Symposium EQ10—Multiferroics and Magnetoelectrics

Single phase multiferroic Bismuth Ferrite (BFO) Nanoparticles (NPs) are potential materials for spintronic devices, photovoltaics, photocatalysis, and memory storage due to their advantageous ferro-electromagnetic properties at room temperature and narrow band gap. We outline the synthesis of BFO utilising pulsed laser ablation technique in liquid (PLAL). Thorough characterization of the ablated NPs including powder X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), high-angle annular dark-field imaging (HAADF), scanning transmission electron microscopy (STEM), and magnetometry have been performed. Laser ablated NPs have spherical morphology and consist of two kinds of NPs: primary and secondary. The sizes of primary particles are around 5 nm and of secondary particles are around 20 nm. After post-thermal treatment at 500 °C for 5 minutes, the NPs are in the size range of 18 ± 5 nm. These NPs exhibit single phase, rhombohedral (R3c) crystal symmetry and are highly crystalline. The HAADF and STEM images unveil a homogeneous elemental distribution and a single ferroelectric domain, respectively for the ablated BFO NPs. Laser ablated BFO NPs show an increase in magnetic moment as compared to sol-gel synthesized BFO NPs (~50 nm) which may be due to the large number of uncompensated spins at the surface in smaller NPs. The laser ablation technique can tailor the NP sizes at low cost. It is quite reproducible approach.

SrTiO₃ (STO) two-dimensional electron gases (2DEGs) can be formed by depositing reactive metals such as Al onto STO single crystals: the metal oxidizes, forming oxygen vacancies in the STO and thus doping it in electrons^[1]. Such 2DEGs can then be used to interconvert spin and charge currents with great efficiency^[2]. In this presentation we will show that the metal oxide (e.g. AlOx) thus formed can be combined with ferromagnetic and heavy-metal-based ultrathin films to generate various types of magnetization configurations with in-plane or out-of-plane anisotropy, depending on their thicknesses. For specific compositions, these multilayers harbor magnetic bubbles, as imaged by magnetic force microscopy (MFM), X-ray photoemission electron microscopy (XPEEM), and NV-center microscopy, that likely have a skyrmionic character, due to the expected Dzyaloshinskii-Moriya interaction caused by the strong broken inversion symmetry. We will finally discuss how the Rashba spin-orbit coupling present in the 2DEG may be used to generate spin-orbit torques to manipulate spin textures.

[1] L. M. Vicente-Arche, S. Mallik, M. Cosset-Cheneau, P. Noël, D. C. Vaz, F. Trier, T. A. Gosavi, C.-C. Lin, D. E. Nikonov, I. A. Young, A. Sander, A. Barthélémy, J.-P. Attané, L. Vila, M. Bibes, Phys. Rev. Materials 2021, 5, 064005.
[2] D. C. Vaz, P. Noël, A. Johansson, B. Göbel, F. Y. Bruno, G. Singh, S. McKeown-Walker, F. Trier, L. M. Vicente-Arche, A. Sander, S. Valencia, P. Bruneel, M. Vivek, M. Gabay, N. Bergeal, F. Baumberger, H. Okuno, A. Barthélémy, A. Fert, L. Vila, I. Mertig, J.-P. Attané, M. Bibes, Nature Materials 2019, 18, 1187.

This project received funding from the ERC Advanced grant “FRESCO” n° 833973 and the ERA-NET QUANTERA European Union’s Horizon H2020 project “QUANTOX” under Grant Agreement No. 731473.

●Introduction of magnetic materials, ferro/piezoelectric materials, and magnetoelectric materials.
●Characterization of magnetic materials, ferro/piezoelectric materials, and magnetoelectric materials.
●Magnetoelectric antennas from VLF to UHF and their applications

●ME antenna: State of the art
●ME antenna: Design and fabrication
●ME antenna: Measurement and result
●ME antenna: Further works and discussion

●Chu’s theory in radiation of electrically small antennas
●Roles of mechanical, magnetic and magnetoelectric materials in electrically small antennas
●Full-wave modeling of piezoelectric and magnetoelectric effects in antennas
●Examples of electrically small antennas based on mechanical, magnetic and magnetoelectric effects

●Introduction to phase-field models for ferroic materials and devices
●Challenges for developing phase-field model for simulating ME antennas
●Results on phase-field simulations of ME antennas
●Related examples: Phase-field simulations of acoustically mediated spintronic emitters in millimeter-wave and terahertz range

We will first discuss the general connection between broken symmetries and emergnet phenomena, especially magnetoelectricty in multiferroics in the framework of symmetry operational similarity (SOS). As a specific example of new phenomena associsated with SOS, we will discuss toroidal magnetoelectricity in chiral and polar BaCoSiO4.

Among the large number of correlated oxide materials, there exists a small set of materials which exhibit multiple order parameters; these are known as multiferroics, particularly, the coexistence of ferroelectricity and some form of ordered magnetism (typically antiferromagnetism). The scientific community has been able to demonstrate electric field control of both antiferromagnetism and ferromagnetism at room temperature. Current work is focused on ultralow energy (1 attoJoule/operation) electric field manipulation of magnetism as the backbone for the next generation of ultralow power electronics. We are exploring many pathways to get to this goal. In this talk, I will describe our progress to date on this exciting possibility. My specific focus will be on the materials physics behind getting to 100mV switching of magnetism (and ferroelectricity).

In complex oxides, charge-ordered states involve a complex interplay between charge, spin, orbital and lattice degrees of freedom, and modulate important macroscopic phenomena such as magnetic order, metal-insulator transitions and colossal magnetoresistance. The intra-unit-cell arrangement within charge order superlattices can lead to even richer physics by breaking additional symmetries and allowing novel couplings. In half-doped manganites, for instance, charge order can reside on the sites (site-centered) or on the bonds (bond-centered), with each possibility leading to a distinct orbital and magnetic order arrangement. Previous theroetical work also predicts a third possibility: the formation of a mixed (or intermediate) state combining both site and bond orders [1]. This state leads to the formation of finite electrical dipoles within each supercell and is therefore predicted to be polar and possibly multiferroic. However, determining if such a state exists and distinguishing between various charge-ordered textures remain a challenge.

Here we demonstrate that atomic-resolution cryogenic scanning transmission electron microscopy (cryo-STEM) enables the visualization of charge-ordered states in manganites through the lens of the complex, picoscale lattice distortions that entwine with electronic transitions. In the model charge-ordered system Nd_1/2Sr_1/2MnO₃, we addresse the longstanding question of whether charge order resides on the sites or on the bonds. In addition to observing that the system hosts site-centered charge ordering, we discover the nanoscale coexistence of an intermediate state which mixes site and bond orders. In agreement with the theroetical proposals, we confirm that this intermediate state breaks inversion symmetry. Finally we show that nonlinear couplings of distinct lattice modes locally control the selection between these competing ground states [2].

[1] Efremov, D. V., Van Den Brink, J., & Khomskii, D. I. (2004). Bond-versus site-centred ordering and possible ferroelectricity in manganites. Nature Materials, 3(12), 853-856.

[2] El Baggari, I., Baek, D. J., et al. (2021). Charge order textures induced by non-linear couplings in a half-doped manganite. Nature Communications, 12(1), 1-7.

Applications in advanced, beyond CMOS computing are driving researchers and industry alike to explore a broader set of materials than ever before. While industry has maintained, in some form, their ability to continually meet the demands of Moore’s law by increasing the number of transistors on chip, largely in part from efforts in scaling and size reduction of the transistors themselves and the associated circuit area, the same cannot be said for energy scaling. In the last fifteen years, scaling has deviated from Dennard’s trend which states that the power density of circuits stays roughly the same as transistors get smaller, assuming, concurrent voltage reductions are made. Herein lies the challenge – while we have reduced transistor and circuit size, voltages of operation have not been scaled at the same rate. As a result, power and energy dissipation now stands as one of the most pressing challenges for advanced nanoelectronics. This has motivated renewed attention to a wide variety of routes to reduce the voltage of operation of next-generation logic. Such approaches have thrust materials – including multiferroics and magnetoelectrics – back into the mix as candidates for beyond CMOS computing.

Despite considerable research on multiferroic and magnetoelectric materials in the last decade and advances in our ability to synthesize, control, characterize, and fabricate these materials, the requirements of beyond CMOS computing are already pushing these materials to their limits. Here, we will explore recent efforts to reduce the operating voltages of multiferroic and magnetoelectric devices including exploring both new and old materials in ever decreasing sizes. We will focus on thin-film versions of magnetoelectric composite structures based on piezoelectric/ferroelectric materials. For example, we will explore thin film ferromagnet/relaxor heterostructures and routes to quantify magnetoelectric coupling in small scale devices. Specifically, in Fe_0.5Rh_0.5/(0.68)PbMg_1/3Nb_2/3O₃-(0.32)PbTiO₃ (PMN-PT) heterostructures, we will demonstrate routes towards 100 mV actuation and magnetoelectric coefficients similar to those observed in other composite structures. In turn, we will discuss the implications of thin-film geometries for such applications and how substrate clamping impacts the response of materials. For example, we will explore how different types of relaxors (e.g., PMN vs. BaZr_0.5Ti_0.5O₃) respond differently to substrate constraints and thus provide an impetus for the use of some relaxors over others in thin-film geometries. At the same time, we will explore how releasing such materials from the substrate – thereby producing free-standing versions of them – further changes their response to applied stimuli and could provide a pathway to the desired effects. Finally, we will explore the design of novel thin-film relaxor materials. We will look at how thin-film heterostructuring (namely as superlattices) could provide a novel route to improved relaxor behavior. To end, we will discus where these ideas could lead us next and what metrics can be met using these materials and methods.

Multiferroic materials and nanocomposites offer the promise of electrically-controlled magnetism and a path towards magnetoelectric memory and logic devices. Multiferroicity may be obtained in a single phase material or in a composite material consisting of a magnetic phase and a ferroelectric phase coupled by strain. Most research on single phase multiferroics has centered on BiFeO3, but we show here that room temperature multiferroicity can be produced in a single phase orthoferrite as a result of antisite defects. Ferroelectricity is obtained in thin films of Y-rich YFeO₃, yttrium orthoferrite (YFO), a perovskite-structured canted antiferromagnet. The ferroelectricity is attributed to Y_Fe antisite defects which facilitate a non-centrosymmetric distortion. This mechanism is predicted to work analogously for other rare earth orthoferrites, with a dependence of the polarization on the radius of the rare earth cation. We then demonstrate the growth of two-phase epitaxial nanocomposites consisting of YFO and a magnetic spinel, CoFe2O4 (CFO). A nanocomposite consisting of pillars of CFO embedded epitaxially in a YFO matrix exhibits both robust ferroelectricity and ferrimagnetism at room temperature, as well as a strain-mediated magnetoelectric coupling effect. This work expands the range of materials available for multiferroic and magnetoelectric applications.

Magnetic domain formation reveals fascinating physics and is of great relevance for technological applications. Recent advances in thin film devices adopting magnetic films as sensing layers offer promising routes for sensing ultra-low magnetic signals, e.g. with magnetoelectric (ME) cantilever sensors or surface acoustic wave (SAW) devices. As the sensors are operated by external stimuli of various frequencies, the complex physics of magnetic domain and domain wall activity at these modulation frequencies are one of the most important factors that determine the performance of the magnetic field sensors.
In-operando magnetic domain observation sheds light on the irreversible and hysteretic magnetization changes due to domain nucleation, domain wall resonances, domain wall flexure modes, precessional magnetization effects, and spin-wave like phenomena. Using time-resolved magneto-optical Kerr effect microscopy with time resolutions down to picoseconds, we show the effect of magnetic domains on the noise performance of ME, ΔE-effect, and SAW sensors. With the spatial and temporal domain analysis during sensor operation together with complementary electrical measurements, the micromagnetic origins are determined.
We identify specific magnetic domain wall processes as sensor-intrinsic noise sources. Magnetic noise originating from magnetic domain wall activity is the most relevant figure-of-merit for improving sensor performance. Even minimal magnetic domain activity restricts the achievable limit of detection. On various sensor specific examples, we demonstrate that understanding the magnetic domain physics and actively controlling magnetic domain behavior is key to improving sensor performance. The magnetic domain wall activity is directly related to magnetic losses. The presented data prove the significance of micromagnetic processes for real-world applications. Strategies for the elimination of domain wall activities and magnetic losses will be discussed.
We acknowledge funding through DFG CRC 1261 “Magnetoelectric Sensors: From Composite Materials to Biomagnetic Diagnostics”.

[1] P. Durdaut, E. Rubiola, J.M. Friedt, C. Müller, B. Spetzler, C. Kirchhof, D. Meyners, E. Quandt, F. Faupel, J. McCord, R. Knöchel, M. Höft, Journal of Microelectromechanical Systems 29, 1347 (2020)
[2] N.O. Urs, E. Golubeva, V. Röbisch, S. Toxværd, S. Deldar, R. Knöchel, M. Höft, E. Quandt, D. Meyners, J. McCord, Physical Review Applied 13, 024018 (2020)
[3] P. Hayes, M. Jovičević Klug, S. Toxværd, P. Durdaut, V. Schell, A. Teplyuk, D. Burdin, A. Winkler, R. Weser, Y. Fetisov, M. Höft, R. Knöchel, J. McCord, E. Quandt, Scientific Reports 9, 16355 (2019)
[4] M. Jovičević Klug, L. Thormählen, V. Röbisch, S. Salzer, M. Höft, E. Quandt D. Meyners and J. McCord, Applied Physics Letters 114, 192410 (2019)
[5] A. Kittmann, P. Durdaut, S. Zabel, J. Reermann, J. Schmalz, B. Spetzler, D. Meyners, N.X. Sun, J. McCord, M. Gerken, G. Schmidt, M. Höft, R. Knöchel, F. Faupel and E. Quandt, Scientific Reports 8, 278 (2018)
[6] R.B. Holländer, C. Müller, M. Lohmann, B. Mozooni and J. McCord, Journal of Magnetism and Magnetic Materials 432, 283–290 (2017)
[7] J. McCord, Journal of Physics D: Applied Physics 48, 333001 (2015) <div id="gtx-trans" style="position: absolute; left: 124px; top: 160px;"> <div class="gtx-trans-icon"> </div> </div>

Spin orbital torques (SOT) which are generated by a spin current are key to magnetic switching in spintronic applications. These SOTs can be generated by a spin source material via intrinsic mechanisms such as the spin Hall effect and can switch magnetizations without the need for external magnetic fields. Conventionally, the spin polarizations of these SOTs are limited due to the symmetry of the material allowing only for efficient in-plane magnetic switching. Unconventional SOTs arising from novel spin current polarizations however, have the potential to switch an arbitrary magnetization direction. Such SOTs have been reported in lower symmetry materials such as antiferromagnetic materials and transition metal dichalcogenides (TMDs) [1,2]. Here, we report the control of unconventional spin torques in epitaxial IrO2 thin films. IrO2 films with (001) and (111) orientation were grown by RF magnetron sputtering with a Py ferromagnetic overlayer and characterized using spin torque ferromagnetic resonance (ST-FMR). We find that the (001) orientation heterostructures have a conventional spin Hall conductivity (SHC) consistent with theoretical calculations and other experimental results [3]. The (111) orientation heterostructures show both conventional and unconventional spin torques consistent with trends predicted in our theoretical calculations. When charge current is applied along either of the crystallographic axes ([1-10] or [-1-12]), we see a noticeable unconventional contribution to the ST-FMR angular dependance from spins polarized along the charge current direction. We attribute these unconventional spin torques to the lower symmetry resulting from triclinic distortions in the (111) films. This work could provide further understanding of unconventional spin torques as well as controlling SOTs via epitaxial design.
[1] Nan, T., Quintela, C. X., Irwin, J., Gurung, G., Shao, D. F., Gibbons, J., Campbell, N., Song, K., Choi, S. Y., Guo, L., Johnson, R. D., Manuel, P., Chopdekar, R. V., Hallsteinsen, I., Tybell, T., Ryan, P. J., Kim, J. W., Choi, Y., Radaelli, P. G., Ralph,D.C., Tsymbal, E. Y., Rzchowski, M. S., & Eom, C. B. Nature Communications, 2020, 11(1)
[2] MacNeill, D., Stiehl, G. M., Guimaraes, M. H. D., Buhrman, R. A., Park, J., & Ralph, D. C. Nature Physics, 2017, 13(3), 300–305
[3] Bose, A., Nelson, J. N., Zhang, X. S., Jadaun, P., Jain, R., Schlom, D. G., Ralph, D. C., Muller, D. A., Shen, K. M., & Buhrman, R. A. ACS Applied Materials and Interfaces, 2020, 12(49), 55411–55416
This work was supported by a Vannevar Bush Faculty Fellowship (N00014-20-1-2844).

Recent studies on oxide spintronics rely exceedingly on the efficient spin-charge interconversion property of the two-dimensional electron gases (2DEGs) possessing a finite Rashba spin-orbit coupling. SrTiO₃-based 2DEGs are usually generated by the deposition of epitaxial oxides like LaAlO₃ or of reactive metals such as Al [1, 2]. Such 2DEGs are highly promising for the interconversion between charge and spin currents through the direct and inverse Edelstein and spin Hall effects. Here, we compare the formation and properties of 2DEGs generated in SrTiO₃ by the growth of Al, Ta, and Y ultrathin films by dc magnetron sputtering [3]. Both in-situ and ex-situ X-ray photoelectron spectroscopy studies revealed the quantification of reduced Ti states associated with 2DEG formation, their reoxidation by exposure to the air, and the transformation of the metal into its binary oxides. The carrier densities are extracted from magnetotransport measurements. Finally, we investigate the spin-charge conversion as a function of gate voltage by performing spin-pumping ferromagnetic resonance experiments on similar metal/SrTiO₃ samples with a capping of an extra NiFe layer. The trends for the spin to charge conversion efficiency for different metal/SrTiO₃ systems provide a systematic study as a function of the carrier density and the transparency of the metal oxide tunnel barrier [3]. Our results strongly influence the path to choose the correct metal/SrTiO₃ interface to achieve higher spin to charge conversion for future spin-based information readout e.g. Magneto Electric Spin-Orbit (MESO) logics proposed by Intel corporation.

[1] A. Ohtomo and H. Y. Hwang, Nature (London) 427, 423 (2004).
[2] T. C. Rödel et al., Adv. Mater. 28, 1976 (2016).
[3] L. M. Vicente-Arche, S. Mallik, et al., Phys. Rev. Mater. 5, 064005 (2021).

