis suggests that c-FeSi represents a Mott-Hubbard antiferromagnet.
References:
[1] Igor Altfeder, Wei Yi, and V. Narayanamurti, “Spin Polarized Scanning Tunneling Microscopy of the Room Temperature Antiferromagnet c-FeSi”, Rapid Communication, Physical Review B 87, 020403(R) (2013)
[2] J. M. Pruneda, R. Robles, S. Bouarab, Ferrer, and A. Vega, “Antiferromagnetic interlayer coupling in Fe/c-SiFe/Fe sandwiches and multilayers”, Phys. Rev. B 65, 024440 (2001)
[3] S. Loth, S. Baumann, C. P. Lutz, D. M. Eigler, A. J. Heinrich, “Bistability in Atomic-Scale Antiferromagnets”, Science 335, 196 (2012)

The properties of ferromagnetic nanoclusters can be tuned in a wide range by adjusting their size and shape. However, many fabrication methods only yield ensembles of magnetic clusters which are oriented at random in a host material or on a substrate. Thus, most technological applications of such materials have been restricted to macroscopic devices, i.e. where the mean distance between clusters is much smaller than the characteristic device size. A macroscopic size in case of working with random cluster arrangements is essential to avoid problems due to statistical fluctuations in cluster number, size, etc and thus allows one to tune the average clusters properties only. These considerations imply for fabricating miniaturized devices based on magnetic nanoclusters, it is desirable to have means to accurately control position, orientation, size and shape of the clusters.
Selective-area metal-organic vapor-phase epitaxy (SA-MOVPE), which combines top-down structuring and epitaxial growth, enables one to fabricate sub-100 nm-structures of well-defined size and shape on specified positions on a substrate. The areas where the cluster growth shall take place are defined by a nanostructuring process which generates openings in a growth-inhibiting layer on the substrate. Amongst many different materials, the growth of ferromagnetic hexagonal MnAs nanoclusters is possible with SA-MOVPE on (111) GaAs substrates [1]. This approach can be used to define planar magnetic sensor devices consisting of few exactly positioned MnAs clusters of well-defined shape and position [2].
In this contribution, the authors report on the successful SA-MOVPE growth and the electrical contacting of nanocluster arrangements consisting of single-domain MnAs nanoclusters, and present magnetoresistance measurements demonstrating the dependence of the overall resistance of these arrangements on the orientation of the magnetizations of the individual nanoclusters. In these measurements, discrete resistance changes of the single-domain MnAs nanocluster arrangements can be observed as the external magnetic field is varied. The switching behavior of the cluster magnetizations and its effect on the magnetoresistance characteristics of the MnAs nanocluster arrangements are explained by accounting for the interplay of crystalline anisotropy of hexagonal MnAs, of the form anisotropy of the clusters and of the coupling between clusters.
[1] Wakatsuki, T. et al.: Growth Direction Control of Ferromagnetic MnAs Grown by
Selective-Area Metal-Organic Vapor Phase Epitaxy, JJAP 48 (2009) 04C137
[2] Heiliger, C. et al.: Magnetic Sensor Devices Based on Ordered Planar Arrangements of
MnAs Nanocluster, IEEE Trans. Mag. 46 No. 6 (2010)

Study of Magnetic nanodots is at central in the field of nanomagnetism. Besides the interesting properties caused by dimensionality effect, magnetic nanodots can induce many novel phenomena when forming heterostructures with other electronic materials. In this work, I will use several examples to demonstrate their effect. These examples include 1) Collective ferromagnetic behavior of magnetic nanodot arrays on 2-dimensional electron gas, 2) Colossal magnetoresistance of organic thin films induced by inserting a layer of magnetic nanodots, and 3) Dramatic enhancement of metal-insulator transition temperature in manganites capped by a layer of magnetic nanodots. All these fascinating phenomena originate directly from the interaction between magnetic nanodots with the electronic structures of the host materials. Their underlying mechanism will also be discussed based model calculations.

We report growth of various phase architectures of self-assembled BiFeO3-CoFe2O4 (BFO-CFO) thin films on differently oriented SrTiO3 (STO) substrates. CFO forms segregated rectangular, strip and triangular nanopillars embedded in a coherent BFO matrix on (001), (110) and (111) oriented STO substrates, respectively. Nanostructures with an aspect ratio of up to 5:1 with a prominent magnetic anisotropy were obtained on both (001) and (110) STO along out-of-plane and in-plane directions, respectively. Magnetic easy axis rotation from in-plane to out-of-plane directions was realized through aspect ratio control. These studies established a detailed relationship of magnetic anisotropy with specific shape and dimensions of ordered magnetic arrays. The results suggest a way to effectively control the magnetic anisotropy in patterned ferromagnetic oxide arrays with tunable shape, aspect ratio and elastic strain conditions of the nanostructures.

Thin film magnetoelectric oxide heterostructures are among the most promising materials for a new generation of spintronic devices, in which spin transport is used to convey information. However, prevailing theories of magnetoelectric coupling have failed to fully describe the behavior of these composites, particularly in thicker device structures. Here we present evidence for a crossover from charge- to strain-mediated interfacial coupling in heterostructures of the piezoelectric PbZr_xTi_1-xO₃ (PZT) and the half-metal La_1-xSr_xMnO₃ (LSMO). Using polarized neutron reflectometry we reveal the presence of a graded magnetization, which is associated with local strains and interfacial charge transfer directly measured by aberration-corrected transmission electron microscopy. We then perform density functional theory calculations to show that strain acts to suppress magnetization locally through a change in Mn-e_g orbital polarization. Our results suggest a radically new model of coupling in these materials and we offer ways to tune the performance of a composite by selecting appropriate layer geometries.
This research was supported in part by Oak Ridge National Laboratory's Shared Research Equipment (ShaRE) User Program, which is sponsored by the Office of Basic Energy Sciences, U.S. Department of Energy. Polarized neutron reflectometry experiments were performed at the Spallation Neutron Source at Oak Ridge National Laboratory, managed by UT-Batelle, LLC for the U.S. Department of Energy. Part of this work was conducted in Drexel University&’s Centralized Research Facilities. The SuperSTEM Laboratory is supported by the U.K. Engineering and Physical Sciences Research Council.

The rare-earth orthochromites (RCrO₃), such as DyCrO₃, have been suggested to display both magnetic and ferroelectric order. Being multiferroic with higher Neel temperature than their manganite counterparts (RMnO₃), these RCrO₃ and their solid solutions (that have not been studied yet) are of great interests. However, the structural and magnetic properties of these materials remain as yet poorly understood. In this work, the solution based synthesis of phase pure DyCrO₃ and several solid-solutions (including YCrO₃ HoCrO₃ and NdCrO₃) are presented. X-ray diffraction and Raman spectroscopy were used to analyze the structure of the bulk samples. The valence states of the cations were investigated with x-ray photoelectron spectroscopy. The effect of the substituting ions was seen in the temperature dependence of the magnetic moment. Promising magnetocaloric behavior was extracted from isothermal magnetization measurements with values for the magnetocaloric effect and relative cooling power as high as 8.1 J/kg K and 196 J/kg respectively. Magnetic and ferroelectric properties of these samples will be presented in detail.