Acknowledgments: The authors acknowledge support from the ERC Advanced Grant No. 833973 “FRESCO,” and Intel’s Science and Technology Center – Feinman.

Multiferroic BiFeO₃ possesses a rich phase diagram of metastable phases that allows strain tuning of its room-temperature multiferroic properties in thin-film form. In particular, at large compressive strain a supertetragonal phase with giant polarization can be stabilized alongside the more common rhombohedral structure. Although BiFeO₃ in this mixed-phase regime is recognized as highly technologically relevant for multifunctional device integration, little is known about its ferroelectric state during the thin-film synthesis. Here, we demonstrate the robustness of polarization in ultrathin, compressive-strained BiFeO₃ single layers and heterostructures during epitaxial thin-film growth. Using in-situ optical second harmonic generation (ISHG), we explore the emergence of ferroelectric phases at the strain-driven morphotropic phase boundary of BiFeO₃ in the ultrathin limit. Strikingly, we find that the epitaxial films grow in the ferroelectric tetragonal (T-) phase without exhibition of a critical thickness. The onset of monoclinic distortion of this T-phase and subsequent appearance of coexisting phases characteristic of the morphotropic phase boundary are confirmed only to develop post deposition and upon sample cooling. The robustness of polarization in the high-temperature T-phase against depolarizing-field effects is furthermore demonstrated during the growth of capacitor-like (metal|ferroelectric|metal) heterostructures. Our results show that strain-driven T-phase stabilization in multiferroic BiFeO₃ yields a prominent candidate material for realizing ultrathin ferroelectric devices with a well-defined polarization state.

A fascinating consequence of electron correlation is the emergence of a wide range of quantum phases including superconductivity in perovskite oxides such as cuprates and ruthenates. In partiular, ruthenates are difficult to grow in molecular beam epitaxy (MBE) due to the ultra-low vapor pressure of Ru. Additionally, Ru has high electronegativity which further makes it difficult for achieving complete oxidation, Ru → Ru⁴⁺ state under standard MBE conditions. The former issue has been dealt by supplying Ru using the electron-beam source whereas the oxidation issue is addressed by using highly reactive ozone. The use of electron-beam and ozone has adverse consequences on the stability of Ru beam-flux, which is critical for obtaining electronic-grade materials. Here, we show a novel solid-source metal-organic MBE approach that circumvents these issues by supplying a Ru contaning metal-organic compound as a solid source of Ru for the growth of RuO₂.
Single crystalline, epitaxial RuO₂ films with scalable thicknesses were grown on a variety of substrates including r-Al₂O₃ and TiO₂ (110), (101), (100), and (001) as evident from high-resolution X-ray diffraction and scanning transmission electron microscopy (STEM). RuO₂ films with rocking curve full-width-half-maxima as low as 0.05^o were obtained on TiO₂ (110) at substrate temperature of 300°C. Atomic force microscopy revealed atomically smooth film surfaces with a root mean square (rms) roughness value ~ 3 Å.
All films revealed metallic behavior down to 1.8 K with room temperature resistivities being comparable to bulk (35 μΩ-cm) for films as thin as 10 nm. The effect of film thickness, orientation, strain, and defects such as oxygen vacancies on the electrical properties will be discussed.
Finally, we also show the versatility of this novel MBE technique by demonstrating the growth of epitaxial and atomically smooth (001) platinum (Pt) films on SrTiO₃ (001). These films exhibited room-temperature resistivity of ~ 15 μΩ-cm, which is again comparable to that of the bulk single crystals. Importantly, these films showed a residual resistivity ratio (RRR) of 27, which is the highest reported value among Pt films grown to-date. We argue that the solid source MOMBE approach may potentially open up new physical vapor deposition (PVD) pathways (not just MBE) for “stubborn” materials such as delafossites, iridates, tungstates which contain elements that have low vapor pressures and are hard to oxidize in low pressure PVD approaches.

Spin-to-charge conversion has been designated by Intel as a viable solution for beyond-CMOS computing elements [1], with very favourable scaling laws. In this context, ferroelectric Rashba semiconductors (FERSCs) provide a way for the non-volatile electric control of spin-to-charge conversion, of great importance for such computing elements.

The parent compound of FERSCs is germanium telluride (GeTe), a CMOS-compatible semiconductor characterized by a giant Rashba-type spin-orbit coupling at room temperature, arising from the symmetry breaking induced by ferroelectric displacement. Importantly, calculations revealed that the spin orientation of each Rashba sub-band is inverted upon ferroelectric polarization reversal [2]. Such prediction has been already demonstrated experimentally in epitaxial GeTe(111) thin films on silicon, by our spin and angle resolved photoemission measurements on two surfaces with inward or outward spontaneous polarization [2]. Thus, FERSCs allow for ferroelectric control of the spin-resolved band structure in semiconductors, with intriguing implications on the spin transport properties.

Here we demonstrate the non-volatile, ferroelectric control of spin-to-charge conversion at room temperature in GeTe epitaxial films [3]. First, we demonstrate the feasibility of the ferroelectric switching, despite the high density of free carriers. The switching is induced by voltage pulses applied to a gate electrode, while the readout of the written state is performed by electro-resistive measurements of the GeTe/metal junction. Using spin-pumping experiments, we measured a sizeable spin-to-charge conversion efficiency comparable to Pt. We show that the sign of the generated charge current is reversed for two opposite ferroelectric states controlled by a gate. Density functional theory calculations reveal the consistency of these observations with spin Hall effect in thin GeTe films.

Our results open a route towards devices combining spin-based logic and memory integrated into a silicon-compatible material, where ferroelectricity can be employed as a state variable to tune the spin-to-charge conversion.

C.R. acknowledges the project TWEET, grant no. 2017YCTB59 funded by MIUR.
Work supported by the ERC Advanced Grant No. 833973 “FRESCO”'

[1] S. Manipatruni et al., Nature 565, 35–42 (2019)
[2] D. Di Sante et al., Adv. Mater. 25, 509-513 (2013)
[3] C. Rinaldi et al., Nano Letters, (2018); DOI: 10.1021/acs.nanolett.7b04829
[4] S. Varotto et al., arXiv:2103.07646 (2021)

Contact-less biomagnetic sensing constitutes the next frontier for advanced healthcare, creating novel diagnostic abilities using multichannel magnetocardiography (MCG) and magnetoencephalography (MEG) either as a single source of information for rapid patient screening or as fusion with established methods such as electrocardiography and electroencephalography as a source for additional information. The main obstacle towards clinical biomagnetic diagnosis using magnetic imaging techniques is the lack of readily applicable sensor technology offering extremely low magnetic noise floors; realtime MCG measurements demand for a limit of detection (LOD) lower than 10 pT/√Hz, reaching below 100 fT/√Hz enables even MEG signal acquisition. Such extremely minute amplitudes which are six to seven orders lower than earth’s permanent magnetic field, demand lowest noise sensor technology as the low frequency signal regime below about 500 Hz is strongly affected by 1/f-noise.

In the presented work the piezoelectric (PE) phase of thin film magnetoelectric (ME) composites is actively excited, thus exploiting the converse ME effect [1], remedying shortcomings of the direct ME effect. The experimental work involves bulk micromachined thin film cantilever composites of mesoscopic scale (25mm x 2.5mm x 0.35 mm) of which high frequency mechanical resonances at about 500 kHz are studied. These resonances show high mechanical quality factors of about Q≈1000. This mechanical oscillation, being rigidly coupled into the magnetostrictive (MS) material phase leads to a voltage induced in a pickup coil surrounding the sensor composite. This converse ME voltage response with respect to small external fields shows high sensitivities in the order of kV/T and linearity up to a field magnitude of several µT, no external magnetic driving field is required.

The signal components of MCG lie in the range from 3 to about 150 Hz with vanishingly small amplitude < ~ 100 pT, this regime is targeted by this research. The entirely passive readout combined with low power requirements makes the presented ME sensor system suitable for integration into sensor arrays as anticipated for future biomagnetic imaging purposes. Using single layer MS FeCoSiB material, the measured noise floor in the low frequency regime shows an abrupt increase as soon as an excitation threshold of about 200 mV is reached. Intense magnetization activity in the MS phase poses the dominant source of noise, practically limiting the LOD at 10 Hz to about 70 pT/√Hz [1]. Using sophisticated magnetic layer systems such as exchange bias[2] enables to effectively lower the magnetically dominated noise, even at high drive levels, while maintaining high magnetic sensitivities.

In this contribution, we demonstrate the performance of electrically modulated thin film ME composites, using magnetron sputtered exchange biased, amorphous FeCoSiB films driven by aluminum nitride. This approach allows composite magnetoelectrics to detect low DC as well as AC magnetic fields of amplitudes down to ~ 10 pT/√Hz @ 10 Hz without the requirement of an external magnetic AC or DC bias field. In a human patient trial conducted in a magnetically shielded room, using averaging it was even possible to perform an MCG recording.

Funding via DFG, SFB1261 “Magnetoelectric Sensors: From Composite Materials to Biomagnetic Diagnostics” is gratefully acknowledged.
[1] P. Hayes, M. Jovičević Klug, S. Toxværd, P. Durdaut, V. Schell, A. Teplyuk, D. Burdin, A. Winkler, R. Weser, Y. K. Fetisov, M. Höft, R. Knöchel, J. McCord, E. Quandt, Converse Magnetoelectric Composite Resonator for Sensing Small Magnetic Fields. Scientific reports. 9, 16355 (2019)
[2] M. Jovičević Klug, L. Thormählen, V. Röbisch, S. D. Toxværd, M. Höft, R. Knöchel, E. Quandt, D. Meyners, J. McCord, Antiparallel exchange biased multilayers for low magnetic noise magnetic field sensors. Appl. Phys. Lett. 114, 192410 (2019)

In this work, we use molecular dynamics simulations to investigate the polarization switching dynamics in the single domain and 180^o bidomain (i.e., including the preexisting domain wall) models in BaTiO₃ and KNbO₃. In a single domain study, for both materials, we observed that the hysteresis loop is essentially no-existent in the highest temperature non-cubic phase. We attribute this behavior to the observation of spontaneous local polarization fluctuations leading to the elimination of the nucleation barrier. Interestingly, in the case of the case of the bidomain structure, while we observe domain migration driven by the electric field, at high fields new domain nucleation is severely suppressed by comparison with the single domain simulations. This behavior is explained by the suppression of simulation dimension fluctuations due to the presence of the two domains. We give further credence to this conjecture by demonstrating that artificial suppression of the simulation cell dimension fluctuations in the case of a single domain switching also suppresses new domain nucleation.

The newly demonstrated very low frequency (VLF) magnetoelectric (ME) antennas [1] based on two-phase ME heterostructure have shown their capability and advantages in device miniaturization, ultra-sensitive magnetic field detection and communication in harsh environment. In this work, the authors utilized the non-linear electric field versus strain response of electrostrictive phase to achieve frequency mixing effect and intermodulation in the ME antenna. The proposed modulation scheme required simply an electrically applied superposition of carrier signal and baseband signal. To make the experimental demonstration, 5 layers of highly magnetostrictive Metglas foil were stacked and epoxy-bonded on both sides of a PMN-PT slab with a dimension of 50×5×0.2 mm, forming a two-phase ME heterostructure. The ME antenna had its maximum ME coefficient under 5.7 Oe DC magnetic field bias and the corresponding resonance frequency was 28. 73 kHz. With a 30 Vpp carrier signal at 28.73 kHz and a baseband signal at 5 kHz with various amplitudes applied across the PMN-PT slab, two modulated frequency components at 23.73 kHz and 33.73 kHz were observed in picked-up near field signal. The analog modulation depth can be controlled by the amplitude of the baseband signal. The digital modulation test was also performed with a binary digit sequence of “0101…” at a data rate of 5 kbps as baseband signal, the picked-up modulated signal was successfully demodulated by an on-board demodulator, which indicated the potential application of high data rate communication using the proposed modulation scheme and PMN-PT material-based VLF ME antenna.

[1]. Dong, Cunzheng, Yifan He, Menghui Li, Cheng Tu, Zhaoqiang Chu, Xianfeng Liang, Huaihao Chen et al. "A portable very low frequency (VLF) communication system based on acoustically actuated magnetoelectric antennas." IEEE Antennas and Wireless Propagation Letters 19, no. 3 (2020): 398-402.

Hysteresis is a limiting factor for the actuation accuracy of PbZr_1-xTi_xO₃ (PZT) ceramics and thin films. Explanation of hysteresis for these materials using phenomenological models such as Rayleigh model is a common practice. On the other hand, application of Rayleigh model and its variants results in significant discrepancies in explaining the hysteresis behavior in epitaxial thin films. Here, hysteresis in application-relevant, epitaxial, monoclinic PbZr_0.55Ti_0.45O₃ films were investigated for non-switching AC excitation fields at zero and 20 kVcm^-1 DC bias cases. The hysteresis, loss and nonlinearity in the strain and polarization response of the films show unique features that is not expected from the Rayleigh model. The strain exhibits a hysteretic behavior and is linear with the excitation amplitude. However, the polarization is hysteretic and highly nonlinear over the investigated frequency range from 70 Hz to 5 kHz. The application of the bias shows almost no change for the strain response, whereas the loss tangent and nonlinearity of the polarization are significantly reduced.

The observations are explained by a new model, the so called polarization rotation model. This model describes the film properties as a result of the nonlinear rotation of the polarization vector within the monoclinic unit cell in response to the applied electric field, accompanied by a viscous interaction of the domains. The model can describe the scaling of the loss tangent and (non)linearity of the strain and polarization in an amplitude and frequency range that is far exceeding the applicable range of the Rayleigh model. It is shown that the nonlinear response and the hysteretic loss can be ascribed to two separate processes. The nonlinearity stems from the nonlinear nature of the rotation of the polarization vector. On the other hand, the hysteresis and ferroelectric loss stems from viscous interactions of domains during the rotation of the polarization vector.

[1] Lucke, P., Bayraktar, M., Birkhölzer, Y. A., Nematollahi, M., Yakshin, A., Rijnders, G., . . . Houwman, E. P. (2020). Hysteresis, Loss and Nonlinearity in Epitaxial PbZr_0.55Ti_0.45O₃ Films: A Polarization Rotation Model. Advanced Functional Materials, 30(52), 2005397.
doi:https://doi.org/10.1002/adfm.202005397

[2] Lucke, P., Bayraktar, M., Schukkink, N., Yakshin, A. E., Rijnders, G., Bijkerk, F., & Houwman, E. P. Influence of DC Bias on the Hysteresis, Loss, and Nonlinearity of Epitaxial PbZr_0.55Ti_0.45O₃ Films. Advanced Electronic Materials, 2100115. doi:https://doi.org/10.1002/aelm.202100115

Topological Insulators (TI) possess Dirac Surface Conducting States which are protected by Time Reversal Symmetry (TRS) [1]. However, a gap opening in the surface state bands through braking of TRS is needed for realization of Quantum Anomalous Hall Effect (QAH) which in turn can give rise to and Axion Insulator States and topological magnetoelectric effect [2-3]. Magnetic phase has been reported in TIs using doping [4-5] and proximity effect [6]. Recently, intrinsic magnetic TI (MTI) phase has been theoretically predicted in MBi₂Te₄ (M = Mn, Ni, Eu, V) family of compounds [7-8] and experimentally observed in MnBi₂Te₄ [9]. Here, we report growth of highly c-axis oriented TI, Bi₂Te₃ with nominal surface roughness (~1.3 nm) using sputtering and formation of a topological antiferromagnetic phase at the interface of Bi₂Te₃/Ni₈₀Fe₂₀ (Permalloy, Py) heterostructures. We grew samples of 40 nm c-axis oriented Bi₂Te₃ at 250 C followed by 20 nm of Py at room temperature. Since the growth of the FM on TI is carried out without breaking vacuum, the topologically nontrivial surface states are accessible to the FM species in their pristine condition. We observed large diffusion of Ni from Py and formation of distinct interfacial phase of Ni_xBi_yTe_z. TSS catalyzed reaction [10] possibly leads to formation of a topologically non-trivial interfacial phase because of solid state reaction between Ni and Bi2Te3 and proximity effect. Zero Field Cooled (ZFC) magnetic hysteresis loop measurements at low temperatures showed a large spontaneous exchange bias which suggests presence of FM/AFM coupling in the samples. Further, Field Cooled (FC) measurements with field cooling at 1 T showed enhanced exchange bias field values of ~160 Oe. We characterized the Neel Temperature of the AFM phase at ~63 K. We further identified the antiferromagnetic phase at Ni intercalated Bi₂Te₃ interfacial layer using Polarized Neutron Reflectometry (PNR). The large exchange bias in the Ni intercalated Bi2Te3 interfacial layer clearly suggests a large exchange energy which is characteristic of super-exchange mediated VdW MTI compound [8]. The observation of magnetic phase at the interface layer further raises the possibility of QAH state and paves the way for further exploration of TI/FM based heterostructures and applications for energy efficient spintronic devices. Also, growth of high crystalline quality TI using sputtering opens up the path towards integration of crystalline TIs in industrial CMOS devices.
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10. G. Li and C. Felser et al., Appl. Phys. Lett. 116, 070501 (2020).

A field-dependent transition from the superparamagnetic-to-superparamagnetic blocked state is realized during magnetization (M) versus temperature (T) measurements by controlling the size of the iron (Fe) particles in the confined layers of titanium nitride (TiN) thin-film matrix. The nanoscale Fe particles of uniform size and shape are embedded in a TiN/Fe/TiN multilayered pattern using a pulsed laser deposition (PLD) method. Crystallographic studies carried out using x-ray diffraction have indicated the epitaxial nature of TiN film. Applying the Maxwell relation to the M-T curves at various fields, δM/δT vs. H plots are acquired, the integration of which provides quantitative information about the isothermal entropy change in the Fe nanoparticulate system. Our experimental data shows that a positive magnetocaloric effect (MCE) with a maximum entropy change of ∼2.27x10³ J/K m³ can be achieved for the magnetic field in the range of 0.002 - 0.5 T for the TiN/Fe/TiN sample having Fe particles of the size ~15 nm. All the change in entropy curves clearly shows a peak in the vicinity of blocking temperature. A broad isothermal entropy change over a wide range of temperature has been found for the same sample with a refrigeration capacity value of 8.9x10⁴ J/m³.

NdNiO3 (NNO) is a member of the perovskite rare-earth nickelates whose phase diagrams host metal-insulator transitions and various magnetic orders [1]. These transitions can be tuned using a variety of knobs such as rare-earth substitution, epitaxial strain and dimensionality. Even more, reducing the dimensions of NNO devices can modulate the drops and jumps in resistivity across the metal-insulator transition [2]. Here we perform temperature-dependent electrical transport measurements to further examine the effect of the lateral dimensions of thin films of NNO on the metal-insulator transition. Focused ion beams microstructuring is used to reduce the dimensions of the device towards sizes comparable to the metallic and insulating domains near the transition temperature. In addition to measuring the temperature-dependent resistivity, we image the domain structure of NNO films at low temperatures using scanning transmission electron microscopy to investigate the relationship between the metal-insulator transition and the structural changes at the atomic level. These complementary measurements may reveal the interplay between atomic-scale changes, nanoscale domain arrangements, and the macroscopic properties of rare-earth nickelates and transition-metal oxides in general.



References
[1] Mattoni, G., Zubko, P., Maccherozzi, F. et al. Striped nanoscale phase separation at the metal–insulator transition of heteroepitaxial nickelates. Nat Commun 7, 13141 (2016).
[2] Hong Lee et al. Imaging and Harnessing Percolation at the Metal−Insulator
Transition of NdNiO3 Nanogaps. Nano Letters 19 (11), 7801-7805 (2019).

We studied the angular momentum of phonons in BaTiO3 from first principles. We find that in the tetragonal phase of BaTiO3 the average phonon angular momentum is significantly larger in directions perpendicular to polarization than parallel. Furthermore, there is additional anisotropy within the plane perpendicular to polarization. We analyze the interatomic force constants and build a simple effective model to reveal the origin of the anisotropies. Finally, we suggest experiments using BaTiO3 as an electric field controllable phonon angular momentum reservoir.

*This work was supported by grant NSF DMR-1848074.

Magnetoelectricity (ME) is the property of certain materials to exhibit a coupling between their electric and magnetic properties. This enables the control of magnetization with electrical fields, a process whose energy cost is order of magnitude lower than the direct control of magnetization with magnetic fields, paving the way towards low-power devices. Nowadays, ME has been reported in a large variety of materials but most of them are characterized by weak coupling constants. Furthermore, the effect only appears at low temperatures, putting heavy limitations for practical applications. One of the main challenges in this field is to develop materials with stronger coupling with higher operating temperatures.
The strength of the magnetoelectric coupling is given by the tensor α_ij which describes the magnitude of the polarization (magnetization) built by a magnetic (electric) field. The magnetoelectric coupling can be computed nowadays with ab initio techniques within the density functional theory (DFT) framework. Here, we present results on the ab initio computation of the ME tensor on a few known ME materials such as Fe₂Mo₃O₈, GaFeO₃ or Ni₃TeO₆. We compare the results to experimental data and discuss the importance of the electronic, ionic and strain components to the magnetoelectric tensor.

Solution-processable molecular ferroelectrics, which show ferroelectric properties approaching inorganic perovskites, have amassed much recent attention due to their lightweight, tunable electro-optic and electromechanical coupling effects (1, 2). Spontaneous polarization and the ability to switch the electromechanical activity by an external electric or mechanical stimulus is of prime importance, establishing the basis for many metamaterial technologies (3, 4). Over the past decade, elastic metamaterials with resonant inclusions have gained significant traction owing to their unique response to incident dynamic loads, ranging from subwavelength band gaps, back-scattering immune wave guides, and topological pumps, to the design of logic gates, nonreciprocity, and diode-like formations. Despite their theoretical promise, such mechanical metamaterials remain hindered by the lack of adaptive stimuli-responsive materials which can be effectively tuned “on-demand” across the time and length scales dictated by such metamaterials. We present a continuous rapid printing strategy for the volumetric deposition of water-soluble molecular ferroelectric metamaterials with precise spatial control in virtually any three-dimensional geometry by means of an electric-field–assisted additive manufacturing (5). We demonstrate a scaffold-supported ferroelectric crystalline lattice that enables self-healing and a reprogrammable stiffness for dynamic tuning of mechanical metamaterials with a long lifetime and sustainability. A molecular ferroelectric architecture with resonant inclusions then exhibits adaptive mitigation of incident vibroacoustic dynamic loads via an electrically tunable subwavelength-frequency band gap. The findings shown here pave the way for the versatile additive manufacturing of molecular ferroelectric metamaterials.

Reference:
1. Z. Zhang et al., Tunable electroresistance and electro-optic effects of transparent molecular ferroelectrics. Sci. Adv. 3, e1701008 (2017).
2. D.-W. Fu et al., Diisopropylammonium bromide is a high-temperature molecular ferroelectric crystal. Science 339, 425-428 (2013).
3. W. Zhou, P. Chen, Q. Pan, X. Zhang, B. Chu, Lead-Free Metamaterials with Enormous Apparent Piezoelectric Response. Adv. Mater. 27, 6349-6355 (2015).
4. L. Van Lich, T. Shimada, S. Sepideh, J. Wang, T. Kitamura, Multilevel hysteresis loop engineered with ferroelectric nano-metamaterials. Acta Mater. 125, 202-209 (2017).
5. Y. Hu et al., A 3D-printed molecular ferroelectric metamaterial. Proc. Natl. Acad. Sci. U.S.A. 117, 27204-27210 (2020).

It is generally accepted that most electro-optical materials are transparent and, often, piezoelectric by nature. Electrical switching of ferroelectric domains and consequent domain wall motion promotes a strong piezoelectric activity. However, light scatters strongly at refractive index discontinuities such as found at domain wall boundaries. Thus, simultaneously achieving high piezoelectric properties along with a high transparency remains elusive, until now. Here, we applied electrical and mechanical stresses to manipulate and reconfigure the domains in perovskite Pb(In_1/2Nb_1/2)O₃ Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ domain-engineered single crystals. We demonstrated that, via the stress induced ferroelectric-ferroelectric phase switching, a highly transparent crystalline state develops as the polydomain structure reconfigures to a monodomain ferroelectric state. The unprecedented optical properties are achieved at very low electric fields (less than 1.5 kV/cm) with the piezo crystal retaining an excellent piezoelectric performance. We demonstrate a robust method to enhance or suppress light scattering at ferroelectric domain boundaries, manipulated by external electrical and mechanical stresses, realizing on-demand optical transmissivity and piezoelectricity outperforming state-of-the art materials.

In this paper, detailed magnetic and electrical transport properties of composite thin films consisting of antiferromagnetic (AFM) NiO and ferromagnetic (FM) Ni have been studied. Highly oriented NiO thin films were grown on sapphire (0001) substrate using pulsed laser deposition technique varying the growth parameters such as the oxygen pressure of the chamber and the substrate temperature. To obtain various percentage mixtures of Ni and NiO by means of introducing point defects (oxygen vacancies) into NiO crystal structure, the as-grown films were annealed in forming gas (Ar:96% and H2:4%) varying the temperature from 3000 C to 5000 C with annealing time from 10 minutes to 100 minutes. The X-ray diffraction (XRD) data show that the as-grown NiO films have the rock salt structure and are preferentially grown on the sapphire substrate along the (111) planes. The phase mixture of Ni/NiO was obtained by annealing and was confirmed by XRD, Raman spectroscopy, and SEM-EDS. Additionally, the film transmittance degraded as a conspicuous amount of NiO crystal structure was converted into preferentially oriented pure Ni detected by XRD as well as Raman spectroscopy. The very sharp peak at 44.4° on XRD indicates the face-centered cubic crystal structure of Ni. Detailed magnetic properties were investigated using a superconducting quantum interference device (SQUID) magnetometer. The temperature and field-dependent magnetization M (H, T) data show that pure NiO and Ni are antiferromagnetic and ferromagnetic, respectively. The Ni-rich samples show ferromagnetic behavior with lower saturation magnetization (1.2 x 10-3 emu) and higher coercivity of 150 Oe than pure Ni film. Ferromagnetism has also been observed in Ni-rich samples in the ferromagnetic resonance (FMR) experiment, and the FMR linewidth increases with decreasing Ni phase in the composite thin films. Pure NiO samples exhibit a very high resistivity of 812 Ohm-m, whereas, pure Ni just behaves like a normal metallic ferromagnet. However, the value of resistivity sharply decreases with increasing oxygen vacancies in NiO, and it behaves like a semiconductor. Finally, the behavior of the composite Ni/NiO thin films resemble a magnetic semiconductor leading to a promising application in the arena of spintronic devices.

Ferromagnetic and multiferroic nanostructures are promising for next generation memory, logic, sensors, actuators, and nanoelectronics. In this work, we apply advanced scanning probe microscopy techniques including Magnetic Force Microscopy (MFM), Piezoresponse Force Microscopy (PFM), and Tomographic Atomic Force Microscopy (T-AFM) for nanoscale characterization of three multiferroic systems: homogeneous BiFeO₃ thin films, self-assembled BiFeO₃-CoFe₂O₄ (BFO-CFO) vertically aligned nanocomposites, and Lanthanum Strontium Manganite (LSMO) nanoarrays. Correlations between domain nanostructure and microstructural and engineered feature geometries are studied. Additionally, correlations between ferroelectric polarization and interfacial strain are directly investigated in 3 dimensions. Such correlated, multi-modal nanoscale imaging is promising for future optimization of multiferroic devices.

Magnetoelectric coupling phenomena in multiferroics were initially treated almost exclusively as bulk effects exhibited by homogeneous materials. In recent years, attention has moved to the nanoscale. On the one hand, magnetoelectric phase control ultimately happens on the level of the mostly sub-micrometer-sized domains. On the other hand, domain walls, interfaces and thin films are nanoscale objects that have been proving to be the source of intriguing magnetoelectric coupling effects. In my talk, I will therefore explore the fascinating world of magnetoelectric correlations at the nanoscale in a number of examples, the diversity of which is intended to demonstrate the enormous potential of the nanoscale perspective on multiferroics.

[1] We show that in multiferroics with separately emerging magnetic and ferroelectric order (type I), the microscopic magnetoelectric coupling can be intrinsically strong, even though the macroscopic leading-order magnetoelectric effect is forbidden by symmetry. In hexagonal manganites like YMnO₃ an imbalance in the competing superexchange contributions couples the antiferromagnetic to the ferroelectric domains. The associated domain walls are a source of topological vortex-like magnetic singularities. --- Nat. Comms. 12, 3093 (2021).

[2] In certain multiferroics with jointly emerging magnetic and ferroelectric order (type II), like (Tb,Dy)FeO₃, the multiferroic bulk state can be converted into multiferroic domain walls in a non-multiferroic environment. Electric or magnetic fields act on these confined multiferroic objects or return them to the bulk phase. --- Nat. Comms. 12, 2755 (2021).

[3] In epitaxial type-I multiferroic YMnO₃ thin films the coexistence of magnetic and ferroelectric order roots in the improper nature of the electrically polarized state. We find that its lattice-distortive origin permits to control the ferroelectric order via the mechanical clamping of the film to the substrate. This control is effective in the regime from a single monolayer to about 100 nm. Improper ferroelectrics hence offer additional degrees of freedom for tuning the magnetoelectric coupling in multiferroics. --- Nat. Comms. 10, 5591 (2019).

[4] As room-temperature multiferroic, BiFeO₃ is most promising for device applications. La alloying reduces the coercive field for magnetoelectric switching to the CMOS regime, but it also destroys the nano-domain patterning that is crucial for voltage-induced magnetization reversal. We show how the functionality of (La,Bi)FeO₃ films can be restored to the full potential of the BiFeO3 parent compound by an electric-field training procedure. The training also initiates a giant net-polarization enhancement that survives in a device-like capacitor geometry. --- submitted.

The integration of magnetoelectric multiferroics, hosting coexisting and coupled ferromagnetic and ferroelectric orders, into magnetoelectric spin-orbit logic devices holds promises for unprecedented performance and energy consumption reduction by several orders of magnitude.^[1,2] While static properties such as the coercive electric field are thoroughly studied and optimized, investigations on the evolution and coupling of the ferroic orders during electric-field cycling are sparse.^[3,4,5,6]
Here, we study the evolution of the ferroelectric order of the technologically most relevant magnetoelectric material, BiFeO₃, and its chemically engineered relative La_0.15Bi_0.85FeO₃. Using operando optical second-harmonic generation microscopy, we directly access the polarization of the films integrated in technologically relevant capacitor heterostructures during electric-field cycling. We demonstrate that electric-field training results in a spontaneous domain ordering and a giant enhancement of the net polarization in La_0.15Bi_0.85FeO₃. Furthermore, we track the evolution of this ordered domain configuration operando in the device architecture during repeated switching and find that it is stable for more than 10⁶ switching cycles. In conclusion, our operando investigations give unprecedented insights into the polarization evolution during real device conditions and, therefore, reveal hitherto hidden opportunities to probe the polarization dynamics and to optimize BiFeO₃-based integrated devices.
[1] S. Manipatruni et al., Nature 565, 35 (2019).
[2] M. Trassin, J. Phys. Condens. 28, 033001 (2016).
[3] J. Heron et al., Nature 516, 370 (2014).
[4] Y. Huang et al., Nat. Commun. 11, 2836 (2020).
[5] B. Prasad et al., Adv. Mater. 32, 2001943 (2020).
[6] E. Parsonnet et al., Phys. Rev. Lett. 125, 067601 (2020).

For different concepts of magnetoelectric sensors with sputter deposited magnetostrictive layers it could be shown that they can achieve low detection limits in the range of 100 pT/√Hz and below when used as magnetic field sensors in biomagnetically interesting frequency ranges [1-3]. To reach a further improved detection limit, during sensor operation a better control of the magnetization state of the magnetostrictive phase is required. This enables a reduction in magnetically induced noise contributions while maintaining the sensor sensitivity.
In the presented work, sensors are considered whose magnetostrictive phase is formed by (Fe₉₀Co₁₀)₇₈Si₁₂B₁₀ single or multilayers. Such magnetic layers show comparatively low magnetic anisotropy together with a significantly high magnetostriction. Process-related influences during sensor fabrication, layer stresses for example, can therefore affect the magnetic properties. In this context, we compare amorphous, magnetostrictive FeCoSiB films prepared by RF and DC sputter deposition. The chemical and magnetic properties determined by elastic recoil detection and magnetooptical magnetometry, respectively, are correlated with the resulting magnetoelectric sensor properties such as noise level and detection limit.

[1] S. Salzer, V. Röbisch, M. Klug, P. Durdaut, J. McCord, D. Meyners, J. Reermann, M. Höft, R. Knöchel, "Noise Limits in Thin-Film Magnetoelectric Sensors With Magnetic Frequency Conversion", in IEEE Sensors Journal, vol. 18, no. 2, pp. 596-604, 2018.
[2] Zaeimbashi, M., Nasrollahpour, M., Khalifa, A. et al., "Ultra-compact dual-band smart NEMS magnetoelectric antennas for simultaneous wireless energy harvesting and magnetic field sensing", Nat. Commun. 12, 3141, 2021.
[3] V. Schell, C. Müller, P. Durdaut, A. Kittmann, L. Thormählen, F. Lofink, D. Meyners, M. Höft, J. McCord, E. Quandt, "Magnetic anisotropy controlled FeCoSiB thin films for surface acoustic wave magnetic field sensors", Appl. Phys. Lett. 116, 073503, 2020.

This work was funded by the German Research Foundation (DFG) through the Collaborative Research Centre CRC 1261 "Magnetoelectric Sensors – From Composite Materials to Biomagnetic Diagnostics".

Relaxor ferroelectrics, such as (1-x)Pb(Mg_1/3Nb_2/3)O₃-(x)PbTiO₃ (PMN-xPT), exhibit giant piezoelectricity which arises from the presence of multiple symmetries near a Morphotropic Phase Boundary (MPB). The MPB in PMN-xPT lies around x = 35% and separates a Rhombohedral (R) phase, with spontaneous polarization along the <111> crystallographic directions, from a Tetragonal (T) phase whose polarization lies along <100>. [1] For compositions near the MPB, the presence of bridging Monoclinic (M) and Orthorhombic (O) phases help to facilitate large lattice distortions as the spontaneous polarization rotates between the competing symmetries in order to align with the applied field.
Bulk studies on (011) oriented PMN-PT single crystals have demonstrated non-volatile (i.e. permanent) 71/109 degree switching between in-plane and out-of-plane R polarization directions. [2] Such switching results in distinct strain states and forms the basis of low-power, ultrafast, memory storage devices when coupled with a ferromagnetic (FM) material. [3] Achieving low power, however, necessitates the use of thin films of PMN-PT. Additionally, the films must be removed from their substrates in order to prevent substrate clamping that eliminates their giant piezoelectricity. Here we demonstrate growth of epitaxial (011) PMN-PT thin films, followed by removal of the substrate via etching of a Sr₃Al₂O₆ (SAO) sacrificial layer. [4] By coupling with a FM Nickel overlayer, we demonstrate manipulation of in-plane magnetic anisotropy as seen by the Magneto-Optic Kerr Effect (MOKE). The coupling only requires 3V applied bias across the PMN-PT membrane, which is much lower than the >100V needed using bulk PMN-PT. However, we do not see evidence of the 71/109 degree bi-stable switching as observed in the bulk studies. Instead we find that the piezo-driven coupling is dominated by polarization rotation from the nominally <111> oriented R states towards the [011] O state along the applied field direction. Our findings have significant implications on the design of strain-mediated devices based on relaxor-ferroelectrics such as PMN-PT.

[1] Noheda, B.; Cox, D. E.; Shirane, G.; Gao, J.; Ye, Z.-G. Phys. Rev. B 2002, 66 (5), 054104
[2] Wu, T.; Bur, A.; Zhao, P.; Mohanchandra, K. P.; Wong, K.; Wang, K. L.; Lynch, C. S.; Carman, G. P. Appl. Phys. Lett. 2011, 98 (1), 012504
[3] Hu, J.-M.; Li, Z.; Chen, L.-Q.; Nan, C.-W. Nat. Commun. 2011, 2, 553
[4] Lu, D.; Baek, D. J.; Hong, S. S.; Kourkoutis, L. F.; Hikita, Y.; Hwang, H. Y. Nat. Mater. 2016, 15 (12), 1255–1260

Acknowledgement: This work was supported by the Army Research Office through Grant W911NF-17-1-0462. MOKE measurement at the University of Wisconsin–Madison was supported by the US Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences (BES), under award number DE-FG02-06ER46327.

Surface acoustic wave devices have found a wide field of applications since it was discovered that they can be excited by interdigital transducers on piezoelectric substrates in 1965. One of these applications gaining more interest in the recent years is the sensing of magnetic fields. Especially for the measurement of biomagnetic signals surface acoustic wave devices are promising candidates due to their ability to detect magnetic fields below 100 pT/Hz^1/2 over a large frequency range of 1 Hz to 100 kHz [1]. This large bandwidth in combination with a wide dynamic range make them also interesting for artificial current sensing.