One of the novel tools for studying spin-electron-lattice phenomena in functional materials is the x-ray free electron laser [1]. This is especially true in the soft x-ray range, where the L-edges exist for the transition metal oxide materials which can be studied using resonant techniques. These techniques are used to directly investigate the magnetic, charge, or orbital structure of manganites [2], magnetites [3], nickelates [4], and cupric oxides [5]--the building blocks for high temperature superconductors. Most importantly is the new available information in the time domain that using an x-ray FEL offers. By using pump-probe developments with ultra-short laser and x-ray pulses, dynamics on femtosecond time scales can be observed that yield information about the time scales of the order parameters involved among the spin-electron-lattice phenomena. The coupling between the different mechanisms can be measured in these materials, and followed in real time. We review recent results in these systems and discuss the new understanding that is forming in the study of magnetic, charge, and orbital dynamics in these quantum materials.
[1] Nature Photonics 4, 641 - 647 (2010).
[2] Physical Review B 84, 241104(R) (2011); Phys. Rev. B 86 064425 (2012).
[3] Nature Materials 12, 882-886 (2013).
[4] Nature Communications 3 838 (2012); Phys. Rev. Lett. 110 127404 (2013).
[5] Phys. Rev. Lett. 108, 037203 (2012).

We have recently completed a survey of L2₁ structure Heusler alloys which have composition A₂BC where A and B are transition metals but C is not. The L2₁ structure can be understood by considering the cases, (i) A=B=C for which the L2₁ structure is equivalent to bcc and (ii) B=Cne;A for which the L2₁ structure is equivalent to B2 or CsCl. Finally, if every other B atom in the B2 structure is replaced by a C, atom one obtains L2₁. Our survey of the L2₁ structure Heusler alloys included all alloys for which A=(Cr, Mn, Fe, Co, Ru, or Rh), B=(Ti, V, Cr, Mn, or Fe) and C=(Al, Ga, In, Si, Ge, Sn, P, As, or Sb). We find that there is an almost universal tendency among these alloys to form in a state which has an electronic structure such that either the “up” or “down” spin channel has exactly three occupied states per atom. This has the effect of causing the spin-moment per formula unit to be given by M=Z-24, where Z is the number of valence electrons per formula unit. Often there will be a gap in the density of states for the spin channel with precisely 12 filled electronic states per formula unit (3 per atom). This gap adds an additional stability to the phase. Evidence for this can be seen when one plots the energy versus magnetic moment for a given lattice constant. There is typically a minimum near M=Z-24 and for the systems with a gap at the Fermi energy, there is a discontinuity in the slope of the E vs. M curve leading to a characteristic “v” shape. This tendency for the transition metal atoms to shuffle electrons between spin channels to satisfy the Slater-Pauling condition of 3 electrons per atom in one of the spin channels often leads to multiple solutions of the DFT equations, some of which may be physically accessible via changes in magnetic field or in lattice volume. Another intersting feature is the existence of with 100% polarization of the density of states at the Fermi energy and zero magnetic moment. Such materials would be expected to have extremely large domains, no shape anisotropy and extremely fast magnetization dynamics. These materials offer the potential for materials design since they allow one to adjust the lattice through isovalent variation of the C element, to adjust the magnetization, and to adjust the anisotropy through layering of different half-metallic alloys along the (001) direction.
This work was supported in part by NSF DMREF Grant number 1235396 and by C-SPIN, one of the six SRC STARnet Centers, sponsored by MARCO and DARPA

By separating spin- from electrical-currents, nanoscopic lateral non-local spin valves (NLSVs) [1] provide a direct probe of spin transport in metals. In addition to their potential to further our fundamental understanding of spin transport in metals, NLSVs are also relevant to future spintronic devices such as CPP GMR sensors and have even been proposed as a disruptive technology for HDD read heads [2].
In our recent work [3] we have performed an extensive investigation of nanoscopic NLSVs, employing transparent interface limit devices fabricated via e-beam lithography, in situ multi-angle deposition, and UHV e-beam evaporation. Multiple ferromagnet/normal metal (F/N) combinations—using Co, Ni, NiFe, Fe, Cu and Al—have been explored, providing a wide picture of NLSV spin transport, including the importance of impurity scattering, and a critical assessment of the general adherence to Elliot-Yafet spin relaxation. One issue in particular that has eluded explanation is the non-monotonic T dependence of the NLSV spin accumulation signal, ΔR_NL(T), e.g. in Ni80Fe20/Cu devices. In this work we elucidate its origin and demonstrate a simple means to mitigate this suppression. Remarkably, our experimental results clearly show that this non-monotonicity cannot be attributed to the properties of a single F or N material. In fact, we find that the downturn in ΔR_NL occurs only for materials combinations where the F is known to form a local moment as a dilute impurity in N. We show detailed analysis of the data provides strong evidence of the importance of Kondo physics in these systems, a factor that is not typically considered. Significantly, careful studies using techniques such as polarized neutron reflectometry rule out alternative possibilities related to ordering/freezing of interfacial magnetic phases.
Systematic fitting of ΔR_NL(T) as a function of F separation, and direct quantitative comparison with Hanle effect measurements, reveal that the downturn in ΔR_NL is in fact predominantly due to suppression of the injected current polarization, α, which we argue to be a direct manifestation of the Kondo effect in a spin transport device. Good agreement is found between the onset of depolarization and the Kondo temperature, T_K, and a logarithmic decrease in ΔRNL is found around T_K, as might be anticipated if Kondo physics is important. Moreover, using a judiciously chosen thin (5 nm) interlayer we demonstrate that by preventing formation of local moment impurities at the F/N interface this effect can be mitigated and the low-T α almost completely restored.
This work thus highlights the dramatic effect interfacial-moment-forming impurities have on spin injection and relaxation in metallic systems, and presents an important new challenge for the theoretical description of these spin transport devices.
[1] Jedema, et al., Nature 410, 345-8 (2001).
[2] Hitachi GST, US Patent 0296202 (2010)
[3] O&’Brien, Crowell, Leighton et al., in preparation (2013).

In the research field of spin caloric transport one of most the prominent and still not understood effects is the spin-Seebeck effect (SSE) in magnetic insulators [1]. Different mechanisms have been put forward to explain the SSE including parasitic surface effects. Due to this gap of knowledge the underlying mechanism has to be proved.
We present a systematic study of the SSE in Yttrium Iron Garnet (YIG) films of different thicknesses [2]. From the thickness dependence of the measured inverse spin Hall Voltage we can unambiguously identify the origin of the measured longitudinal SSE signals. A comparison with the thickness dependence of the recently discovered spin Hall magneto resistance (SMR) [3] allows us to estimate the influence of the interface on the SSE signal and to identify the genuine origin of the SSE in the bulk of the YIG. Corresponding simulations on atomistic length scales allow us to explain the thickness dependence of the inverse spin Hall Voltage from a finite propagation length of the thermally excited magnons, that are identified as the source of the SSE. Eventually these could be used to manipulate domain walls very efficiently [4].
[1] K. Uchida et al., Nature Mater. 9, 894 (2010)
[2] A. Kehlberger et al., arXiv:1306.0784 (2013)
[3] H. Nakayama et al., Phys. Rev. Lett. 110, 206601 (2013)
[4] P. Möhrke et al., Solid State Commun.,150, pp. 489-491 (2010)