For the sensing of magnetic fields Love waves give much higher sensitivities than Rayleigh wave based sensors, due to the strong confinement of the acoustic wave in the magnetostrictive film [2]. Love waves are horizontal shear waves which are guided to the surface by a layer of higher acoustic impedance than the substrate. Operating in a delay line configuration the magnetoelastically induced change of shear modulus of the magnetostrictive layer yields in a corresponding phase shift of the acoustic wave. The magnetically generated 1/f noise of this magnetostrictive film on top of the guiding layer turned out to be the limiting factor regarding the limit of detection of the sensors. One approach to reduce magnetic noise is to exchange bias the ferromagnetic phase by the coupling to an antiferromagnetic phase [3, 4]. With this, a single or nearly single domain state can be achieved and domain walls reduced or eliminated as they are the main source of phase noise in these devices.

In this work a layer system consisting of (Fe₉₀Co₁₀)₇₈Si₁₂B₁₀ / Ni₈₀Fe₂₀ / Mn₇₀Ir₃₀ is used, where (Fe₉₀Co₁₀)₇₈Si₁₂B₁₀ is the magnetostrictive layer and Ni₈₀Fe₂₀ is a seed layer for the antiferromagnetic Mn₇₀Ir₃₀ which induces the exchange bias. All films are deposited by means of magnetron sputtering on ST-cut Quartz substrates and ST-cut Quartz based SAW sensors while a magnetic field is applied during deposition to induce the exchange bias. Challenges concerning the deposition process on piezoelectric substrates, the magnetic behavior as well as the noise performance of the sensors will be discussed. The successful reduction of magnetically induced 1/f noise in the low frequency regime brings SAW magnetic field sensors closer towards biomagnetic applications.

[1] V. Schell, C. Müller, P. Durdaut, A. Kittmann, L. Thormählen, F. Lofink, D. Meyners, M. Höft, J. McCord, and E. Quandt, “Magnetic anisotropy controlled FeCoSiB thin films for surface acoustic wave magnetic field sensors,” Applied Physics Letters, 116, 2020.
[2] A. Kittmann, P. Durdaut, S. Zabel, J. Reermann, J. Schmalz, B. Spetzler, D. Meyners, N. X. Sun, J. McCord, M. Gerken, G. Schmidt, M. Höft, R. Knöchel, F. Faupel, and E. Quandt, “Wide Band Low Noise Love Wave Magnetic Field Sensor System,” Scientific Reports, vol. 8, no. 1, 278–287, 2018.
[3] E. Lage, C. Kirchhof, V. Hrkac, L. Kienle, R. Jahns, R. Knöchel, E. Quandt & D. Meyners, “Exchange biasing of magnetoelectric composites”, Nature materials, 11, 523–529, 2012.
[4] M. Jovicevic Klug, L. Thormählen, V. Röbisch, S. D. Toxværd, M. Höft, R. Knöchel, E. Quandt, D. Meyners & J. McCord, “Antiparallel exchange biased multilayers for low magnetic noise magnetic field sensors”, Applied Physics Letters, 114, 2019.

This work was funded by the German Research Foundation (DFG) through the Collaborative Research Centre CRC 1261 “Magnetoelectric Sensors – From Composite Materials to Biomagnetic Diagnostics”.

The stability of magnetism in reduced dimensions is a major scientific agenda in the pursuit of implementing magnetic nanostructures into spintronic devices. Methods to probe and control magnetization states of such structures in a deterministic manner include use of spin polarized currents, photon absorption and relatively recently, electric fields that tailor magnetoelectric coupling in multiferroic based structures. In theory, a short electric pulse is able to generate localized magnetic fields that can couple to the local magnetic dipoles electrodynamically. Here, using the Landau-Lifshitz-Gilbert (LLG) formalism of magnetism dynamics combined with continuum Maxwell relations we study the response of a ferromagnetic permalloy nanodisc to nanosecond electric field pulses [1]. The dynamics of the magnetic order during this process are examined and discussed. Ferromagnet nanodiscs can relax to a vortex phase as the ground state due to the demagnetizing field. Our results demonstrate that the planar chirality of such a ferromagnet nanodisc can be switched via a time-wise asymmetric electric field pulse on the order of a few ns duration that generates radially varying tangential magnetic fields. These fields couple to the vortex state of the nanodisc ferromagnet electrodynamically, revealing an effective and robust method to control chirality. The kinetics of the chirality switching as a function of disc size is analyzed. We also go on to discuss the possibility to generate programmable metasurfaces based on the approach we develop where local magnetic states can be rapidly written onto thin magnetic films with ease and extraordinary precision.

[1] W. A. S. Aldulaimi, C. Akaoglu, M. B. Okatan, K. Sendur and I. B. Misirlioglu, "Chirality switching in ferromagnetic nanostructures via nanosecond electric pulses", under revision in Annalen der Physik, June 2021.

The intriguing low temperature softening of the high magnetic anisotropy epsilon Fe₂O₃ (eFO) nanoparticles [1] has been also observed in epitaxial stabilized eFO thin films on MgAl₂O₄(111) and flexible fluorphlogopyte mica (001) substrates. The selection of an adequate substrate and buffer layer leads to an excellent film quality with sharp (00L) eFO X-ray diffraction peaks and the formation of 3 in-plane twin domains compatible with the 3-fold symmetry of the substrate surface. At room temperature, the M(H) hysteresis loops show a strong magnetic anisotropy and absence of step-like magnetization drop due to soft components. A reduction of the magnetic anisotropy and coercive field occurs on cooling below 200 K, which is reminiscent of that found in randomly distributed nanoparticles. The H_c collapse is less pronounced in the epitaxial films (-40%) than in nanoparticles (-96%), but the (00L) out of plane-texture allows a better observation of temperature dependence of frequency shifts of Raman modes. The trends primarily agree with the anharmonic model at temperatures above ~ 200 K, whereas a marked phonon softening can be observed below this temperature for different modes. The transition has been also studied by X-ray absorption spectroscopy (XAS) measurements in total electron yield and fluorescence modes and by X-ray magnetic circular dichroism (XMCD) and transport measurements.


[1] M. Gich, C. Frontera, A. Roig, E. Taboada, E. Molins, H. R. Rechenberg, J. D. Ardisson, W. A. A. Macedo, C. Ritter, V. Hardy, J. Sort, V. Skumryev, and J. Nogués, High- and Low-Temperature Crystal and Magnetic Structures of ε-Fe2O3 and Their Correlation to Its Magnetic Properties, Chem. Mater. 18, 3889 (2006).

After 50 years of exponential increase in computing efficiency, the technology of today’s electronics is approaching its physical limits, with feature sizes smaller than 10 nm. New schemes must be devised to contain the ever-increasing power consumption of information and communication systems, which requires the introduction of non-traditional materials and new state variables. As recently highlighted, the remanence associated with collective switching in ferroic systems is appealing to reduce power consumption¹. A particularly promising approach is spintronics, which relies on ferromagnets to provide non-volatility and to generate and detect spin currents. However, magnetization reversal by spin transfer torques is a power consuming process. This is driving research on multiferroics to achieve a low-power electric-field control of magnetization, but practical materials are scarce and magnetoelectric switching remains difficult to control. In this talk, we will propose an alternative strategy to achieve low-power spin detection and generation, in non-magnetic systems combining ferroelectricity and Rashba spin-orbit coupling². We will describe various materials strategies based on single materials, multilayers and two-dimensional electron gases. In particular we will show results on SrTiO₃-based electron gases in which a large electric field induces a ferroelectric-like state that enables a non-volatile control of its spin-orbit properties and spin-charge conversion capabilities³. These observations open the way to the electric-field control of spin currents and to ultralow-power spintronics, in which non-volatility would be provided by ferroelectricity rather than by ferromagnetism.

1. Manipatruni, S. Beyond CMOS computing with spin and polarization. Nature Physics 14, 338 (2018).
2. Varignon, J., Vila, L., Barthélémy, A. & Bibes, M. A new spin for oxide interfaces. Nature Phys 14, 322–325 (2018).
3. Noël, P. et al. Non-volatile electric control of spin–charge conversion in a SrTiO₃ Rashba system. Nature 580, 483–486 (2020).

This work received support from the ERC Advanced grant n° 833973 “FRESCO”.

Antiferromagnetic materials hold great promise for next-generation spintronics due to the combination of stability in external magnetic fields and the potential for ultrafast switching. Here, we explore a new antiferromagnetic metal for spintronic applications. We begin with a correlated insulator, NdNiO3. NdNiO3 has a metal-to-insulator transition that is concomitent with a paramagnetic to antiferromagnetic transition. We demonstrate that electron doping can induce a new antiferromagnetic metallic phase in NdNiO3. We use a combination of neutron diffraction and resonant x-ray scattering to probe the magnetic order and angle resolved photoemission spectroscopy to demonstrate a new Fermi surface in the antiferromagnetic metal phase. Our materials show a large zero-field planar Hall effect and could find applications in heat-assisted magneto-recording.

Crystal symmetry in condensed-matter materials largely dictates their micro- and macroscopic properties, and new material functionalities can arise when inversion symmetry is broken and when symmetry changes. Thus, the ability to control crystal inversion symmetry on demand is of both fundamental and technological importance. Perhaps the most pervasive example of the manifestation of broken inversion symmetry is in ferroelectrics, where crystal structures that inherently break inversion symmetry directly lead to a switchable spontaneous electrical polarization. Recently, the ability to synthesize heteroepitaxial ferroelectric superlattice systems has enabled new methods to control ferroic order^1–4 and even crystal symmetry^5,6. Here, by using BiFeO₃/TbScO₃ superlattices as our model system, we take advantage of (i) the discontinuity of the spontaneous polarization; (ii) the lattice mismatch; and (iii) the octahedral tilt frustration between the two layers to engineer crystal inversion symmetry. We stabilize both a non-centrosymmetric polar and a centrosymmetric antipolar phase mediated by a first order phase transition at room temperature. Both phases are identified and characterized using a combination of high-resolution and four-dimensional (4D) scanning transmission electron microscopy (STEM), piezoforce microscopy (PFM), and confocal second harmonic generation (SHG) over atomic and mesoscopic length scales. Moreover, applying orthogonal in-plane electric fields to this mixed-phase system results in the deterministic, nonvolatile interconversion of the centrosymmetric and noncentrosymmetric phases, resulting in a three order of magnitude change in the non-linear optical (SHG) response of the system. Demonstration of such an optically addressable, multi-state memory device presents new avenues for cross-functional devices which take advantage of the interconversion between ferro- and anti-ferroelectric states.

1. Tang, Y. L. et al. Observation of a periodic array of flux-closure quadrants in strained ferroelectric PbTiO3 films. Science. 348, 547–551 (2015).
2. Yadav, A. K. et al. Observation of polar vortices in oxide superlattices. Nature 530, 198–201 (2016).
3. Das, S. et al. Observation of room-temperature polar skyrmions. Nature 568, 368–372 (2019).
4. Wang, Y. J. et al. Polar meron lattice in strained oxide ferroelectrics. Nat. Mater. 19, 881–886 (2020).
5. Dong, W. et al. Emergent Antipolar Phase in BiFeO3-La0.7Sr0.3MnO3Superlattice. Nano Lett. 18, 15 (2020).
6. Mundy, J. A. et al. A high-energy density antiferroelectric made by interfacial electrostatic engineering. arXiv:1812.09615 (2018).

Ferroics, namely ferroelastics, ferroelectrics, ferromagnets and multiferroics (more than one ferroic order), have been the subject of increasingly intense research over the best part of half a century; having applications as memory devices, mechanical switches and acousto-optic devices. All ferroics, when cooled below a critical temperature (Tc) between a high-symmetry paraphase and a low-symmetry ferrophase, develop energetically equivalent but differently orientated states (domains). Domains are separated by interfaces (domain walls), which exhibit properties unique to that of the bulk and often constitute the active component for novel devices.
Importantly, Ferroelastics play a vital role in the fundamental understanding of multiferroic switching, as a large majority of multiferroics exhibit a ferroelastic component which mediates many of the observed properties, such as (super-)conductivity¹ and magnetoelectricity².
Analogous to other ferroics, ferroelastics are defined by their hysteresis loop (strain vs stress). On a macroscopic scale the hysteresis can appear smooth, yet on a finer scale, is actually formed of small discrete jumps or ‘jerks’.
Most driven systems exhibiting small discrete events, cover a broad range of sizes dubbed as ‘crackling noise’³. Such distributions are examples of avalanches, where Mean-field theory allows for the statistical characterisation of domain wall movement by predicting the power law distribution of jerks as a scale-free process of the form g(x)dx~x^-γdx, whereby g(x) describes the probability (g) of a jump (x) and the critical exponent is characteristic of that system⁴. Such exponents can be implemented to describe the statistics of certain observables, such as the energy released (Ε) or area spanned (α) by events.
These power-law critical exponents are scale invariant (no characteristic size/ time scale) and demonstrate a behaviour that is independent of the microscopic properties of that system. As such, systems that share the same critical exponent values are of the same ‘universal class’, where the statistical behaviour of one class can be utilised to garner insight into another system that macroscopically appears to be much different or more intractable, such as tectonic activity and the crackling of sweet wrappers⁵.
This study presents a new insight into the role that the aspect ratio (AR) has on of ferroelastic domain dynamics, by probing two LaAlO₃ (LAO) samples with different AR (high vs low). In-situ heating polarised optical microscopy was used on LAO bulk where the samples were heated from room temperature (RT) to beyond its Tc (~545C°) and back.
It was observed that the AR influences i) the resultant domain pattern at RT, where a clear preference is given to the orientation parallel to the longer axis of the high AR (HAR) sample vs the low AR (LAR) sample, which exhibits a more even split and complex configuration of domain variants that meet at the centre and ii) each AR display some similarities in their he dynamical behaviour at high temperatures while cooling down from Tc. At lower temperatures the AR induces distinct dynamic behaviour: the HAR samples display a propagating domain front that forms above RT and below Tc, leading to a global reconfiguration of the microstructure. This behaviour is explained in terms of thermal stress and the anisotropy present.
Moreover, statistical analysis shows that not only is there a difference in the overall criticality between the two samples, LAR:Ε=1.6 vs HAR:Ε=1.4, but that the HAR sample demonstrates a ‘mixing’ of the critical exponent before and after the global reconfiguration (Ε=1.66 to 1.33). This latter point is highlighted in the analysis of the maximum likelihood method statistics.
¹ T. Rojac, et al., Nat. Mater. 16(3), 322(2017).
² D. I. Khomskii, J. Mag. & Mag. Mater. 306(1), 1(2006);
³ J. P. Sethna, et al., Nature 410 (6825), 242(2001).
⁴ B. Casals, et al., Nat. Comms. 12(1)(2021).
⁵ F. Bohn, et al., Sci. Rep. 8(1)(2018).

Recently, an abundance of novel transport physics has been found at interfaces, much of which promises to be key in the development of future technologies. The unusual electronic and structural environment at the interface can lead to behaviours which deviate wildly from that in the individual bulk constituents, while remaining confined to the interface. Electrically conducting homointerfaces, such as the domain walls that separate different orientations of electrical polarisation within the same band-insulator [1,2], are of particular interest, as they boast the ability to be created, moved and erased on demand [3,4]. Thus, they offer an additional dynamic degree of freedom when compared to conducting heterointerfaces (between dissimilar band insulators, e.g. LaAlO₃/SrTiO₃ [5]), which are fixed in place. Despite this, understanding of charge transport and fundamental aspects of conduction at domain walls is lacking in comparison to the current state-of-the-art in heterointerfaces. Here, we try to address this disparity by reporting on a geometric magnetoresistance measurement of carrier mobility in approximately conical 180° domain walls, which occur in partially switched ferroelectric thin film single crystal lithium niobate. We show that the domain microstructure, which is usually a hurdle to overcome when performing transport measurements on domain walls, results in a domain wall geometry which is equivalent to that of the “Corbino disc”; a textbook sample arrangement for geometric magnetoresistance measurements. Analysis of our magnetoresistance data reveals that these domain walls exhibit an extremely high room temperature Hall mobility of ~ 3,000cm²V^-1s^-1. This value much larger than that seen at bulk or interface oxides before. Finally, we discuss the insight this measurement gives us in terms of understanding the transport at lithium niobate conducting domain wall homointerfaces.

[1] Seidel, J., Martin, L., He, Q. et al. Conduction at domain walls in oxide multiferroics. Nature Mater 8, 229–234 (2009).
[2] Schröder, M. et al. Conducting Domain Walls in Lithium Niobate Single Crystals. Adv. Funct. Mater. 22, 3936–3944 (2012).
[3] McQuaid, R., Campbell, M., Whatmore, R. et al. Injection and controlled motion of conducting domain walls in improper ferroelectric Cu-Cl boracite. Nat Commun 8, 15105 (2017)
[4] McGilly, L., Yudin, P., Feigl, L. et al. Controlling domain wall motion in ferroelectric thin films. Nature Nanotech 10, 145–150 (2015).
[5] Ohtomo, A., Hwang, H. A high-mobility electron gas at the LaAlO₃/SrTiO₃ heterointerface. Nature 427, 423–426 (2004)

Magnetoelectrics are witnessing an ever-growing success toward the voltage-controlled magnetism derived from inorganic materials. However, these inorganic materials have predominantly focused on the ferroelectromagnetism at solid-to-solid interfaces and suffered several drawbacks, including the interface-sensitive coupling mediators, high-power electric field, and limited chemical tunability. Here, we report a promising design strategy to shift the paradigm of next-generation molecular magnetoelectrics, which relies on the integration between molecular magnetism and electric conductivity though an in situ cross-linking strategy. Following this approach, we demonstrate a versatile and efficient synthesis of flexible molecular-based magnetoelectronics by cross-linking of magnetic coordination networks that incorporate conducting chain building blocks. The as-grown compounds feature an improved critical temperature up to 337 K and a room-temperature magnetism control of low-power electric field. It is envisaged that the cross-linking of molecular interfaces is a feasible method to couple and modulate magnetism and electron conducting systems.