Our experiments focus on spin-orbit torque (SOT) in the ferromagnetic semiconductor (Ga,Mn)As. Due to the bulk inversion asymmetry, SOT effects are possible in a uniform ferromagnetic system, removing any spin transfer torque effects that may come from the spin-hall effect. An electrical current in this spin-orbit coupled ferromagnet leads to a spin polarization of the charge carriers and consequently a torque is applied to the magnetization. This torque can lead to reversible switching of the magnetization [1,2], but in our experiments we apply a microwave frequency alternating current to resonantly drive the magnetization [3]. In such a spin-orbit ferromagnetic resonance technique we are able to simply determine the symmetry and magnitude of the SOT.
The first experiments in (Ga,Mn)As measured a field like torque, the current leading to in-plane components of the carrier spin-polarization (Sx, Sy), which are approximately independent of the magnetization orientation within the sample [4]. More recently we observe an anti-damping torque, with similar magnitude to the in-plane torque, characterized by magnetization-dependent spin polarization, Sz(M). This anti-damping torque is due to the Berry phase acquired by charge carriers accelerating in the spin-orbit coupled bandstructure.
We expect the Berry phase anti-damping SOT to also be present in ferromagnet/paramagnet bilayers with broken structural inversion symmetry (e.g. [5]). Therefore, two scattering-independent relativistic mechanisms (the spin hall effect and SOT) can contribute to the current-induced magnetization switching in those technologically important magnetic structures.
[1] A. Chernyshov et al. Nat. Phys. 5, 656 (2009).
[2] M. Endo, F. Matsukura, and H. Ohno, Appl. Phys. Lett. 97, 222501 (2010).
[3] D. Fang et al. Nat. Nano. 6, 413 (2011).
[4] I. Garate and A. H. MacDonald Phys. Rev. B 80, 134403 (2009).
[5] I. M. Miron et al. Nature 476, 189 (2011).

The coupling of electron spin to real-space magnetic textures leads to a variety of interesting magnetotransport effects. The skyrmion spin textures often found in chiral B20-lattice magnets give rise, via real-space Berry phases, to the topological Hall effect, but it is typically rather small. Here we show that B20-ordered Fe_0.7Co_0.3Si epilayers grown on Si (111) substrates display a giant topological Hall effect due to the combination of three favourable properties: they have a high spin-polarisation, a large ordinary Hall coefficient, and small skyrmions. Moreover they show enhanced ordering temperatures due to the presence of epitaxial strain. The topological Hall resistivity is as large as ~750 nOmega;cm at helium temperatures. Furthermore, we observed a drop in the longitudinal resistivity of ~100 nOmega;cm at low temperatures in the same field range, suggesting it is of topological origin. That such strong effects can be found in material grown in thin film form on commercial silicon wafer bodes well for skyrmion-based spintronics.
We would like to acknowledge funding from EPSRC and the FP7 ITN Q-NET.

Materials with large coercivity and perpendicular magnetic anisotropy represent the mainstay data storage media, thanks to their ability to retain a stable magnetization state over long periods of time and their compliance with increasing miniaturization steps. A major concern is that the same anisotropy properties that make a material attractive for storage also make it hard to write. We address this issue by investigating novel spin torque mechanisms based on spin-orbit effects. It is well known that spin-orbit coupling is ultimately responsible for magnetocrystalline anisotropy and damping. Under certain conditions, however, spin-orbit effects might either induce [1,2,3] or enhance [4,5] specific spin torque mechanisms.
We show that in materials lacking inversion symmetry, the spin accumulation induced by the current creates torques on the magnetization. We analyze the expected symmetry of these torques in uniformly magnetized layers as well as magnetic domain walls, and evidence experimentally their existence in agreement with symmetry arguments. We discuss our results in terms of known Spin Orbit mechanisms such as Rashba or Spin Hall Effect, and show that the complexity of the experimental observations goes beyond the predictions of simple models. 1. A. Manchon and S. Zhang, PRB 78 212405 (2008); 2. I.M. Miron et al. Nature Materials 9, 230-234 (2010); 3. I.M. Miron et al. Nature 476, 189-193 (2011); 4. I.M. Miron et al. PRL 102, 137202 (2009); 5. I.M. Miron et al. Nature Materials 10, 419 (2011).

The spin Hall effect (SHE) is a key ingredient for future spintronic devices since it is the only method to convert charge currents into spin currents or vice-versa with using neither ferromagnets nor magnetic fields. Platinum is one of the most common SHE materials exhibiting reasonably large spin Hall (SH) angle of a few percent. However it is a costly metal, being unsuitable for the practical application. In this work, we demonstrate large extrinsic SHEs can be induced in Cu-based alloys by adding small amount of strong spin-orbit coupling impurity such as Ir and Bi. Our analyses based on the 3 dimensional Valet-Fert model yielded the spin Hall angle of 0.02 for CuIr alloys [1], almost the same as that for Pt, and that of -0.24 for CuBi alloys [2].
[1] Y. Niimi, M. Morota, D. H. Wei, C. Deranlot, M. Basletic, A. Hamzic, A. Fert, and Y. Otani, Phys. Rev. Lett. 106, 126601-1~4 (2011).
[2] Y. Niimi,Y. Kawanishi,D. H. Wei, C. Deranlot, H. X. Yang, M. Chshiev, T. Valet, A. Fert, and Y. Otani, Phys. Rev. Lett. 109, 156602-1~5 (2012).

The recently discovery that a charge current flowing through certain high atomic number (e.g. Pt, Ta, or W) thin film micro- or nano-strips can exert a quite substantial spin torque on an adjacent thin film magnetic material has great potential for spintronics applications. For example the anti-damping spin transfer torque that results from the transverse spin current that can be efficiently generated via the giant spin Hall effect (SHE) in these high Z materials by a longitudinal electrical current has been demonstrated to reversibly switch the magnetization direction of in-plane magnetized nano-magnets, to induce persistent microwave magnetic oscillations in such nano-magnets, and to facilitate the high speed manipulation of domain walls in magnetic nano-strips. The deterministic switching of the magnetization direction of ultra-thin magnetic layers with perpendicular magnetic anisotropy has also been achieved, and attributed by various groups to either the spin torque arising from the spin Hall effect, or to the field-like torque arising from a different, interfacial, spin-orbit interaction, or to a combination of both. Here I will report recent results from our studies of this giant SHE and related spin torque phenomena, including investigations into the characteristics and quantitative strength of the SHE in the aforementioned and other high Z thin film materials, and into the fundamental role that the interfacial spin-mixing conductance plays in determining the effectiveness of the SHE for exerting strong anti-damping spin torques on the adjacent ferromagnet. I will also summarize results from an investigation of the spin-orbit torques found in a variety of NM/FM/Oxide device configurations as determined using three different techniques that have recently been utilized to quantitatively measure the magnitude and the sign of these torques: DC bias measurements, second-harmonic AC signal measurements, and spin-torque ferromagnetic resonance (ST-FMR). I will discuss the variation in the apparent character of the spin-orbit torques as indicated by these different types of measurements, and as found in different materials configurations and as affected by the modification of the electrical and magnetic properties of the multilayer structures.

Multiferroic heterostructures thin films of ferromagnetic and ferroelectric materials have attracted particular interest for their properties which provide a unique opportunity to exploit several functionalities yield have new device concepts, such as ferroelectric tunnel junctions. We have studied thickness effect of ferroelectric PbZr_0.52Ti_0.48O₃ films with thickness range from 100 to 3 nm on La_0.67Sr_0.33MnO₃/(LaAlO₃)_0.3(Sr₂AlTaO₆)_0.7 (LSMO/LSAT) (001) substrates deposited by pulsed laser deposition technique. The x-ray diffraction patterns of the heterostructures show only the (00l) reflections corresponding to the LSAT substrate, PZT and LSMO layers. The Atomic force microscopy of PZT/LSMO/LSAT heterostructures show that the average surface roughnesses (Ra) was very low, less than 2 nm. The frequency dependence of the dielectric permittivities (real (ε&’) and imaginary (ε”)) for different PZT/LSMO heterostructures show the following characterisitics: i) weak frequency dependence below 10⁵ Hz, ii) relatively high ε&’ and low ε” values below 10⁵ Hz, and iii) large ε” dispersion was observed above 10⁵ Hz accompanied with a significant reduction in ε&’ value. The ε&’ (ε”) increased from 68 (9.75) to 463 (39) when the thickness of the PZT films increased from 10 nm to 100 nm measurement at 10 KHz. The grain size and mechanical stress between LSMO/LSAT and PZT films can be the intrinsic effect responsibles for the dielectric properties variation. Another source that can contribute in the variation of the observed values of ε&’ and ε&’&’ is the interfatial layer, which acts as a pinning center affecting the domain wall mobility. Well saturated ferroelectric loops were observed for PZT films with 100, 50 and 25 nm thickness with a remanent polarization (Pr) of 35, 33 and 31 mu;C/cm² respectively. Piezo force microscopy of the ultrathin PZT films shows a clear and reversible out-of-plane phase contrast above ± 2.5 V, which indicates the ferroelectric character of those 7, 5, and 3 nm PZT/LSMO thin films. The Current-Voltage (IV) characteristics of the PZT/LSMO films, with PZT barrier thickness between 7 to 3 nm showed nonlinear IV characteristics indicating tunneling mechanism. The resistance switching behavior was observed from low resistance state (LRS) to high resistance state (HRS) and vice versa by sweeping the voltage from negative to positive and back. At zero bias, the HRS/LRS ratio was ~100. The magnetization properties show ferromagnetic behavior in all samples (100-10 nm) due to the LSMO layer (~60 nm). An Appreciable decrease in saturated magnetization (Ms) was observed with decrease of PZT layer thickness. The average Ms value at 300 K was 170, 150, 100, and 45 emu/cm³ for 100, 50, 25, and 10 nm, respectively. Similar behavior in Ms was observed at 5 K. The enhancement in magnetization with increase in PZT thickness may be due to interface effect between PZT/LSMO interfaces.

Magnetoelectric multiferroic composites have received much attention in recent years due to their large magnetoelectric coupling and have potential applications such as magnetic sensors and transducers, microwave and millimeter wave devices, miniature antennas, etc and also rich in fundamental physics. However, the availability of room temperature multiferroics and hence there is a need to find suitable magnetoelectric multiferroic materials with large electrical-magnetic coupling around room temperature for potential device applications. In this context, we report the structural, dielectric, magnetic and magnetoelectric properties of multiferroic composite ceramics, (1-x) Pb(Fe_0.5Nb_0.5)O₃ - x Co_0.65Zn_0.35Fe₂O₄( x =0.1,0.2,0.3, 0.4). Although the Pb(Fe_0.5Nb_0.5)O₃ (PFN) is known to be a single phase multiferroic material, it shows very good ferroelectric properties, but very small magnetoelectric coupling below room temperature and having low magnetic properties. But Co_0.65Zn_0.35Fe₂O₄ (CZFO) compound shows good magnetic properties around room temperature with good magnetostrictive properties. The X-ray diffraction patterns and the Rietveld refinement results revealed the presence of both PFN and CZFO binary phases in theses composites without any other secondary phase. Detailed analysis of dielectric spectroscopy shows a decrease of dielectric constant (ε_r = 5684, 2485, 2318, 966 for x = 0.1, 0.2, 0.3, 0.4, respectively) and the shifting of the ferroelectric Tc (i.e. Tc =395,429,432,459 K for x = 0.1, 0.2, 0.3, 0.4, respectively) towards the higher temperature in these composites as the concentration of CZFO increases. All these composites show very good ferromagnetic-like behavior at room temperature and their saturation magnetization increases with increasing of CZFO concentration (i.e. Ms =8.7,17.8, 29.5, 36.1emu/gm for x = 0.1, 0.2, 0.3, 0.4, respectively). In addition, these composites have very good magneto-dielectric coupling at room temperature. Our results suggest that the above PFN-CZFO composites show good multiferroic and magnetoelectric properties around room temperature which may be suitable for device applications.

Present day electromagnetic devices rely on a discovery made by Oersted 200 years ago where a current passing through a wire generates a magnetic field. While extremely useful this approach has significant limitations in the small scale. Recent discoveries suggest that a ferromagnetic materials magnetization state can be manipulated with an electric field and thereby overcome the deficiencies associated with Oersted&’s discovery. This presentation describes analytical and experimental work to on nanoscale multiferroic materials demonstrating electric-field induced control of magnetic spin states. The multiferroic consists of nanoscale Ni elements mechanically coupled with a piezoelectric PMN-PT substrate. Analytical modeling uses micromagnetics fully coupled with mechanical equilibrium to guide the design of nanoscale geometries with energy levels permitting electric field induced strain reorientation of magnetic spin states. Experimental data is presented on Ni thin film, Ni nanoscale single domains, and Ni superparamagnetic nanocrystals. MOKE data and MFM measurements on Ni thin film indicate magnetic easy axis reorientation and manipulation of Bloch domain walls with an applied electric field. PEEM measurements on Ni nanoscale single domain structures indicate 90 degree reorientation with an applied electric field. SQUID measurements on Ni superparamagnetic nanocrystals indicate switching from superparamagnetic to single domain ferromagnetic at room temperature with an applied electric field.