Magnetoelectric multiferroic materials are greatly sought for their potential use in new devices. Combining ferroelectricity and magnetism in the same material has been a real challenge especially in perovskites. The search for new ferroelectric structural families beyond perovskites is one path towards the discovery of new multiferroic materials. Here, we report on our search for new ferroelectrics using high-throughput computing and a recently built database of more than 2,000 phonons[1]. Browsing the phonon database, we identify materials exhibiting dynamically unstable polar phonon modes, a signature of a potential ferroelectric. We identify one new family of ferroelectric materials: the anti-Ruddlesden-Popper phases of formula A₄X₂O with A: a +2 alkali-earth or rare-earth element and X: a -3 anion Bi, Sb, As, and P [2]. We show that the unique geometrically-driven mechanism of ferroelectricity involves the movement of an anion in a cationic cage.
This new mechanism leads in the case of Eu₄Sb₂O to a rare example of a material combining ferroelectricity and ferromagnetism.

[1] Petretto, G., Dwaraknath, S., P.C. Miranda, H., Winston, D., Giantomassi, M., van Setten, M. J., Gonze, X., Persson, K. A., Hautier, G. & Rignanese, G.-M. (2018). Sci. Data. 5, 180065.
[2] Markov, M., Alaerts, L., Miranda, H. P. C., Petretto, G., Chen, W., George, J., Bousquet, E., Ghosez, P., Rignanese, G.-M. & Hautier, G. (2021). Proc. Natl. Acad. Sci. 118, e2026020118.

Narrowband terahertz (THz) radiation is crucial for high-resolution spectral identification, but a narrowband THz source driven by femtosecond (fs) laser has remained scarce. Here we computationally predict that a metal/dielectric/magnetoelastic heterostructure enables converting a fs laser pulse into a multi-cycle THz pulse with a narrow linewidth down to ~1.5 GHz, which is in contrast with the single-cycle, broadband THz pulse from the existing fs-laser-excited emitters. We show that such narrowband THz pulse originates from the excitation and long-distance transport of THz spin waves in the magnetoelastic film, which can be enabled by a short strain pulse obtained from fs laser irradiation of the metal film providing that the thicknesses of the metal and magnetoelastic films both fall into a specific range. Our results therefore reveal an approach to achieving optical generation of narrowband THz pulse based on heterostructure design, which also has implications in the design of THz magnonic devices.

Antenna miniaturization is one of the fundamental challenges for decades. Conventional small antennas use electric current for radiation which relies on electromagnetic wave resonance that leads to antenna sizes comparable to the electromagnetic wavelength l₀. Here we demonstrated a new antenna miniaturization approach that acoustically actuated nanomechanical magnetoelectric (ME) antennas with ferromagnetic/piezoelectric thin film resonators could successfully miniaturize the magnitude of 1 to 2 orders with one of the highest passive antenna gain within all nano to micro-scale antennas at a similar frequency. In this talk, we will present the ME antenna theory, calculations, comparison with conventional antennas, and a quick view of a bio-medical application example. These mico-antennas with ultra-high sensitivity, high selectivity of the magnetic field, the integrated capability to CMOS technology, and ground plane immunity have great potential applications for bio-implantable antennas, biomedical applications, the internet of things, etc.

A three-dimensional topological insulator (TI) has spin−momentum locked surface states, in which an interconversion between spin and charge currents occurs. This spin−charge conversion (SCC) enables various applications of TI films in the fields of spintronics. However, when the thickness of the TI film is reduced to a few nanometers, a finite energy gap appears at the Dirac point and the magnitude of spin−momentum locking decreases owing to hybridization between top and bottom surface states. As a result, the weak antilocalization and SCC phenomena, hallmarks of TI surface states, are suppressed.
In this study, we modulated the hybridization effect in ultrathin Bi₂Se₃ films via interface coupling with transition metal oxide HfO_2-x. The unbonded Hf d-orbital states accompanied by oxygen-vacancy defects enhanced spin−orbit coupling and induced asymmetric surface states in Bi₂Se₃, which results in suppression of the hybridization effect. Accordingly, we observed a transition from weak localization to weak antilocalization due to the oxygen defects through magneto-transport measurements. In addition, we simulated the band structures of ultrathin Bi₂Se₃ as a function of the oxygen defects with density functional theory and observed reduced hybridization gap and enhanced spin−momentum locking. To investigate SCC phenomena, we introduced spintronic THz emission spectroscopy that is recently emerged as a unique technique for investigating ultrafast SCC in sub-picosecond timescales. Using the THz spectroscopy, we identified enhancement of SCC efficiency owing to the dehybridization of surface states. These results provide novel methods of controlling surface states of TI and developing TI-based electronic and spintronic devices.

Perovskite nickelates (RENiO₃, RE=Rare Earth, RE≠La) are fascinating materials, well-known, for their metal to insulator transition (MIT) and unique antiferromagnetic ground state [1-3]. In this talk, I will discuss the phase diagram of the system and the physics of the MIT before focusing on NdNiO₃/SmNiO₃ superlattices that display, depending on the thickness of the individual layers, a single or double MIT and a single or double magnetic transition. The electronic state of the materials was revealed at the unit cell scale by STEM [4]. The critical wavelength Λ_cMIT below which a single MIT is observed is 16 unit cells – the corresponding critical wavelength for coupled magnetic transitions is Λ_cAFM = 34 unit cells. This model system allows the origin of the coupling at such oxide interfaces to be studied. The key finding is that the critical length scales are not set by the length scale of the propagation of structural motifs (for instance Ni-O-Ni bond angles) across the interface between the two materials, which ab-initio calculations and STEM analyses suggest is minimal, but rather by the balance between the energy cost of having metallic-insulating – magnetic-non-magnetic – phase boundaries and the energy gain of the bulk phases [5].

[1] M.L. Medarde, Journal of Physics Condensed Matter 9, 1679 (1997)
[2] G. Catalan, Phase Transitions 81, 729 (2008)
[3] S. Catalano et al., Reports on Progress in Physics 81, 046501 (2018)
[4] B. Mundet et al. Nano Letters 21, 6, 2436 (2021)
[5] C. Dominguez et al. Nature Materials, 19, 1182 (2020)

Spintronics is one of the most promising candidates for the next-generation memory and logic applications for integration with the complementary metal oxide semiconductor (CMOS) technology, such as the magneto-resistive random-access memory (MRAM). Compared to the electrical current-driven MRAM, [1,2] such as spin-transfer torque (STT) and spin-orbit torque (SOT), voltage-controlled magnetoelectric random access memory (MeRAM) and alike have the advantages of low-power, high-speed and high-density.
Here, in the first part, we will discuss the fundamental physics of voltage-controlled magnetic anisotropy (VCMA) and the mechanism of VCMA-induced magnetization switching of the perpendicular magnetization; then we will show the progress of VCMA-based MeRAM, [3,4] and the comparison of MeRAM, STT-MRAM, SOT-MRAM. In the second part, we will present the interfacial Dzyaloshinskii–Moriya interaction (DMI) in heterostructures and the voltage-controlled DMI (VC-DMI) effect, and its role on the voltage control of the motion of domain walls and other spin textures such as skyrmions. Based on the VCMA and VC-DMI effects, we develop a method to resolve the need of precise timing of write pulses with the use of chiral symmetry breaking by DMI in the graded magnetic system [5] to achieve the deterministic VCMA switching. Finally, we will discuss the perspective of magnetoelectric ferrimagnetic materials and structures for high-speed applications. The voltage-controlled spintronics has great potential for future low-power and high-density non-volatile memory and logic applications.
References:
[1] K. L. Wang, J. G. Alzate, and P. Khalili Amiri, Low-Power Non-Volatile Spintronic Memory: STT-RAM and Beyond, J. Phys. D. Appl. Phys. 46, (2013).
[2] S. A. Wolf, D. D. Awschalom, R. A. Buhrman, J. M. Daughton, S. Von Molnár, M. L. Roukes, A. Y. Chtchelkanova, and D. M. Treger, Spintronics: A Spin-Based Electronics Vision for the Future, Science.
[3] C. Grezes, F. Ebrahimi, J. G. Alzate, X. Cai, J. A. Katine, J. Langer, B. Ocker, P. Khalili Amiri, and K. L. Wang, Ultra-Low Switching Energy and Scaling in Electric-Field-Controlled Nanoscale Magnetic Tunnel Junctions with High Resistance-Area Product, Appl. Phys. Lett. 108, (2016).
[4] X. Li, T. Sasaki, C. Grezes, D. Wu, K. Wong, C. Bi, P. V. Ong, F. Ebrahimi, G. Yu, N. Kioussis, W. Wang, T. Ohkubo, P. Khalili Amiri, and K. L. Wang, Predictive Materials Design of Magnetic Random-Access Memory Based on Nanoscale Atomic Structure and Element Distribution, Nano Lett. 19, 8621 (2019).
[5] H. Wu, J. Nance, S. A. Razavi, D. Lujan, B. Dai, Y. Liu, H. He, B. Cui, D. Wu, K. Wong, K. Sobotkiewich, X. Li, G. P. Carman, and K. L. Wang, Chiral Symmetry Breaking for Deterministic Switching of Perpendicular Magnetization by Spin-Orbit Torque, Nano Lett. 21, 515 (2021).

In ferroelectric/multiferroic heterostructures, intrinsic polarization and the resulting electric field generate a rich set of localized electrical properties that have recently begun to be studied by differential phase contrast imaging or four-dimensional scanning transmission electron microscopy (4D STEM). Due to the size of the probe in STEM, these emerging techniques are capable of imaging both the polarization and electric field over a wide range of length scales. But there is still much to be learned about how electron probe conditions affect the electric field measured with these techniques. Recently, we developed a real-space imaging technique that can directly map the local electric field and charge density of crystalline materials with sub-angstrom resolution, using scanning transmission electron microscopy (4D STEM) alongside an angle-resolved pixelated fast-electron detector. Using this technique, we image the interfacial charge distribution and ferroelectric polarization in a SrTiO₃/BiFeO₃ heterojunction in 4D, and reveal charge accumulation at the interface that is induced by the penetration of the polarization field of BiFeO₃. By performing Bader charge analysis on the acquired charge density maps, we demonstrate the existence of two-dimensional electron gas or hole gas (2DEG or 2DHG) at BiFeO₃/TbScO₃ interface, indicated by the fact that the charge state of each Fe/O column increases when the polarization points away from the interface and decreases when the polarization points toward the interface. Our charge-density imaging method advances electron microscopy from detecting atoms to imaging electron distributions, providing a new way of studying local bonding in crystalline solids.

By applying 4D STEM to image the electric field in (PbTiO₃)₁₆/(SrTiO₃)₁₆ superlattices, we demonstrate that measurements using electron probes of differing sizes reveal different electric field configurations. Using a picometer sized probe, we find the high-Z lead atomic columns exhibit a strong bias in the electric field correlated with the polarization direction. Using a nanometer sized probe, we find an electric field that is un-correlated with the polarization. Combining these results with phase-field simulations, we show that the dominant contribution of the electric field measured with 4D STEM changes from the dipole of individual unit cells, determined by the bonding structure of PTO, to the total electric field, determined by the strain and electrostatic boundary conditions, depending on the probe size. The separability of two different fields probed by 4D STEM offers possibility to reveal how each contributes to the electronic properties of the film.

Spin textures and their dynamics hold the key to understand and control the properties, behavior and functionalities of novel magnetic materials, which can impact the speed, size and energy efficiency of spin driven technologies. Topology, frustration, and bespoke geometries that impact spin textures have recently attracted significant scientific interest and led to intense research addressing a broad spectrum of challenging scientific and technological questions, including stability, dynamics, nucleation, and transport in novel spin textures, such as chiral bobbers, magnetic hopfions and torons, skyrmion tubes, and curvilinear magnetism [1].
Advanced characterization tools that provide magnetic sensitivity to spin textures, disentangling the role of individual components in heterogeneous material at high spatial resolution, ultimately at buried interfaces and in all three dimensions [2], and at high temporal resolution to capture the spin dynamics across scales, are required to address those questions, and are therefore of large scientific interest.
Various magnetic soft X-ray spectro-microscopies [3] using polarized soft x-rays provide unique characterization opportunities to study the statics and dynamics of spin textures in magnetic materials combining X-ray magnetic circular dichroism (X-MCD) as element specific, quantifiable magnetic contrast mechanism with spatial and temporal resolutions down to fundamental magnetic length, time, and energy scales.
Current developments of x-ray sources aim to increase dramatically the coherence of x-rays opening the path to new techniques, such as ptychography [4] or x-ray photo-correlation spectroscopy (XPCS) [5] that allow unprecedented studies of nanoscale heterogeneity, complexity, and fluctuations.
I will review recent achievements and future opportunities with magnetic x-ray spectro-microscopies. Examples will address static properties and dynamic behavior of various magnetic skyrmion [6,7] and Hopfion [8] textures, including those on curved substrates [9] with potential application to novel magnetic logic and storage devices.
This work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division Contract No. DE-AC02-05-CH1123 in the Non-Equilibrium Magnetic Materials Program (MSMAG).

[1] C.H. Back, et al, J Phys D: Appl Phys 53 (36), 363001 (2020)
[2] P. Fischer et al, APL Materials 8 010701 (2020)
[3] P. Fischer and H. Ohldag, Report on Progress in Physics 78 094501 (2015)
[4] X. Shi, et al, Appl Phys Letter 108, 094103 (2016)
[5] M. H. Seaberg, et al, Phys Rev Lett 119 067403 (2017)
[6] S. Woo, et al., Nature Materials 15 501 (2016)
[7] N. Kent et al, Appl Phys Lett 115 112404 (2019)
[8] N. Kent et al, Nature Comm 12 1562 (2021)
[9] R. Streubel et al, JAP 129 210902 (2021)

Magnetic-field sensors based on piezoelectric and magnetostrictive composite materials offer a low limit of detection and a high dynamic range. Here, a theoretical investigation of the sensitivity behavior of such composite magnetoelectric sensors is presented. Our focus is on biomedical applications targeting magnetocardiography (MCG) and magnetomyography (MMG). For these applications, measurement frequencies on the order of 1 to 100 Hz and a limit of detection in the pT-range are relevant. The behavior of two types of composite sensors is investigated in simulations – resonant cantilever-type sensors and surface acoustic wave (SAW) sensors. By combination of the simulated sensitivity with a noise model the limit-of-detection is predicted. Theoretical predictions are compared to experimental results of fabricated sensors.

Analytic calculations are used to derive the scaling behavior of FeCoBSi-Si-AlN cantilever-type magnetoelectric sensors. Sensitivity, noise, and resulting detection limits display different behavior with functional and substrate layer thicknesses [1]. For an improved limit of detection, a higher quality factor or thicker functional layers are necessary. For higher-mode operation, the finite-element method (FEM) is used to calculate the charge distribution along the cantilever. For higher modes, a change of sign in the charge distribution reduces the sensor sensitivity [2]. To prevent this and obtain maximum performance, sensors with segmented electrode are designed.

1D, 2D, 2.5D, and 3D FEM models for Love-wave propagation in magnetoelectric SAW sensors were established [3]. Theoretical and experimental results show that SAW sensors with lower IDT pitches and thus higher frequencies have a higher sensitivity. With the higher frequency and staying at the same guiding layer thickness, there are higher order Love waves [4]. Their frequencies are even higher, but their sensitivity is lower compared to their fundamental mode. From simulations, it is observed that the wave energy is not as concentrated at the surface for the higher modes and thus has less overlap with the magnetostrictive layer.

Biomedical magnetic fields are typically not homogeneous. To analyze the effect of an inhomogeneous excitation, we conducted simulations of localized excitation by a small coil at different positions along the sensor axis [5]. The simulations predict a rich excitation-position-dependent and frequency-dependent response behavior.

In summary, geometry-dependent effects, higher-mode operation, and inhomogeneous excitation fields are evaluated for magnetoelectric sensors. It is demonstrated that magnetic-field sensors suitable for magnetocardiography and magnetomyography with a limit of detection in the pT-range are achievable. In combination with the high linearity of the composite ME sensors unshielded and uncooled operation for wearable sensor systems is within reach.

This work is supported by the DFG Collaborative Research Center SFB1261.

[1] Krantz, M. C.; Gerken, M. Limit of Thermal-Vibration Noise in Magnetic Field Detection with Magnetoelectric-Composite Cantilevers. Phys. Rev. Appl. 2020, 13 (5), 054047.

[2] Schmalz, J.; Krantz, M. C.; Knies, A.; Lüder, H.; Gerken, M. Signal-to-Noise Ratio Enhanced Electrode Configurations for Magnetoelectric Cantilever Sensors. AIP Adv. 2020, 10 (7), 075314.

[3] Schmalz, J.; Spetzler, B.; Faupel, F.; Gerken, M. Love Wave Magnetic Field Sensor Modeling — from 1D to 3D Model. In 2019 International Conference on Electromagnetics in Advanced Applications (ICEAA); IEEE, 2019; 0765–0769.

[4] Schmalz, J.; Kittmann, A.; Durdaut, P.; Spetzler, B.; Faupel, F.; Höft, M.; Quandt, E.; Gerken, M. Multi-Mode Love-Wave SAW Magnetic-Field Sensors. Sensors 2020, 20 (12), 3421.

[5] Özden, M.-Ö.; Teplyuk, A.; Gümüs, Ö.; Meyners, D.; Höft, M.; Gerken, M. Magnetoelectric Cantilever Sensors under Inhomogeneous Magnetic Field Excitation. AIP Adv. 2020, 10 (2), 025132.

Large conductivity modulation by the coupling of ferroelectric polarization to the strongly correlated oxide LaNiO₃ has been demonstrated at an epitaxial ferroelectric-LaNiO₃ interface. High quality PZT/LNO on STO heterostructures were grown by MBE and off-axis RF magnetron sputtering and fabricated into all-oxide field effect devices. Using operando synchrotron diffraction measurements, we correlate changes in the conductivity with structural changes in the ferroelectric and conducting oxide as the ferroelectric polarization is switched. Comparing crystal truncation rods with half-order LNO Bragg reflections, we determine that the LNO oxygen octahedra are compressed and elongated while inside a comparatively rigid cation sublattice as the PZT ferroelectric polarization is switched. This finding is supported by DFT calculations. The LNO tetragonality change is accompanied by rotations of the LNO oxygen octahedra, with the tilt angles increasing and decreasing to modify the channel conductivity in a manner consistent with theory and transport.