Investigated SrMnO₃ and CaMnO₃ compounds crystallize in cubic perovskite Pm3m and orthorhombic Pnma crystal structure, respectively and exhibit magnetic phase transition to antiferromagnetic G-type phase near 230 K and 120 K, respectively. Recently, first principles calculations predicted that both systems should exhibit a strong spin-phonon coupling and therefore they should become ferroelectric and even ferromagnetic under a biaxial strain, although both materials are paraelectric down to liquid He temperatures.^1,2 The strain effect was not yet confirmed experimentally, but Sakai et al. ³ expanded SrMnO₃ lattice by Ba doping and successfully induced ferroelectricity in
Sr_1-xBa_xMnO₃ (xge;0.45) with T_C asymp; 400 K and canted ferromagnetism with T_N asymp; 200 K. Important fact is that in this case the strong ferroelectricity (P_S = 13 mu;C/cm² ) is driven by displacement of magnetic Mn⁴⁺ cations and the magnetoelectric coupling is therefore the largest yet attained. ³
Infrared reflectivity spectra of SrMnO₃ ceramics reveal 25 cm^-1 stiffening of the lowest-frequency phonon below Néel temperature. The temperature dependence of polar phonon frequency is extraordinarily large and we explain it by the exceptionally strong spin-phonon coupling. The same phonon shift with temperature was obtained from the first principles calculations. Polar phonons become Raman active below T_N, although they are forbidden in the cubic crystal structure. It gives evidence that the cubic symmetry is locally broken due to the strong magnetoelectric coupling. Multiphonon and multimagnon scattering is also seen in Raman spectra. Microwave and THz permittivity is strongly influenced by hopping electronic conductivity, which is caused by a small non-stoichiometry of the sample. THz conductivity unusually increases on cooling, while the DC conductivity markedly decreases. Thermoelectric measurements gave the room-temperature free carriers concentration of n_e = 3.16×10²⁰cm^-3 and the sample composition was determined as Sr²⁺Mn⁴⁺_0.98Mn³⁺_0.02O^2-_2.99.
Our radio-frequency dielectric, THz and infrared studies of CaMnO₃ ceramics reveal metal insulator phase transition at T_N and splitting of some polar phonons below T_N. Both effects give evidence on the strong spin-phonon coupling, which reduces crystal symmetry in this material.
1. J.-H. Lee and K.M. Rabe, Phys. Rev. Lett. vol. 104, p. 207204, 2010.
2. S. Bhattacharjee, E. Bousquet, and P. Ghosez, Phys. Rev. Lett. vol. 102, p. 117602, 2009
3. H. Sakai et al. , Phys. Rev. Lett. vol. 107, p. 137601, 2011

The quest for magnetoelectric multiferroics is driven by the promise of a novel generation of devices combining the best characteristics of ferromagnetic and ferroelectric materials. For instance, those materials would make possible the advent of memories in which information would be written by an applied voltage consuming low power and non-destructively read by magnetic read head. These cherished applications require materials displaying a substantial magnetization and electric polarization which are coupled and coexist well above room temperature. These properties are extremely unusual in single phase materials and firm candidates for the development of these technologies are still sought. In this contribution we will report on the growth of epitaxial thin films of ε-Fe₂O₃ by pulsed laser deposition on SrTiO₃ (111) and present recent data on its structural, magnetic and dielectric characterization. The films are ferromagnetic and ferroelectric at room temperature and display magnetization and polarization values at remanence of about 50 emu/cm³ and 1 µC/cm² with a long retention time, respectively. A magnetocapacitive response with low losses has also been detected indicating that the films present a coupling between both ferroic orders. The benefits and prospectives of this single transition metal oxide will be discussed in the context of the recent literature on single phase magnetoelectric multiferroics.

The microscopic origin of the static magneto-electric coupling, i.e. the change of magnetization with electric field or of electric polarization with magnetic field, is already well understood. Depending on the specific magnetic structure of the investigated materials, the exchange striction, inverse Dzyaloshinskii-Moriya interaction or the spin-dependent covalency between the metal d state and ligand p state may play the key role. In contrast, due to the dynamic magnetoelectric coupling, spin waves (magnons) can be excited by the electric component of the electromagnetic radiation; therefore, these excitations are called electromagnons (EMs). Unlike the antiferromagnetic or ferromagnetic resonances — i.e. magnons from the Brillouin zone (BZ) centre (q=0) excited by the magnetic component of the electromagnetic radiation —, the EMs are magnons with a general value of the wavevector q. In different magnetic structures, EMs can have wavevectors from the Γ point of BZ, from the BZ boundary (q=Q_BZ), q=q_m or q=Q_BZ-2qm (here q_m denotes the modulation wavevector of the magnetic structure).
The reasons of EM activation in the THz permittivity spectra and their properties will be discussed for different compounds; among them, in the canonical spin-induced ferroelectric TbMnO₃ [1]. In this material, the ferroelectricity is induced by a spin-orbit coupling (inverse Dzyaloshinskii-Moriya interaction), and two EMs are activated by the magnetostriction. Second example is multiferroic CaMn₇O₁₂, whose polarization is the highest among all spin-induced ferroelectrics. In this material we observed one EM below the first magnetic phase transition occuring at 90 K and the second EM activates in the THz spectra below 50 K, when the magnetic structure becomes incommensurately modulated by two wavevectors. Finally we will demonstrate that electromagnons are not limited to spin-induced ferroelectrics. We have observed an EM in nanograin ceramics of ε phase of Fe₂O₃. This material is below 490 K a pyroelectric ferrimagnet and the EM activates in the infrared spectra below 110 K, [2] when the magnetic structure becomes incommensurately modulated. We will shown how by combining infrared, THz and inelastic neutron scattering experiments, the EM can be discerned from magnons or phonons. This we achieved for the first time without using a single crystal. In a broader perspective, the EMs are sensitive not only to the static external magnetic field, but also to the external electric field. It opens a promising route for electrically controlled magnonics in the THz region.
1. Takahashi Y. et al. Evidence for an electric-dipole active continuum band of spin excitations in multiferroic TbMnO₃, Phys. Rev. Lett. 2008, 101, 187201
2. Kadlec C. et al. Electromagnons in ferrimagnetic ε - Fe₂O₃ nanograin ceramics. subm. to Phys. Rev. B, arXiv:1305.3064v2

Multifunctional nanostructures of multiferroic materials are potential candidates for future spintronics, nanoelectronics, and high date storage system. Multiferroics exhibit simultaneous ferroelectricity as well as ferromagnetism. This will make them an ideal candidate for multifunctional devices.
In this direction, BiFeO₃ nanostructures, viz., nanoparticles as well nanowires have been synthesized. The synthesized nanostructures have been characterized for their structural, morphological, electric, dielectric and magnetic properties. The nanostructures possess rhombohedral structure with hexagonal phase. Magnetic study shows that synthesized nanostructures exhibit ferromagnetism at room temperature. Electrical study reveals that the synthesized nanostructures display polarization versus applied electric field loops. Magnetoelectric coupling coefficients, measured using dynamic lock-in technique, confirms that nanowires possess high magnetoelectric coupling coefficients as compared to nanoparticles. The observed multiferroic properties have been explained on the basis of nano-size of the synthesized nanostructures.

In multiferroic materials, most studies focus heavily on electrical and magnetic behaviours for obvious reasons related to practical application. The role of strain in these materials is often left uninvestigated, and often holds key information on the coupling behaviour in these complicated systems, with resonant ultrasound spectroscopy a versatile tool in investigating these phenomena. Resonant ultrasound spectroscopy has been used to examine the elastic and anelastic properties of ceramic samples from the BiFeO₃-CaFeO_2.5 (BCFO) and BiFeO₃-NdFeO₃, (BNFO) solid solutions. The BCFO literature shows that it is antiferromagnetic at room temperature (TN ~650 K, essentially identical to BFO, and BNFO). It has an ordered arrangement of dopant ions and oxygen vacancies, and is pseudo-tetragonal. Local strain fields at low doping levels are likely to be responsible for suppressing gradient coupling between order parameters and the cycloidal magnetic ordering typical of BFO, yielding weak ferromagnetism in BCFO. Furthermore, TN is observed to be essentially unrelated to composition in BCFO. While there is a high temperature loss peak in both the dielectric (~350-500 K) and acoustic data (~350-750 K), it appears to be uncorrelated with TN and is thus attributed to the onset of ionic conductivity. An additional low-temperature Debye-like peak in internal friction, Q^-1, is also observed, with a maximum near 100 K. This correlates very closely with a peak in dielectric loss, reported previously. Both low temperature loss features are consistent with movement of known oxygen vacancies, with a spectrum of relaxation times. Below TN, weak stiffening of the shear modulus, scaling as the square of the magnetic order parameter, m, indicates that coupling with strain, e, is biquadratic, lambda;e²m². This implies that conventional coupling of m with symmetry breaking shear strains (lambda;em²) is weak in BCFO. BNFO shows behaviour involving formation of antiferroelectric phases with varying dopant concentrations and temperatures, leading to highly dissipative behaviour at intermediate temperatures. However, as in BCFO, linear stiffening below T_N indicates lambda;e²m² coupling. This shows that the weakness of the coupling of m with symmetry breaking shear strains may be a common feature of BFO-related systems. An examination of spontaneous strains associated with BCFO, and the nature of the phase diagram, suggests that while coupling between order parameters relating to ferroelectricity and R-point tilting is favourable, coupling between tilting and cation/oxygen/vacancy ordering is unfavourable. There is only weak/negligible coupling between shear strain and antiferromagnetic ordering, which probably accounts for the very weak coupling of magnetic ordering with the multiple other order parameters in this system.

Multiferroic materials, showing simultaneous ferroelectric and magnetic ordering, exhibit unusual physical properties for new device applications as a result of the coupling between their dual order parameters. In particular, the use of such multifunctional materials can be foreseen in spintronics and memory multistate devices, addressed either electrically or magnetically.
In this frame, BiFeO₃ (BFO) is the most studied multiferroic system because both polar and magnetic orders coexist at room temperature. The local short-range magnetic ordering of BFO is G-type antiferromagnet, consisting in each Fe+3 spin surrounded by six antiparallel spins on the nearest Fe neighbors. The spins are not perfectly antiparallel, as there is a weak canting moment caused by the local magnetoelectric coupling to the polarization. However, a long-range superstructure consisting of an incommensurate spin cycloid of the antiferromagnetically ordered sublattices has been observed superimposed to this canting. Several experimental measurements, including the presence of a magnetic hysteresis loop in single crystals, suggested that at very low temperatures there could be a weakly ferromagnetic state.
In the last decade, an enhancement of magnetization in BiFeO₃ thin films produced by pulsed laser deposition (PLD) technique could be observed, due to their more disordered and polycrystalline structure.
The epitaxial nature of the films on the (100), (110) and (111) crystallographic directions was confirmed by XRD. The thickness of an individual BFO film oscillates between 100-200 nm.
In this work, epitaxial and polycrystalline BFO thin films were deposited by pulsed laser deposition on Si and SrTiO₃ single crystals with different orientations.
The phase and composition of the films were confirmed by X-ray diffraction and Energy Dispersive X-ray spectroscopy, respectively. The magnetization behavior in the temperature range 5 - 300 K has been studied in all deposited films. Magnetoresistance response (MR) has been measured as a function of temperature (in the same temperature interval) in the field range 0 - 70 kOe with a standard four-point technique. MR ratio for a given value of magnetic field H is defined as MR = [R(H)) - R(H=0)] / R(H=0) x 100. For T < 100 K, all the films display an high-field, non-saturating, negative magnetoresistance response together with a positive maximum at a magnetic field decreasing with measuring temperature. For higher temperatures, the field behavior of magnetoresistance changed into a parabolic shape, typical of ordinary magnetoresistance. Correspondingly, magnetic hysteresis loops changed from a highly ferromagnetic state to a much weaker ferromagnetic contribution superimposed to a diamagnetic phase. The magnetoresistance response will be interpreted in the light of the magnetic state.

The spiral spin structure in BiFeO₃ is an exquisite natural feature in this multiferroic material, but has detrimental consequences for room-temperature applications because it suppresses weak-ferromagnetic behaviour and inhibits the linear magnetoelectric effect [1-6]. Methods to control the spiral structure in bulk BiFeO₃ using chemical doping, electric fields and high pressures have received a great deal of attention recently [1-3]. As the cycloid has a characteristic periodicity of ~ 62 nm, it is also influenced by finite size and truncation effects in nanostructures of BiFeO₃ [4,5] and BiFe_0.5MnO_.5O₃ [1] at shorter length-scales. In this work, we discuss recent elastic neutron diffraction experiments on BiFeOnot;₃ and BiFe_0.5Mn₀₅O₃ nanoscale thin films which have shown the collapse of the cycloid and the rise of a canted antiferromagnetic component with a weak-ferromagnetic behaviour. Using atomistic simulations based on the classical vector spin approach, the current model for BiFeO₃ is extended to investigate the effect of nanostructure morphology and chemical substitution on the molecular fields that give rise to the cycloid. The spin correlation functions for the cycloidal and G-type antiferromagnetic order are calculated for a range of geometries, along with the resulting magnetic hysteresis loops. It is shown that the collapse of the cycloid is a necessary, but insufficient condition, for the appearance of the weak-ferromagnetic behaviour, and there are multiple routes to achieve this.
1. D. L. Cortie et al., Appl. Phys. Lett. 101, 172404 (2012)
2. M. Ramazanoglu et al., Phys. Rev. Lett. 107, 067203 (2011)
3. P. Rovillain, Nature. Mat 9, 9745 (2010)
4. W. Ratcliff II et al. , Adv. Funct. Mater. 21, 1567 (2011)
5. H. Bea et al. , Appl. Phys. Lett. 93, 072901 (2008).
6. J. Jeong et al. Phys. Rev. Lett. 108, 077202 (2012)

Multiferroic materials with simultaneous ferroelectric and magnetic order have drawn significant interest and provide a new fertile ground for the fundamental research due to their multifunctionality.1 Among them, multiphase composites in form of piezoelectric-magnetostrictive components have attracted particular interest because of their naturally stronger magnetoelectric effect (induction of magnetization by electric field or polarization by magnetic field). Most of these combinations use Pb based oxides (toxic for human and environment)2 for piezoelectric component, owing to their superior piezo-performance . In search of its alternative in non lead based piezoelectric materials, modified BNT (Bi0.5Na0.5TiO3) is reported to have excellent piezoelectric response.3 In this work we report our results on two phase particulate composite of CZFMO (Co0.6Zn0.4Fe1.7Mn0.3O4) (magnetic phase)4 and modified BNT (piezoelectric phase).
BNTKNNLTS-CZFMO [1-y](1minus;x)(Bi0.5Na0.5TiO3-xK0.47Na0.47Li0.06Nb0.74Sb0.06Ta0.2O3)- y(Co0.6Zn0.4Fe1.7Mn0.3O4) with x = 0.03, y = 0.1 has been prepared via solid state reaction route. Both the calcinations and sintering temperatures are optimized. XRD results confirmed the formation of the composite. The dielectric constant and tan δ loss of the sample are measured with varying temperature using impedance analyzer model HP 4192 A. Dielectric spectra show a ferroelectric to paraelectric transition around 590 K. Ferroelectric measurements have been performed using ferroelectric tester (Radiant Technologies Precision Premier II). Magnetic properties and magnetoelectric coupling behavior are also investigated in detail, for this composite.
References:
1C. W. Nan, M. I. Bichurin, S. Dong, D. Viehland, and G. Srinivasan, J. Appl. Phys. 103, 031101 (2008).
2Jian-ping Zhou, Hong-cai He, Zhan Shi, Gang Liu and Ce-Wen Nan, J. Appl. Phys.100, 094106 (2006).
3Amrita Singh, and Ratnamala Chatterjee, J. Am. Ceram. Soc. 96, 509 (2013)
4Arti Gupta, A. Huang, Santiranjan Shannigrahi, and Ratnamala Chatterjee, Appl. Phys. Lett. 98, 112901 (2011).