3D scaffolds are essential in biomaterials science to resemble the natural 3D environment of living cells. However, many of these scaffolds are non-transparent or too thick for being imaged with conventional light microscopy. In such applications it would be very beneficial to achieve a magnetic detection of the cells after tagging them with magnetic superparamagnetic iron oxide nanoparticles (SPIONs) so that non-invasive imaging becomes possible. An interesting method in this context is Magnetic Susceptibility Particle Mapping (MSPM), which is able to detect magnetic nanoparticles in a lab scale device. Our system is based on magnetoelectric (ME) sensors utilizing the Delta-E effect in combination with a permanent magnet, which generates a bias field for the sensor and at the same time magnetizes the SPIONs. The permanent magnet is placed above the sensor, and the sample is rotated through the gap in between at a defined frequency. The SPIONs in the sample are magnetized by the permanent magnet and generate an additional magnetic field, which is recorded by the ME sensor. The detection threshold of our MSPM system is about 20 µg SPIONs in a volume of 200 mm³, and the spatial resolution is in the range of a few mm. We have also equipped the MSPM system with a magnetorelaxometry extension by using a self-biased sensor, so that it is possible to distinguish between particles bound to a material and diffusing particles. To demonstrate the feasibility of the MSPM method, we will present data that show the distribution of cells in a hydrogel matrix and of magnetic particles in responsive hydrogels, which are interesting in the context of tissue engineering and soft actuators.

The field of nanoscale ferroelectrics has utilized geometric confinement in thin films, nanodots, nanoislands, and strained superlattices to stabilize flux closures, vortices, skyrmions, and other topologically non-trivial polar states. Ferroelectric nanotubes provide a unique geometry with a large surface-to-volume ratio and vertical to lateral aspect ratio, and from these unique geometric properties, new polarization domain structures can be stabilized. Using phase-field modeling, we simulated the equilibrium polarization domain structure in Pb(Zr_0.52Ti_0.48)O₃ nanotubes under different height and wall thickness conditions, and three unique domain structures were found. Each domain structure comprises an array of periodic flux-closures and anti-flux-closures pairs formed to minimize elastic and electrostatic energy. These domain structures differ by the frequency of the flux-closure/anti-flux-closure pairs along the perimeter and height. We demonstrate that the thermodynamic stability of these domain structures can be tuned by changing the nanotube geometry with wall thickness and height.

Integration of high-performance and voluminous hard magnetic materials into microelectromechanical systems (MEMS) structures have prevalent potential applications in the areas of energy harvesting, loudspeaker, biomedical, and various other markets [1]. However, miniaturization of bulk hard magnets to hundreds of micrometers employing conventional top-down (micromachining, bonding, etc.) and bottom-up (sputtering, electro-deposition, etc.) approaches is challenging because of issues like high-temperature processes, patterning, the realization of complex structures, etc [2].

A novel technique based on the agglomeration of powder materials into rigid structures using atomic layer deposition (ALD), best adapted for MEMS fabrication processes, has been used to integrate high-performance and organic-free NdFeB micromagnets into MEMS structures [3,4]. 75nm of ALD deposited Al₂O₃ layer is ample to fully agglomerate the particles which are dry filled into the etched Si cavities (aspect ratios (t/l) of 0.25 to 5). Since the ALD process is a gas phase CVD technique it has the advantage to coat the particles detailedly with a homogeneous and conformal layer, as a result, the layer binds the particles together moreover protects them from corrosion.

In this study, we have investigated the salient factors and process parameters influencing the magnetic properties of the ALD agglomerated NdFeB micromagnets. A vibrating sample magnetometer was used to measure the magnetic properties. Low temperature (75°C) thermal ALD process of Al₂O₃ is developed to circumvent issues like oxidation during the deposition process and effects of thermal expansion mismatch. Besides fabricating micromagnets comprising of single hard magnetic material (NdFeB, SmCo) we fabricated hybrid micromagnets by mixing (in weight ratio) NdFeB, SmCo magnetic materials to improve the effective magnetic properties of the micromagnets. The hybrid micromagnets displayed better reversible temperature coefficients over a specified temperature range. We analyzed the susceptibility curves and recoil loops to understand the magnetic mechanism. Additionally, magnetic interactions in the agglomerated particles were interpreted from Henkel plots by measuring dc demagnetization remanence and isothermal remanence. The δM plot indicates negative values elucidating magnetostatic type of interactions.

1. Niarchos, D. "Magnetic MEMS: key issues and some applications." Sensors and Actuators A: Physical 109.1-2 (2003): 166-173.
2. Arnold, David P., and Naigang Wang. "Permanent magnets for MEMS." Journal of microelectromechanical systems 18.6 (2009): 1255-1266.
3. Lisec, T., et al. "A novel fabrication technique for MEMS based on agglomeration of powder by ALD." Journal of Microelectromechanical Systems 26.5 (2017): 1093-1098.
4. Bodduluri, Mani Teja, et al. "High-performance integrated hard magnets for MEMS applications." MikroSystemTechnik 2019; Congress. VDE, 2019.

Strain-engineering is a powerful technique to control the properties of perovskite oxides. The effect of bi-axial strain is effectively a rotation/tilt of the oxygen octahedra, altering bond angles and lengths. Antiperovskite materials, functional materials that can possess properties such as antiferromagnetism, superconductivity and topological effects, have a perovskite crystal structure. However, a cation is occupying the oxide anion position while the perovskite B-site is occupied by an anion. Recent progress in thin film deposition has advanced heterostructures of antiperovskites with oxide perovskites and other quantum materials. Here, a density functional theory (DFT) study of the effect of bi-axial strain on the cubic antiperovskite Mn₃GaN is presented. Mn₃GaN is antiferromagnetic with the Mn spins ordered in a Kagome Γ^5g structure. Contrary to perovskite oxides, the cation-octahedra of Mn₃GaN are resilient to strain and the system prefers to adjust into a centrosymmetric tetragonal structure. In order to understand this resilience, the implications of the intermetallic nature of antiperovskites are addressed, and a detailed mapping of how the chemical bonds are affected by strain in Mn₃GaN is compared to the effect of strain on the ionic bonds in the canonical cubic oxide perovskite SrTiO₃. The stiffness of the Mn₃GaN phonon structure to strain will also be compared to phonon modes that cause octahedral rotations in perovskites. Lastly, the effect of biaxial strain of Mn₃GaN on electronic properties will be addressed via band structure investigations.

Ferroelectric (FE) bubble-like domains contain specially ordered electric dipoles. These objects are considered as possible multifuntional topological defects^[i],[ii] that may find revolutionizing applications in the field of microelectronics, due to their exceptional electronic and mechanical behavior. These nanoscale objects arise only in ultrathin complex oxide ferroelectric-dielectric-ferroelectric heterostructures that are epitaxially clamped with flat substrates. The parameter space (epitaxial strain, screening, built-in field, etc.) that favors their formation is quite narrow, and epitaxial growth is necessary. Due to the need for stringent growth conditions, the formation of bubble domains is therefore only possible on select complex oxide platforms. As such, integration of these functional objects with mainstream microelectronics substrates such as silicon and germanium is beyond the current capability of direct synthesis technique. Recently discovered epitaxial layer transfer techniques ^{[iii],[iv],[v]} can be a possible remedy in this regard. These techniques enable the epitaxially grown complex oxides to be freestanding by selectively detaching them from the host substrate. The freestanding complex oxides can be subsequently transferred to an arbitrary substrate.
In this work,^[vi] we demonstrate that ferroelectric bubble domains can be stabilized in a freestanding epitaxial FE – DE – FE complex oxide heterostructure. We find that the FE bubbles are invariant to surface bending and mechanical perturbations up to a limit – suggesting the presence of an energy barrier to their breakdown. Local piezoresponse force mapping, together with field-dependent capacitance tuning and its agreement with calculations of capacitive response, confirm that the FE bubbles conserve their general electromechanical characteristics — this is in spite of a surface that is heavily rippled. We anticipate these results to be the starting point of a new paradigm for the exploration of electric skyrmions with arbitrary boundaries and physically flexible topological orders in ferroelectric curvilinear space.


[i] Y. Nahas, S. Prokhorenko, L. Louis, Z. Gui, I. Kornev, L. Bellaiche, Nat Commun. 2015, 6, 8542.


[ii] Q. Zhang, L. Xie, L. Guangqing, S. Prokhorenko, Y. Nahas, X. Pan, L. Bellaiche, A. Gruverman, N. Valanoor, Adv. Mater. 2017, 29, 1702375.

[iii] S. R. Bakaul, C. R. Serrao, M. Lee, C. W. Yeung, A. Sarker, S.-L. Hsu, A. K. Yadav, L. Dedon, L. You, A. I. Khan, J. D. Clarkson, C. Hu, R. Ramesh, S. Salahuddin Nat. Commun. 2016, 7, 10547.

[iv] S. R. Bakaul, C. R. Serrao, O. Lee, Z. Lu, A. Yadav, C. Carraro, R. Maboudian, R. Ramesh, S. Salahuddin Adv. Mater. 2017, 29, 1605699.

[v]. D. Lu, D. J. Baek, S. S. Hong, L. F. Kourkoutis, Y. Hikita, H. Y. Hwang, Nat. Mater. 2016, 15, 1255.

[vi] S. R. Bakaul, S. Prokhorenko, Q. Zhang, Y. Nahas, Y. Hu, A. Petford-Long, L. Bellaiche, N. Valanoor, Adv. Mater. (accepted, 2021)

The discovery of single-phase magnetoelectric materials and studies of their coupling mechanisms between spin and polarization are important for next generation logic and memory devices. Highly (001) oriented 0.55Pb(Zr_0.52Ti_0.48)O₃-0.45Pb(Fe_0.67W_0.33)O₃ (0.55PZT-0.45PFW) multiferroic thin films were deposited on La_0.67Sr_0.33MnO₃ (LSMO) buffer layer coated on (LaAlO₃)_0.3(Sr₂AlTaO₆)_0.7 (001) substrates by following two subsequent laser ablation processes in oxygen atmosphere employing pulse laser deposition technique. The 0.55PZT-0.45PFW films were found to grow in a tetragonal phase with orientation along (001) plane as inferred from x-ray diffractometry analysis. The temperature dependent dielectric measurements (80-730 K) on metal-ferroelectric-metal heterostructure capacitors in the frequency range of 10²–10⁶ Hz indicated broad and diffused dielectric peaks over a wide range of temperature 320-730 K and exhibits high dielectric constant ~2100 at 10 kHz at room temperature. The well saturated slim polarization hysteresis loop of the thin film capacitors suggests nano-scale ferroelectric ordering (relaxor type) in these films in accord with diffuse dielectric response results. An excellent high energy storage density with efficiency of ~62 % was estimated at an applied electric field of 1200 kV/cm. Magnetization studies involving Zero Field Cool and Field Cool magnetization curves on the thin films using PPMS down to 4 K indicated an irreversible magnetic ordering in the thin films. A saturated magnetization M-H loop with P_s value of 32 emu/cm³ was observed at room temperature. The magneto-dielectric studies suggest robust direct evidence of magneto-electric coupling via strain at room temperature. These results suggest that our thin films are multiferroic (ferroelectric-ferromagnetic) at room temperature. These results will be presented in detail at the MRS Fall meetings.

So far, the orbital moment has not been seriously considered as an information carrier due to the orbital quenching in ordinary solid-state matters. However, as claimed in recent theoretical studies, we can expect nonzero orbital moment polarization against the orbital quenching whenever the orbital texture is formed in systems, e.g., surfaces or bulks [1,2]. Notably, the formation of orbital texture is irrelevant to the spin-orbit coupling strength (SOC). Thus we expect efficient orbital moment polarization even in a system composed of light elements. The magnetization of a magnetic layer feels a torque if the orbital polarization is transported to a magnetic layer. Hence, it is called orbital torque.
We systemically observe the orbital torque in ferromagnetic metal (FM)/Cu/Al₂O₃ trilayers [3]. Here we believe the Cu/Al₂O₃ interface is responsible for the orbital polarization. All samples in this study are prepared by the electron beam evaporation technique and photo-lithography. We utilized the ferromagnetic resonance technique to observe the torque efficiency. Interestingly, the orbital torque strongly depends on the FM materials. When the FM is CoFe or Fe, the torque efficiency reaches ~0.13, comparable to that for Ta or Pt. In contrast, the torque efficiency becomes almost zero when the FM is a Ni-based material.
Moreover, we find a significant annealing temperature dependence in the torque efficiency. After annealing at 400 C, the torque efficiency for the CoFe/Cu/Al₂O₃ trilayers becomes ~0.25, which is comparable to that for W. All these strong dependencies of torque efficiency on the FM materials and the annealing temperature reflect a characteristic transport of orbital moments. Commonly the change of the FM materials and the heat treatment modifies the interfacial/layer condition through the intermixing or separation. Since localized electrons transport information of the orbital moment, it is more sensitively affected by the material condition than spins.
Finally, we note that the spin Hall conductivity in the FM/Cu/Al₂O₃ trilayers is 3.4×10⁴ Ω^-1cm^-1, about 10-100 times larger than that for spin Hall materials or topological insulators. Thus, we believe the FM/Cu/Al₂O₃ trilayers are superb for spin manipulation with low power consumption.
We will give further discussions during the symposium.


[1] S. R. Park et al., Phys, Rev, Lett. 107, 156803 (2011).
[2] D. Go et al., Phys. Rev. Lett. 121, 086602 (2018).
[3] J. Kim et al., Phys. Rev. B 103, L020407 (2021).

Bismuth ferrite is a well-known multiferroic ceramic. It shows ferroelectric and antiferromagnetic ordering at a Curie temperature of ~1100 K and a Néel temperature of ~653 K. Bismuth ferrites properties can be altered by varying shape and size of particles and also by doping other elements. In this study, bismuth ferrite was synthesized using three methods for comparison – a conventional high-energy shaker mill, a nano-agitator bead mill, and precipitation followed by calcination. Gadolinium (10%) and cobalt (10%) were used as dopants in the calcination method. The composition, morphology, and magnetic properties of the resulting particles were characterized with SEM, XRD, and VSM. The powders produced by both milling methods were weakly ferromagnetic. The particles produced from the nano-agitator mill process had a more uniform distribution of grain sizes than the conventional shaker mill process. The particles produced through calcination method were large polygons covered in smaller facets. This method is faster, but there are few options available for substitution of the B-site cation. The milling methods show fewer impurity phases than the calcination method.

Magnetic refrigeration has drawn the attention of many researchers due to its advantages over significant drawbacks of conventional cooling methods. The search is for a material with substantial relative cooling power (RCP), high magnetic entropy change(ΔS_m) and can operate at room temperature. La_0.7Ca_0.3MnO₃ exhibits high RCP of 243.1 J/Kg [1] and maximum ΔS_m of 6.99 J/Kg.K [1]. It has been pre-established that the composite formation undergoes multiple magnetic phase transition resulting in increase of RCP and ΔS_m [2]. For example, it is reported that ΔS_m and RCP is 2.32 J/Kg.K and 299 J/Kg for La_0.45Nd_0.25Sr_0.3MnO₃-2.5%NiO composite [2] and ΔS_m and RCP is 1.95 J/Kg.K and 298 J/Kg for La_0.45Nd_0.25Sr_0.3MnO₃-2.5%CoO composite [2]. All of these are improvement over ΔS_m and RCP value of 1.79 J/Kg.K and 296 J/Kg for pure La_0.45Nd_0.25Sr_0.3MnO₃ sample. In case of La_0.45Nd_0.25Sr_0.3MnO₃-2.5%CuO composite, significant increase in ΔS_m is reported to 3.95 J/Kg.K in compensation for decrease in RCP to 245 J/Kg which is attributed to the proximity between the critical temperature of La_0.45Nd_0.25Sr_0.3MnO₃, 240K and the Neel temperature of the CuO, 230K [2]. The MCE study on LCMO-MO (metal oxide) composites is not widely reported and with superior ΔS_m value of 6.99 J/Kg.K for La_0.7Ca_0.3MnO₃ [1] as compared to that of 1.79 J/Kg.K for La_0.45Nd_0.25Sr_0.3MnO₃ [2] better results are expected for LCMO-MO composite. Also, the recorded critical temperature of La_0.7Ca_0.3MnO₃ of 256K is significantly different from Neel temperature of used NiO, CuO and CoO which should yield higher ΔS_m and RCP. Other literatures have also recorded the performance improvement in manganite by composite formation with metal oxides [4]. This study presents the structural and magnetocaloric study of La_0.7Ca_0.3MnO₃ composite with metal oxides; NiO, CuO, CoO.
The (1-x)La_0.7Ca_0.3MnO₃-xMO (x=0, 0.25, 0.5 ; M=Ni, Cu, Co) nanocomposite samples were synthesized through autocombustion using nitrate salts followed by calcination. X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) were used to study structural characterize the samples. The XRD plots show no secondary phases and respective increase of the metal oxide phases with increase of their content. A grain refinement of LCMO phase was observed with the increased metal oxide phase in the composite. To study the magnetocaloric properties of the composites, magnetic isotherms were collected in the temperature range 100-300K and field up to 5T. The data is currently being analyzed to compare the magnetocaloric performance of the composites.



References
1. Wei Tang, Wenjian Lu, Xuan Luo, Bosen Wang, Xuebin Zhu, Wenhai Song, Zhaorong Yang, Yuping Sun, Particle size effects on La0.7Ca0.3MnO3: size-induced changes of magnetic phase transition order and magnetocaloric study. Journal of Magnetism and Magnetic Materials. 322-16, 2360-2368 (2010)
2. D. Neupane, L. Hulsebosch, A. K. Pathak and S. R. Mishra, Magnetocaloric Study of La0.45Nd0.25Sr0.3MnO3/MO (MO=CuO, CoO, and NiO) Nanocomposites. IEEE Transactions on Magnetics. (2021)
3. Sheng-Bo Tian, Manh-Huong Phan, Seong-Cho Yu, Nam Hwi Hur, Magnetocaloric effect in a La0.7Ca0.3MnO3 single crystal. Physica B: Condensed Matter. 327-2-4, 221-224 (2003)
4. M.S. Anwar, Faheem Ahmed, Rehan Danish, Bon Heun Koo, Impact of Co3O4 phase on the magnetocaloric effect and magnetoresistance in La0.7Sr0.3MnO3/Co3O4 and La0.7Ca0.3MnO3/Co3O4 ceramic composites. Ceramics International. (2014)

The discovery of robust ferroelectricity in ultrathin HfO₂-based materials has aroused surging interests in developing silicon-compatible ferroelectric nanoelectronics. To understand the fundamental origin of the ferroelectricity, epitaxial thin films of HfO₂-based oxides such as Hf_0.5Zr_0.5O₂ (HZO) have been successfully grown, wherein different polar phases have been detected, including the orthorhombic Pca2₁ phase and a newly discovered rhombohedral phase. However, it is not well understood how the misfit strains imparted by the substrate materials influence the stability of the polar phases. Here, we present a comprehensive thermodynamic model, developed based on our recently proposed strain phase separation theory, to predict and understand the stability and coexistence of polar and nonpolar phases in HZO epitaxial thin films. We demonstrate that the misfit strains imposed by substrates of different crystallographic structures (e.g., fluorite, perovskite, and hexagonal) can serve as a viable way to stabilize different polar phases. We establish film thickness – lattice constant phase diagrams for HZO thin films on different types of substrates, which can be utilized to understand the origin of ferroelectricity and guide the strain engineering of HfO₂-based ferroelectric thin films.