It has been of great interest of the microwave electronics community to integrate microwave passive devices, such as circulators, isolators, phase shifters, filters, etc. with semiconductor device platforms. Such an achievement would meet the demands of increasing systems integration concomitantly and enhancing performance and functionality, while concomitantly reducing device profile, volume, and weight.1 Previous attempts to grow ferrite thick films (micron scale) onto Si and GaAs failed in that the high temperatures required for the processing of low microwave loss ferrite materials resulted in the films&’ microwave performances degraded. Nowadays, there are new options for high power microwave semiconductor materials, which include the wide bandgap materials, e.g., GaN and Gadolinium Gallium Garnet (GGG), which have demonstrated advantages in power handling and high frequency operation.2
In our study, single-crystal films of Ba-Y type ferrites have been epitaxially grown on GGG (111) substrates from a molten-salt solution by vaporizing the solvent. A 5-50 µm thick film of Zn2Y (Ba2Zn2Fe12O22) or Co2Y (Ba2Co2Fe12O22) was successfully grown on a GGG substrate with an area of 1 cm2 area. The ferrite crystal constituents and anhydrous sodium carbonate are melted on a (111) cleavage surface of a GGG substrate at 1100 to 1250°C for 5-20 hrs. With evaporation of the solvent, single crystal films were gradually formed, indicating (001) orientation perpendicular to film surface. It was verified by X-ray Diffraction (XRD). Vibrating sample magnetometer (VSM) measurement presents a magnetic easy basal plane on the films. Furthermore, ferromagnetic resonance (FMR) measurements were performed to determine microwave characteristics of these films. The ferrite thick films on GGG can realize the integration of high performance ferrite microwave passive devices.
1. V. G. Harris, “Modern Microwave Ferrites”, IEEE TRANSACTIONS ON MAGNETICS, 48, NO. 3, (2012);
2. I. Zaquine, H. Benazizi, and J. C. Mage, J. Appl. Phys. 64, 5822 (1988).

Ferromagnetic shape memory alloys are multi-functional materials exploitable in new-concept smart devices thanks to their extraordinary properties (e.g. giant magnetoelastic, magnetocaloric, barocaloric effects) [1]. Nanoscale materials have recently drawn a lot of interest: they are promising for the realization of MEMS, micro/nano-actuators, energy harvester and micror/nano-refrigerators. On the other hand, nanolithography techniques are extensively employed to prepare submicrometer magnetic dots. In this frame, polystyrene nanosphere (PN) lithography has been recently exploited for magnetic thin films nanostructuring on large scale. Such a technique is presently considered a valid alternative to conventional e-beam lithography due to its low-cost and large covering area for a variety of applications (i.e. spintronics, magnetic sensing, ultrahigh-density magnetic recording).
In this work, novel results in arrays of magnetic nanostructures obtained by patterning NiMnGa thin films using polystyrene nanosphere lithography. Film having thickness of 75, 100, 200 nm, were grown by RF sputtering on a MgO substrate covered with a 50 nm thick Cr buffer layer. All the continuous films are ferromagnetic and martensitic at room temperature. Patterned films were obtained by assembling commercially available PN nanospheres monolayer on continuous thin films. (starting mean diameter ranging in the interval 200 divide; 800 nm, subsequently reduced by plasma matrix). Dot arrays were obtained by submitting the film to sputter etching with Ar+ ions (dots diameter 120 divide; 700 nm) [1].
Sample morphology was studied by means of SEM and AFM microscopy. Magnetic measurements were performed by high sensitivity magnetometry (Vibrating Sample Magnetometer in the temperature interval 70 - 400 K) in all patterned samples to study the martensite-austenite transformation and the magnetization processes. MFM microscopy was exploited to study the magnetic domain pattern (in the remanent and demagnetised state) in nanodots. The effect of patterning (i.e. dot/hole diameter and mutual distance) on the magnetic properties thin film will be discussed.
[1] Ll. Mañosa et al., Nature Materials 9, 478-481(2010).
[2] P. Tiberto, G. Barrera, L. Boarino, F. Celegato, M. Coïsson, N. De Leo, F. Albertini, F. Casoli, and P. Ranzieri, J. Appl. Phys. 113, 17B516 (2013)

Hexagonal REMnO3 (RE= rare earths) with RE=Ho-Lu, Y, and Sc, is an improper ferroelectric where the size mismatch between RE and Mn induces a trimerization-type structural phase transition, and this structural transition leads to three structural domains, each of which can support two directions of ferroelectric polarization. We reported that domains in h-REMnO3 meet in cloverleaf arrangements that cycle through all six domain configurations, Occurring in pairs, the cloverleafs can be viewed as vortices and antivortices, in which the cycle of domain configurations is reversed. Vortices and antivortices are topological defects: even in a strong electric field they won&’t annihilate. These ferroelectric vortices/antivortices are found to be associated with intriguing collective magnetism at domain walls, reflecting the multiferroic nature of vortices.
We have found that an intriguing, but seemingly irregular network of a zoo of multiferroic vortices and antivortices in h-REMnO3 can be neatly analyzed in terms of graph theory, and this graph theoretical analysis reveals the emergence of Z2×Z3 symmetry in the vortices/antivortices network. In addition, poling or self-poling due to a surface charge boundary condition induces global topological condensation of the network through breaking of the Z2 part of the Z2×Z3 symmetry. The opposite process of restoring the Z2 symmetry can be considered as topological evaporation.
It was also discovered that bound states of vortices and antivortices can form, and the force between a vortex and an antivortex can be attractive and remain constant with increasing separation, similar with what happens in a meson with one quark and one antiquark. We will touch upon the analogy of quark confinement in quantum chromodynamics with what happens in the bound states of vortices and antivortices.
The spontaneous trapping of topological defects in the process of undergoing a continuous phase transition is important to understand the early stage of universe after big bang. The so-called Kibble-Zurek mechanism was proposed for the trapping process of topological defects. It appears that the Kibble-Zurek mechanism is also responsible for the network formation of multiferroic vortices, and thus, hexagonal REMnO3 is a test bed for the development of this cosmos.