Ferroelectrics are usually inflexible oxides that undergo brittle deformation. We synthesized freestanding single crystalline ferroelectric BTO(BaTiO₃), BFO(BiFeO₃) and BTO/LSMO bi-layer membranes and with a damage-free lifting-off process. Our ferroelectric membranes can undergo a nearly 180° folding during in-situ bending test, demonstrating a super-elasticity and ultra-flexibility¹. We found that the origin of the super-elasticity was from the dynamic evolution of ferroelectric nanodomains in BTO and phase change in BFO. High stresses modulate the energy landscape dramatically and allow the dipoles to rotate continuously. A continuous transition zone is formed to accommodate the variant strain and avoid high mismatch stress usually causing fracture. BTO/LSMO shows a self-assembled spring shape due to the lattice mismatch as free-standing in the air. Such spring can undergo up 50% of the length change upon a mechanic forced is loaded. The phenomenon should be possible in other ferroelectrics systems through domain engineering. The ultra-flexible epitaxial ferroelectric membranes could enable many magnetoelectric applications such as flexible sensors, memories and electronic skins. 1. Dong, et al., Science 366, 475-479 (2019)

Recently, acoustically actuated antennas based on the thin-film magnetoelectric heterostructures have been demonstrated with 1-2 orders of magnitude miniaturization over the conventional counterparts. However, it remains challenging to improve their radiation efficiency and develop the high quality antenna modulation technique. Here, we computationally demonstrate a fully coupled model describing the radiation mechanism, including Newton, Maxwell and Landau-Lifshitz-Gilbert (LLG) equations, together with piezoelectric and magnetostrictive constitutive relations. We theoretically propose that magnons, the quanta of spin waves, can be utilized to strongly enhance the radiation efficiency of acoustically actuated antennas when magnon and phonon modes are coupled. In the strong coupling region, our model shows that the radiation efficiency can be 100-time enhanced, and antenna modulation can be substantially simplified. Additionally, a right-hand circular polarization (RHCP) radiation field can be observed.

Polar vortex domains have recently become an emergent research field due to the abundant physical phenomena and potential applications in high-density memories. Here, we explore the mechanisms of creating polar vortex domains in the BiFeO₃ membranes subjected to different boundary conditions using phase-field simulations. A major difference is that the vortex in membrane can be stabilized even under short-circuit electrical boundary conditions compared to vortex in other systems, such as thin film or superlattice. We found that (i) the formation of polar vortex domains at the membrane interior under bending is mainly driven by the reduction of elastic energy under short-circuit boundary condition, and the vortex chirality (namely, clockwise and counterclockwise) could be identified by n-shape and u-shape bending; (ii) in the unbent open-circuit BiFeO₃ membrane case, exotic trapezoid-shaped vortex nanodomains form at the terminations of 109^o domain walls (DWs) and partially charged 71^o DWs, which is driven by the local depolarization field and the interplay among electrostatic, elastic, and gradient and Landau energies. We also examine Kittel’s law by establishing the dependence of vortex periods on the membrane thickness. These results give further understanding of the effect of boundary conditions on the formation of polar vortex domains, guiding experimental designs of vortex-based high-density memories.

Realization of ultrasmall ferroics with nontrivial topological field textures such as vortices, skyrmions, and merons holds promise in novel technological paradigms. Such nontrivial ferroelectric orders and their functionalities, however, inevitably disappear below a critical size of several nanometers. In addition, very few topological structures can exist in ferroelectrics due to the lack of non-collinear interaction among electric dipoles, unlike the Dzyaloshinskii-Moriya (DM) interaction among spins in ferromagnetics. Here, we demonstrate from first-principles that “Polar Skyrmions and Merons” are formed in doped SrTiO₃ heterostructures. Doped (excess) electrons in SrTiO₃ are localized and form a polaronic state in the heterostructures (surfaces and grain boundaries), and give rise to skyrmionic and meronic dipole moments around the polaron formation sites due to the cooperative symmetry breaking of polarons and heterostructures. We further show that the topological number of polaronic state can be tailored by applied mechanical strain, i.e., strain engineering for polar topologies. Our discovery overcomes physical limitations of the critical size of 3-5nm where ferroelectricity disappears and the inability to form topological field (skyrmions, merons) of polarization due to absence of chiral interaction among electric dipoles, and realizes unique polar topological orders at ultimately electron(polaron)-scale. The clarified mechanism that local symmetry breaking via polaron formation coupled with heterostructures provides a novel approach to realize ultimate miniaturization of ferroic materials and opens up new fields to create the polar topological objects. Our result therefore adds a new class of functional polaron families as “Topological Polarons”.

Magnetoelectric hexagonal iron sulfide (h-FeS) exhibits a fascinating coexistence of metal-insulator, structural and magnetic transitions, reflecting an intimate interplay of its spin, phonon, and charge degrees of freedom. Using comprehensive neutron and X-ray scattering measurements supported by first-principles electronic structure simulations, we show how a subtle competition of energetic and entropic free-energy components governs its thermodynamics and the sequence of phase transitions upon cooling.

We identify the critical role of the coupling between antiferromagnetic ordering and the instabilities of anharmonic phonons in the metallic phase in driving the metal-insulator transition. The antiferromagnetic ordering enables the emergence of two zone-boundary soft phonons, whose coupling to a zone-center mode drives the lattice distortion opening the electronic bandgap. Simultaneously, spin-lattice coupling opens a gap in the magnon spectrum that controls the entropy component of the metal-insulator transition-free energy. These results reveal the importance of spin-phonon coupling to tune anharmonic effects, thus opening new avenues to design novel technologically important materials harboring the metal-insulator transition and magnetoelectric behaviors.

D. Bansal, J.L. Niedziela, S. Calder, T. Lanigan-Atkins, R. Rawl, A. Said, D.L. Abernathy, A.I. Kolesnikov, H. Zhou, and O. Delaire. “Magnetically driven phonon instability enables the metal-insulator transition in h-FeS.” Nature Physics. Vol. 16, pg. 669-675, 2020

Funding: US Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences, and Engineering Division, under the Early Career award no. DE-SC0016166 (PI: O. Delaire).

This work deals with the structural, magnetic and magneto-caloric properties of double perovskite Pr₂MnNiO₆ (PMNO) material. The bulk material was prepared in a single phase using the solid-state reaction method. The structural characterization showed monoclinic symmetry with the P21 / n space group. The temperature-dependent magnetic measurements were performed in presence of an applied magnetic field of 500 Oe, which showed a phase transition from paramagnetic to ferromagnetic at ~ 200K. The saturation (ZFC) magnetization shows the change in the slope below ~ 50 K, which may be attributed to the strong interaction between A-site (Pr) and B-site (Mn-Ni network) ions. We carried out the isothermal M vs H (hysteresis curve) measurements in the temperature range of 2K-190 K in the presence of a 0-10 T magnetic field to determine the change in magnetic entropy. The estimated value of magnetic entropy (-ΔS_M) is 1.6 J/Kg-K at 23 K under 10 T magnetic field. Interestingly, our report highlights the presence of low-temperature magneto-caloric effect in PNMO material.

Antiferromagnetic (AFM) spintronics is an emerging field of research which exploits the antiferromagnetic order parameter, known as the Néel vector, to control spin- and orbital-dependent transport properties. This talk will address three novel approaches to functionalize the Néel vector in collinear compensated AFM metals via their response in transverse or longitudinal conductivity. The first approach utilizes room-temperature AFM metal MnPd₂ that allows the electrical control of the Dirac nodal line by the Néel spin-orbit torque [1]. The reorientation of the Néel vector leads to switching between the symmetry-protected degenerate state and the gapped state which modulates the spin Hall conductivity. The second approach involves the nonlinear anomalous Hall effect that can be used to detect the Néel vector in most compensated antiferromagnets supporting the antidamping spin-orbit torque [2]. The magnetic crystal group symmetry of these non-centrosymmetric antiferromagnets, such as CuMnSb, combined with spin-orbit coupling produce a sizable Berry curvature dipole and hence the nonlinear anomalous Hall effect. The third approach highlights antiferromagnets exhibiting a non-spin-degenerate Fermi surface and thus momentum-dependent spin polarization which can be functionalized in AFM tunnel junctions [3]. Using RuO₂ as a representative example of such antiferromagnets, a giant tunneling magnetoresistance effect is predicted for RuO₂-based AFM tunnel junctions. These results broaden the scope of materials and approaches, which can be exploited in AFM spintronics.

1. D.-F. Shao, G. Gurung, S.-H. Zhang, and E. Y. Tsymbal, “Dirac nodal line metal for topological antiferromagnetic spintronics,” Physical Review Letters 122, 077203 (2019).
2. D.-F. Shao, S.-H. Zhang, G. Gurung, W. Yang, and E. Y. Tsymbal, “Nonlinear anomalous Hall effect for Néel vector detection,” Physical Review Letters 124, 067203 (2020).
3. D.-F. Shao, S.-H. Zhang, M. Li, and E. Y. Tsymbal, “Spin-neutral currents for spintronics,” arXiv:2103.09219 (2021).

Nonreciprocity, the property of merit for a variety of RF components such as isolators and circulators, is typically difficult to achieve with the magnitude required for applications. Many systems which demonstrate non-reciprocity are not suitable for scaling to the size, weight, and power required for applications, or do not exhibit a sufficient intensity of the effect for applications relevance. One such physical system which generally does not exhibit non-reciprocity is that of acoustic waves. Acoustic waves are an important medium for information transport, but they are inherently symmetric in time. By achieving non-reciprocity in an acoustic system, the decades of industrial research in this relatively mature field can be leveraged for new applications. The most promising route to accomplish this is by incorporating magnetic materials into surface acoustic wave (SAW) devices, as in the field of acoustically driven ferromagnetic resonance. While this field remained relatively small since the initial discovery, in the past year, many new reports investigating magnetic materials in SAW devices have been published. Motivated by this expanding field, I will discuss our recent work at the Air Force Research Laboratory on the discovery of giant nonreciprocity in the transmission of surface acoustic waves (SAWs) on a lithium niobate substrate coated with ferromagnet/insulator/ferromagnet (FeGaB/Al₂O₃/FeGaB) multilayer structure. I will discuss how this structure, with a unique asymmetric band diagram, can be exploited for very large non-reciprocal behavior. I will expand on magnetoelastic coupling theory to show how the magnetic bands couple with acoustic waves only in a single direction. In our devices, we measure 48.4 dB (ratio of 1:100,000) isolation which outperforms current state of the art microwave isolator devices in a novel acoustic wave system that facilitates unprecedented size, weight, and power reduction. Finally, I will discuss how these results offer a promising platform to study nonreciprocal SAW devices and other such magnetoacoustic devices.

A complex structure of magnetic subsystem in antiferromagnets (AFMs) determines challenges and technological perspectives for both, fundamental research and their applications for spintronic and spin-orbitronic devices [1]. In this respect, properties of the confined samples are of key interest because of the possibility to tune magnetic responses via effects of boundary and geometrical curvature [2]. Here, we consider textures in (i) AFM slabs with the Dzyaloshniskii-Morya interaction (DMI) of bulk symmetry [3] and (ii) the intrinsically achiral curvilinear spin chains arranged along space curves [4].

We derive a transition from spin lattice of G-type AFM to the sigma-model with the respective boundary conditions for the AFM order parameter [3]. The DMI influences a texture via boundary conditions modifying the ground state, domain wall shape and skyrmion profiles. Approaching the boundary in the slab with easy-axis anisotropy, the domain wall becomes broader and of mixed Bloch-Neel type near the top surface. Near the edges of the sample, the domain wall plane possesses and additional twist. Note, that the edge twists appear in achiral AFMs as well if the domain wall plane lies at an angle to the side faces [5]. Similarly, skyrmions of any radius become of the Bloch-Neel type approaching the top/bottom surfaces of the sample. The radius of narrow skyrmions changes up to 10% due to the boundary effects.

AFM spin chains arranged along space curves can model the simplest curvilinear nanoarchitectures. Their geometry is described by the curvature and torsion, determining local bends and twists of the curve. The geometry-driven anisotropy and inhomogeneous DMI render them as chiral helimagnets [6]. In addition, the exchange interaction generates the weakly ferromagnetic response, scaling linearly with curvature and torsion. The inter- and single-ion anisotropies in curvilinear AFM chains lead to the additional anisotropic contributions, scaling with curvature. The single-ion anisotropy leads to the homogeneous DMI mixing normal and tangential components of ferro- and antiferromagnetic vector order parameters. Both anisotropy models contribute to the additional easy axes, which determine the direction of the order parameters in spin-flop phase [4].

[1] V. Baltz et al, Rev. Mod. Phys. 90, 015005 (2018); A. Manchon et al, Rev. Mod. Phys. 91, 035004 (2019)
[2] P. Fischer et al, APL Mat. 8, 010701 (2020); R. Streubel et al, J. Appl. Phys. 129, 210902 (2021); D. D. Sheka, Appl. Phys. Lett. 118, 230502 (2021)
[3] O. V. Pylypovskyi et al, Phys. Rev. B 103, 134413 (2021)
[4] O. V. Pylypovskyi et al, Appl. Phys. Lett. 118, 182405 (2021)
[5] N. Hedrich et al, Nat. Phys. 17, 574 (2021)
[6] O. V. Pylypovskyi, D. Y. Kononenko et al, Nano Lett. 20, 8157 (2020)

Resonant Elastic X-Ray Scattering (REXS) is a powerful technique that combines X-Ray Diffraction (XRD) and X-Ray Absorption (XAS), which has elemental sensitivity to both the physical and electronic structure of materials. Due to the cross-section enhancement near an absorption edge, one can select for spin, charge, and orbital order. We apply this technique to rhenium-based double perovskite thin films, which are well known to possess an intricate interplay between structure and orbital electronic configuration. Especially, Sr2CrReO6 (SCRO) has been theoretically predicted to have order of the dxy and dyz orbitals on the rhenium sites. REXS has been performed on three SCRO thin films in different state of strain, compressive -1% on a (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT) substrate, tensile +1% on a relaxed SrCr0.5Nb0.5O3 buffer layer on LSAT and free of strain on SrTiO3 (STO). We will discuss the presence and absence of orbital-order induced Bragg peaks in the three samples and compare Bragg peaks intensities as functions of the synchrotron energy through the L-II and L-III edges of Re in order to determine crystal site specific x-ray absorption spectra.

Polar vortices in PbTiO₃/SrTiO₃ superlattices are unique topological textures where local electric dipoles are arranged in a spiral fashion.¹ These structures are formed via careful control of strain, electrical, and electrostatic boundary conditions.¹ The unique arrangement of electric dipoles results in two major emergent properties: negative capacitance and chirality.^2,3 While negative capacitance in this system is theoritically expected, the presence of the chirailty is unexpected as these textures are formed from achiral objects. In this work, we have investigated the origins of chirality in this system by examining the three dimensional polarization network. To extract three dimensional information, conventional high-resolution scanning transmission electron microscopy (HR-STEM) can't be used directly since it is based on two dimensional projection. Recently, it has been shown that the depth dependence of four dimensional scanning transmission electron microscopy (4D-STEM) can be utlized to extract the underlying polarization.⁴ Utilizing 4D-STEM, we map out the three dimensional polarization network (lateral and axial polarization) in such textures. We found that the presence of the domain boundaries and dislocations influence the local handedness in these textures. The presence of the chirality is related to the local curvature of the domain boundary. We also find the ability to switch chirality is closely related to how the local polarization varies across the domain boundary. The emergence of chirality from the combination of non-chiral materials and ability to control the handed-ness has far-reaching implications for new electronics based on topological ferroelectric bits.

References:
1. Yadav, A. K. et al. Observation of polar vortices in oxide superlattices. Nature 530, 198–201 (2016)
2. Yadav, A. K. et al. Spatially resolved steady-state negative capacitance. Nature 565, 468–471 (2019)
3. Shafer, P. et al. Emergent chirality in the electric polarization texture of titanate superlattices. Proc. Natl. Acad. Sci. 115, 915–920 (2018)
4. Nguyen, K. X. et al. Transferring Orbital Angular Momentum to an Electron Beam Reveals Toroidal and Chiral Order. ArXiv201204134 Cond-Mat Physics(2020).

Over the past two decades, new discoveries and major advances have been made in the synthesis of oxide-based ferroic thin films and understanding their physical properties. Interface engineering, strain engineering, and defect engineering have been widely used to manipulate the physical properties of the ferroic films in classical lateral heterostructures. As a unique thin-film architecture, vertically aligned epitaxial nanocomposite films have also been widely studied in the past decade and great success has been achieved. However, strain-defect-microstructure-function correlation in such nanocomposite films has not been well-established yet. In this talk, we will overview our strategies in synthesis and characterization of vertically aligned epitaxial nanocomposite ferroic films. Using controlled synthesis, advanced probing, and theoretical modeling, we are able to understand the effect of interface strain on the induced ferroelectric properties in the vertically aligned epitaxial nanocomposite SrTiO₃ films.

Magneto-ionic control targets a change in the magnetic properties of materials by electrochemical reactions, which usually occur at low voltage and at room temperature (1). The most actively studied ions for magneto-ionic systems are oxygen (2,3) and hydrogen (4,5). In most cases, the effect of either oxygen or hydrogen is studied, despite the fact that a dual ion-switch, in which the hydrogen- and oxygen-based mechanisms are combined, may increase the versatility of magneto-ionics. A dual-ion switch has so far been studied only in oxide systems, and in this case major effects were restricted to low temperatures. A transfer to magneto-ionic metal systems with high Curie temperature would boost the perspectives for practical applications (6).
We report combined hydrogen- and oxygen-based magneto-ionic control of electrodeposited nanocrystalline nickel films in an aqueous alkaline electrolyte. To determine and analyze the magneto-ionic effect, in situ SQUID and in situ MOKE magnetometry measurements were used.
We demonstrate simultaneous voltage-control of magnetic moment and coercivity, caused by hydrogen- and oxygen-based mechanisms, respectively. The significant change in coercivity (up to 52 %) is associated with the removal of oxide pinning centers at the surface. It shows a training effect and becomes stronger with an increased number of cycles. The relatively small changes of magnetic moment (about 0.26%) are in a good agreement with the amount of hydrogen loaded into the Ni films in 1 M KOH (7). Both changes are reversible and repeatable at room temperature and require only a low voltage difference of about 1 V.
This combined ionic mechanism to tune magnetic metals may initiate a new direction in room-temperature magneto-ionics and expand the range of its practical applications.