Strongly correlated transition metal oxides (TMOs) have been attracted much attention because of their huge external-field-induced response originating from the strong coupling between spin, charge, and lattice degrees of freedom. An illustrative example can be found in the perovskite manganite system, which is fascinating for having numerous gigantic properties, such as colossal magnetoresistance (CMR) with metal/ferromagnetic-insulator/antiferromagnetic phase transition. During the metal-insulator transition, the ferromagnetic metal and the charge ordering insulator phases often coexist in the state of electronic/spin domains in nano-scale. Nanostructuring strongly correlated TMOs is a frontier to deliver a new class of functional nano-materials. In order to investigate and mastermind these specific properties in spatially nano-confined structures, we report a new nano-fabrication technique of three-dimensional (3D) nanotemplate pulsed laser deposition (PLD) technique [1-4] by the combination of the PLD thin film growth technique with 3D-nanotemplates prepared by the nano imprint lithography technique. This technique enables us to produce nanostructures whose wall-width is well defined as the 10-nm scale on a large area while maintaining initial template positions and shapes exactly. In this study, the accurately wall-width-controlled and well aligned (La_0.275Pr_0.35Ca_0.375)MnO₃ (LPCMO) nanobox array structures, were successfully constructed. The hard X-ray photoemission spectroscopy revealed electrical and ferromagnetic properties with metal-insulator transition phenomena in nanobox array structures. The appearance of the satellite on the Mn 2p3/2 peak corresponding to the starting of the MIT was observed at ~203 K in nanobox array structures which was about 50 K higher than that in LPCMO film. This indicates the enhancement of magnetic transition temperature in the LPCMO nanoboxes which offers a way to tune physical properties in colossal magnetoresistive oxide. The 3D nanotemplate PLD technique can construct tunable wall-width or the diameter of box, and the distance between boxes down to 10 nm so that this technique will open up new way of not only tunable superior electronic/magnetic properties in correlated oxides, but also novel devices with the designed location of nano-materials, such as magnetically and/or temperature tunable photonic crystals taking advantage of the huge electronic/spintronics phase transition.
References [1] Nanotechnology 23 (2012) 485308. [2] Nanotechnology 22 (2011) 415301., [3] Appl. Phys. Express 5 (2012) 125203. [4] Jpn. J. Appl. Phys. 52 (2013) 015001.
References [1] Nanotechnology 23 (2012) 485308. [2] Nanotechnology 22 (2011) 415301., [3] Appl. Phys. Express 5 (2012) 125203. [4] Jpn. J. Appl. Phys. 52 (2013) 015001.

We report on the temperature dependent Raman spectroscopic studies of orthorombic distorted perovskite YCrO₃ in the temperature range of 20-300K. The single phase formation of polycrystalline samples has been confirmed by X-ray diffraction and Rietveld analysis. Temperature evolution of the peak positions of A_g modes (associated with the octahedral rotation in the unit cell) revealed anomalous phonon shift near Cr³⁺ magnetic ordering temperature (T_N ~ 142K). Additional phonon anomaly was identified at ~ 55-60K, reflecting the spin-reorientation transition, plausibly due to the change in Cr-spin configuration. Temperature dependence of DC magnetization measurement under field cooled and zero field cooled modes confirmed the T_N and spin-reorientation temperature. Moreover, the differential magnetic susceptibility versus temperature curve indicates a clear discontinuity near ~ 60K, which is in agreement with the Raman results. Magnetization isotherm recorded below T_N at 125K shows clear loop opening without magnetization saturation up to 20kOe, indicating the coexistence of antiferromagnetic interaction with weak ferromagnetic phase. Dielectric measurements showed the anomaly in dielectric constant and loss near T_N. The correlation between spin-lattice system and observed dielectric anomalies elucidate the spin-phonon coupling owing to the multiferroic phenomenon in YCrO₃.

We will present results obtained by linear dichroism in soft x-ray absorption (XAS-LD) and hard x-ray photoemission spectroscopy (HAXPES) on CaCuO₂/La_0.7Sr_0.3MnO₃ (CCO/LSMO) superlattices (SLs). This is a new system that can be considered as the parent compound for superconducting/magnetic heterostructures previously reported in literature[1,2]. In fact, CCO is the simplest undoped antiferromagnetic parent compound of cuprate superconductors. By comparison with the case of LSMO thin films we elucidate the role of the interface reconstruction in cuprate/manganites systems. In particular, XAS-LD data for CCO/LSMO SLs grown in strongly oxidizing conditions show increased 3dz2-r2 orbital occupation of Cu and Mn valence electrons due to extra apical oxygen ions at the interface. The apical oxygen localizes extra holes in the optimally doped LSMO block. This, in turn, favors the formation of the AF phase that is seen in the magnetotransport properties [3], as in the case of the dead-layer in single layer LSMO thin films [4].
The interface reconstruction mediated by apical oxygen formation can also explain the variations in the Mn 3s exchange splitting measured by HAXPES. In LSMO thin films an increase of the Mn 3s splitting is observed for samples with reduced valence and covalence [5]. The Mn3s splitting in CCO/LSMO SLs increases with the enhanced Mn4+ content and with the expected AF insulating phase in the dead-layer of LSMO. The effect of hole localization in the O 2p orbital was reported in the literature to explain the exchange splitting in mixed valence manganites with different doping concentration [6].
[1] J. Chakhalian, et al. Nature Physics 2, 244 (2006)
[2] J. Chakhalian, et al. Science 318, 1114 (2007)
[3] Nan Yang, et al. J. Appl. Phys. 112, 123901 (2012)
[4] A. Tebano, et al. Phys. Rev. Lett. 100, 137401 (2008)
[5] C. Schlueter, et al. Phys. Rev. B 86, 155102 (2012)
[6] V.R. Galakhov, et al. Phys. Rev. B 65, 113102 (2002)

Distortions of BO6 octahedra in perovskite structure (ABO3) can strongly affect magnetic and electronic properties of oxides. The tilted structure is also coupled to stoichiometry and epitaxial strain in thin films. For example, LaCoO3 (LCO) thin films, unlike the bulk, show ferromagnetic ordering at low temperatures. However, little is known about the behavior of the tilted octahedral structure as a function of film thickness. Recently, May et al. have reported that the octahedral rotations in (LaNiO3)n/(SrMnO3)m superlattice depend on the length of the LaNiO3 and SrMnO3 blocks.[1]
We study the rotations of BO6 in LCO thin films of different thicknesses on SrTiO3 and LSAT (formula goes here) substrates. LCO thin films were grown by pulsed laser deposition. X-ray photoelectron spectroscopy studies of the films have demonstrated that Co 3p edges shift up to 2 eV for 15 u.c. and 20 nm films, indicating possible presence of 2D electron gas at the interface. High angle annular dark field (HAADF) and annular bright filed (ABF) images can reveal strain and octahedral tilt behavior with unit cell resolution. [2]
Octahedral tilts were examined for films of different thicknesses using ABF imaging. We found that from 15 u.c.thin film had fully developed tilted structure, with indications of tilt starting from the last 2 unit cell of STO and gradually increasing into LCO; these results were also supported by in-plane lattice spacing modulations observed in HAADF images. However, similar behavior was not observed in the 5 u.c. film, which is apparently not tilted. EELS mapping at the interface reveals some Ti/Co intermixing, which was however identical in the two films, suggesting that the tilts are responsible for the difference in properties. Density Functional Theory calculations of the system, as well as implications with respect to interface conductance will also be discussed.
* Research supported by the U.S. Department of Energy (DOE), Basic Energy Sciences (BES), Division of Materials Sciences and Engineering, and through a user project supported by ORNL&’s Shared Research Equipment (ShaRE) User Program, which is also sponsored by DOE-BES.
[1] S.J. May et al., Phys. Rev. B 83 153411 (2011).
[2] Y. Kim et al. Advan. Mater. 25 2497 (2013).

Magnetoelectric composites promise much higher magnetoelectric coupling coefficients than intrinsic multiferroics, at least at room temperature. 2-2 composites are the lead choice for sensor applications due to the very high coupling coefficients in resonance. They are well enough understood that