1. Nichterwitz M, Honnali S, Kutuzau M, Guo S, Zehner J, Nielsch K, et al. Advances in magneto-ionic materials and perspectives for their application. APL Mater. 2021 Mar 1;9(3):030903.
2. Nichterwitz M, Honnali S, Zehner J, Schneider S, Pohl D, Schiemenz S, et al. Control of Positive and Negative Magnetoresistance in Iron Oxide–Iron Nanocomposite Thin Films for Tunable Magnetoelectric Nanodevices. ACS Appl Electron Mater. 2020 Aug 25;2(8):2543–9.
3. Zehner J, Soldatov I, Schneider S, Heller R, Khojasteh N, Schiemenz S, et al. Voltage-Controlled Deblocking of Magnetization Reversal in Thin Films by Tunable Domain Wall Interactions and Pinning Sites. Adv Electron Mater. 2020 Nov;6(11):2000406.
4. Tan A, Huang M, Avci C, Büttner F, Mann M, Hu W, et al. Magneto-ionic control of magnetism using a solid-state proton pump. Nat Mater. 2019 Jan;18(1):35–41.
5. Gößler M, Albu M, Klinser G, Steyskal E, Krenn H, Würschum R. Magneto-Ionic Switching of Superparamagnetism. Small. 2019 Nov;15(46):1904523.
6. Lu N, Zhang P, Zhang Q, Qiao R, He Q, Li H-B, et al. Electric-field control of tri-state phase transformation with a selective dual-ion switch. Nature. 2017 Jun;546(7656):124–8.
7. Proost J, Delvaux A. In-situ monitoring of hydrogen absorption into Ni thin film electrodes during alkaline water electrolysis. Electrochimica Acta. 2019 Nov;322:134752.

The stability and evolution of mesoscale structures in solids often involve not only lattice structural and chemical inhomogeneities but also inhomogeneous distributions of electronic, atomic, and ionic defects. We present a general phase-field framework of coupled defect and microstructure evolution of perovskite ferroelectric oxides. In particular, starting from well-established classic thermodynamics, it offers an explicit formulation to obtain the electronic and lattice defect contributions to the thermodynamic properties using statistical/classical thermodynamics. In this model, we treat the ferroelectric perovskite oxide as a wide band-gap semiconductor and we derive all well-known thermodynamic relations related to defect reactions, equilibria including redox and electronic recombination/generation reactions. The model is applied to BiFeO₃ as a prototypical ferroelectric perovskite oxide system to study the interplay between mobile charges and stability of ferroelectric domains. It is demonstrated that the charged domain walls in BiFeO₃ are more likely to be stabilized with a larger amount of mobile charged defects, and the conductivity on domain walls can be tuned by the interactions and types of the charges within the ferroelectric system. It is also found that the mobile defect charges are responsible to the formation of local conductive paths and the bound charge separation at domain walls. The model would give insights to explore the electric properties and applications in ferroelectric perovskite oxides.

Charge carriers including electrons and holes as well as their dynamics play a significant role in determining material behaviors. Ferroelectric materials are quite commonly semiconductors, meaning that upon excitation by various stimuli such as light pulses, their carrier concentrations can be transiently raised by orders of magnitude from the equilibrium values, thus, giving rise to material properties and phenomena vastly different from those at equilibria. Here we develop a phase-field model for the dynamical phenomena of ferroelectric domain structures involving coupled charge carrier evolution and structural responses under ultrafast external stimuli. As an example, we study the carrier and structural dynamics of various ferroelectric PbTiO₃ domain walls excited by a femtosecond above-band-gap light pulse. We find that the carrier concentration around charged domain walls relaxes with a two-component process at picosecond and nanosecond time scales for free carriers and carriers trapped at the domain walls, respectively. Meanwhile, a strong transient strain response is induced, which contains two fast oscillational components and a slower relaxational component, signifying rich dynamics involving domains, domain walls, and charges. The present work provides a general theoretical framework for studying carrier-mediated ultrafast processes in ferroelectric domain structures and thus for exploring dynamical functionalities of ferroelectric materials.

Understanding the phase transitions and switching behaviors of mesoscale topological structures in ferroic materials is critical to realize their potential applications in next-generation high-performance storage devices. In this work, we study the kinetic evolution of a mesoscale supercrystal with 3-D nanoscale periodicity in a PbTiO₃/SrTiO₃ superlattice under thermal and electrical stimuli using a combination of phase-field simulations and X-ray diffraction experiments. We construct a phase diagram of temperature versus polar state, showing the formation of the supercrystal from a mixed vortex and a-twin state and a temperature-dependent erasing process of a supercrystal returning to a classical a-twin structure. Under an electric field bias at room temperature, the vortex topology of the supercrystal irreversibly transforms to a new type of stripe-like supercrystal under an in-plane electric field. Under an out-of-plane electric field, the vortices inside the supercrystal undergo a topological phase transition to polar skyrmions. These results demonstrate the potential for the on-demand manipulation of polar topology and transformations in supercrystals using electric fields. The findings provide a theoretical understanding which may be utilized to guide the design and control of mesoscale polar structures and to explore novel polar structures in other systems and their topological nature.

The flexibility of manipulating the magnetic ordering and ferroelectric polariozation at room temperature through the magnetoelectric effect in BiFeO₃ has attracted great attention to potential device applications. However, basic properties such as the electronic structure of BiFeO₃ is not yet well established. Reported band gap of multiferroic BiFeO₃ has been a subject of debate, with values reported in wide range of 2.5 - 3.1 eV. Presence of oxygen vacancy defects is a common occurrence in the synthesis of multiferrioc BiFeO₃, which leads to room temperature conductivity that is often linked to polaron hopping. Using hybrid density functional theory calculations, we find a band gap of 3.4 eV for R3c BiFeO₃ , which is slightly larger than a recent reported band gap of 3.1 eV for single crystals based on optical absorption and photoluminescence measurements. Our calculations reveals a strong dependence of the band gap and band edge positions on the antiferrodistortive rotations of the FeO₆ octahedra and ferroelectric displacement of Fe ions. The band gap decreases from 3.4 eV for the R3c, to 2.9 eV for the paraelectric R3-c, and to 1.6 eV for the cubic antiferromagnetic BiFeO₃ , by significant lowering of the conduction band due to reduced O-2p Bi-6p coupling in the more symmetric structures, with simultaneous slight raising of the valence band that is dependent on Fe 3d-O 2p and Bi 6s-O 2p interactions. Lastly, we explored the formation of small hole and electron polarons in BiFeO₃ single crystals and propose a charge transport mechanism via polaron hopping, which can explain the observed room temperature conductivity in the material.
Work supported by the NSF NSF Early Career Award grant no. DMR-1652994.

Voltage-controlled switching of remnant magnetic states paves the way towards ultra-low power and non-volatile spintronics. In this presentation, I report on a journey, which took us from isothermal electric switching of exchange bias with the help of simultaneously applied electric and magnetic fields¹ to pure voltage-controlled antiferromagnetic spintronics in zero magnetic field and at CMOS compatible temperatures.²
Nonvolatile Néel vector reorientation in the absence of an applied magnetic field, H, is demonstrated at CMOS compatible temperatures in prototype device structures which exploit the multi-functional properties of thin films of boron (B) doped Cr₂O_3. Boundary magnetization associated with the Néel vector orientation serves as state variable, which is read via magnetoresistive detection in a Pt Hall bar adjacent to the B: Cr₂O₃ film. Switching of the Hall voltage between zero and non-zero values implies Néel vector rotation by 90-degrees. Combined magnetometry, spin resolved inverse photoemission, electric transport and scanning probe microscopy measurements reveal B-dependent T_N and resistivity enhancement, spin-canting, anisotropy reduction, dynamic polarization hysteresis and gate voltage dependent orientation of boundary magnetization. The combined effect enables H=0, voltage controlled, nonvolatile Néel vector rotation at temperatures as high as 400 K. Theoretical modeling estimates switching speeds of about 100 ps making B: Cr₂O₃ a promising multifunctional single-phase material for energy efficient nonvolatile CMOS compatible memory applications.



REFERENCES

¹He, X., Wang, Y., Wu, N., Caruso, A. N., Vescovo, E., Belashchenko, K. D., Dowben, P. A. & Binek, C. Robust isothermal electric control of exchange bias at room temperature. Nat Mater 9, 579-585, doi:10.1038/Nmat2785 (2010).
² Mahmood, A., Echtenkamp, W., Street, M., Wang, J.-L., Cao, S., Komesu, T., Dowben, P. A., Buragohain, P., Lu, H., Gruverman, A., Parthasarathy, A., Rakheja, S. & Binek, C. Voltage controlled Néel vector rotation in zero magnetic field. Nature Communications 12, 1674, doi:10.1038/s41467-021-21872-3 (2021).

We acknowledge financial support by ARO through MURI W911NF-16-1-0472, the subsidiary of SRC nCORE through AMML and NSF through ECCS 1740136 and EPSCoR RII Track-1: Emergent Quantum Materials and Technologies (EQUATE), Award OIA-2044049, the Nebraska Nanoscale Facility: NNCI, and the Nebraska Center for Materials and Nanoscience.

The high demand for the development of multifunctional devices as well as the interesting mechanisms behind the coexistence of ferroic order parameters within the same phase in multiferroic materials has launched a new wave of research in recent years. The unique properties of some multiferroics such as magnetoelectric (ME) coupling, which allows the cross control of electrical dipole moments by applying magnetic fields (or conversely magnetic moments by electric field), open opportunities for designing novel sensors and memory devices with significantly lower power consumption and higher efficiency. Moreover, the existence of domains and excitations, such as electromagnons, awards multiferroic systems with richer multifunctionalities as well as the potential of enhancing ME responses and designing novel devices such as electrically-driven configurable magnonic circuits. Here we report the prediction of dynamical couplings between magnons and optical phonons in systems possessing ferroelectric domain walls that leads to the emergence of new hybrid quasi-particles, namely domain-wall-induced electromagnons. These quasi-particles induce THz resonances in magnetoelectric responses and preferentially localize either near the domain walls or near the middle of domains as a result of scatterings. Such features can be exploited to reach strikingly large ME conversion and designing more reliable and ultrafast ME devices with less energy consumption and using, e.g., local probes.

Acknowledgments. The authors are thankful to the Vannevar Bush Faculty Fellowship (VBFF) Grant No. N00014-20-1-2834 from the Department of Defense, DARPA Grant No. HR0011727183-D18AP00010 (under the TEE Program) and ONR Grants No. N00014-17-1-2818 and N00014-21-1-2086. B.X. further acknowledges the financial support from National Natural Science Foundation of China under Grant No. 12074277, the startup fund from Soochow University, and the support from Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions.

Cobalt cations in oxides can have multiple electronic configurations. While Co in CoO is in a 2+, high spin (S=3/2, t_2g⁵ e_g²) state, LaCoO₃ (LCO) has Co³⁺ that has multiple spin states depending on temperature: low-spin (S=0, t_2g⁶ e_g⁰), intermediate-spin (S=1, t_2g⁵ e_g¹), and high-spin states (S=2, t_2g⁴ e_g²), which have nearly degenerate ground state energies. Recently, we have synthesized LaCoO₃/LaTiO₃ superlattices exhibiting strong orbital polarization [1]. Theoretical calculations show that Co is 2+ in these superlattices with two relevant low-energy spin states (low-spin S=1/2, t_2g⁶ e_g¹ and high-spin, S=3/2, t_2g⁵ e_g²) [2,3].
Here, we combine Resonant Inelastic X-ray Scattering (RIXS), a powerful tool of local electronic states and low-energy excitations, with a DFT+U based theoretical approach to compute RIXS spectra. This approach is capable of describing both the broad, fluorescent features in RIXS (e.g., which are strong features for RIXS of LCO) as well as localized excitations (e.g., characteristic RIXS features of CoO). We find that this theoretical approach allows us to interpret the RIXS spectra for bulk materials and superlattices, and it also describes the changes of the experimental RIXS spectra as a function of the thickness of the cobaltate layers in the superlattices. We will briefly compare the DFT+U RIXS method to atomic and cluster calculations which do not describe the band-like features in condensed matter systems.

[1] S. J. Lee, A. T. Lee et al., Phys. Rev. Lett. 123, 117201 (2019)
[2] A. T. Lee and S. Ismail-Beigi, Phys. Rev. B 101, 144423 (2020)
[3] A. T. Lee, H. Park, and S. Ismail-Beigi, Phys. Rev. B 103, 125105 (2021)

Organic functional materials, particularly multiferroics, are very novel, because of their simultaneous magnetic and ferroelectric properties. These type of materials have many state of the art applications including flexible electronics, data and storage devices [1-4]. In these materials ferroelectricity can be tuned by applying magnetic fields. Since flexibility and multi-ferroicity rare combinations, organic multi-ferroics are candidates of intense research promising of designing new materials fit for the purpose. For these reasons these materials are in the focus of current research not only for fundamental scientific interests, but also of their potential use in new emerging technology, such as flexible electronics devices.

Generally, charge or proton transfer across chemically distinct donor-acceptor pairs, or movements of those protons in the centro symmetric to non-centrosymmetric positions are considered as the key mechanisms of the functionality of these materials [3]. In this context the dynamics of proton or the charge carriers are important to understand to do bespoke designing. Some of hydrogen bonded solids, namely croconic acid (C₅O₅H₂), 1-cyclobutene-1,2-dicarboxylic acid (CBDC, C₆H₈O₄) and 2-phenylmalondialdehyde (PhMDA, C₉H₈O₂) show high polarizability at room temperature [5]. As a continuation of our on-going effort in understanding dynamics of protons in ferroelectricity [6-10], in this work we investigate the dynamics of proton in tetrathiafulvalene-p-bromanil [(H₂C₂S₂C)₂ - C₆Br₄O₂] (TTF-BA). [2], which has shown its potential to be multiferroic at low temperatures. The motivation of this work to help not only to understand the finger print dynamics of ferroelectricity of these materials, but to design new organic ferroelectrics for technological use.

We have used inelastic neutron scattering (INS) to explore proton dynamics in TTF-BA. The INS experiment has been done at ISIS neutron and Muon Source. DFT based first-principles simulations have been done to model the INS spectrum to understand the mechanism of the multiferroic properties of this material. Both spin polarised and non-spin polarised calculations have been done using CASTEP code. PBE + GGA functionals have been used to determine the ground state structures. Quasi harmonic calculations have been performed to understand the mechanism of ferroelectricity. The responsible modes for ferrolectricity have been identified by normal mode and soft phonon analysis of these calculations.


The neutron spectroscopic experiment and DFT simulations indicate that C-H—Br bending modes carry the finger print signature of ferroelectricity of the molecule. In the talk the effect of spin polarisation on this mode will also be discussed.

References
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[10] S.Mukhopadhyay et al.., J. Phys. Commun.3, 113001 (2019).

Here we present a comparative study of the structural, magnetic, and dielectric properties of 960 nm thick film and bulk pellet of single-phase polycrystalline GdFe_0.5Cr_0.5O₃. The film was fabricated on a platinized-silicon substrate by solution deposition and spin-coating methods. The bulk pellet was synthesized by the citrate solution route. Magnetic measurements show Néel temperature~ 270 K for both the bulk and the film whereas dielectric measurements show ferroelectric to paraelectric transition with T_C ~ 525 K for bulk and 450 K for the film. The frequency dependent dielectric data of both samples is found to follow Vogel-Fulcher relation that implies relaxor-type diffusive ferroelectric behavior. Interfacial Maxwell–Wagner polarization was also observed at low frequencies. The electric polarization hysteresis loops, although leaky, are observed at room temperature revealing multiferroic nature. Details and discussions of these results will be presented.

Magnetoelectric materials (ME) are multifunctional systems that combine magnetic and ferroelectric behaviors [1]. With the ability to switch ferroelectric (magnetic) domains by small external fields, ME create the basis for the development of low-consumption devices, including high-density data storage, spintronics, and optoelectronics [1]. For these applications, ME must have strong magnetoelectric coupling at room temperature and above, which rarely observed for classical inorganic ME materials such as metal oxides [1, 2]. Synthesis of such inorganic ME at the atomic level is also a difficult task [1, 2]. However, there are several studies showing that molecular ME based on metal-organic complexes can be an alternative to the currently actively studied ME systems, in particular, transition metal oxides [3-5]. In addition, molecular ME have a wide functionality due to their flexibility, non-toxicity, and high manufacturability. Earlier, we showed the presence of a strong ME-coupling in a new molecular ZnYb complex, which combines ferroelectric and paramagnetic properties at room temperature [5]. This work represents further required fundamental study on the determination of ferroelectric Curie temperature (Tc) in the ZnYb compound and its distinct ferroelectricity type (uniaxial or multiaxial).
Using nonlinear optical microscopy, we investigated anisotropic dependencies of the second harmonic generation (SHG) in the ZnYb single crystal. The ZnYb complex at room temperature is a monoclinic system described by the polar point group 2 [5]. In this case, the electric-dipole term is the main contribution to the SHG signal. Nonzero nonlinear susceptibility tensor components were defined for ZnYb complex. Temperature dependence of the SHG intensity in the range from 293 K to 493 K was obtained. To approximate thisdependence, we used the classical Landau theory. Found values of T_c and critical exponent β are 473 K and 0.5, respectively. The nonzero SHG signal in the region above T_c is because chiral structures can crystallize only in noncentrosymmetric space groups. According to Aizu notation [3], the paraelectric phase in ZnYb belongs the species of orthorhombic noncentrosymmetric group 222 among the 88 ferroelectric species. The 222 → 2 phase transition characterizes the ZnYb compound as a uniaxial ferroelectric system. This results is also in good agreement with the Folk's theory, where the value of the critical exponent β = 0.5 corresponds strongly to ferroelectric system with uniaxial dipole interaction [6].

The reported study was supported by the Russian Science Foundation (project No. 20-79-10233). E.D. M. acknowledges the support of the Ministry of Education and Science of the Russian Federation (state task No. FSFZ-0706-2020-0022).

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