Magnetic control of ferroelectric polarization

Tailoring the multiferroic behavior in BiFeO3 nanostructures by Pb doping

Substituting Pb²⁺ for Bi³⁺ in BiFeO₃ can induce lattice distortions and structural transitions to tune the lone-pair activity for ferroelectricity and neutralized oxygen vacancies to valence Fe²⁺/Fe³⁺ ions for ferromagnetism.

Ferroelectricity and magnetism in metal-formate frameworks of [NH4][M(HCOO)3] (M = Sc to Zn): a first-principles study

A comprehensive first-principles calculations reveal the multiferroicity and tunable magnetism via carrier doping in [NH₄][M(HCOO)₃] (M = 3d transition metal).

Multiferroic PVDF–Fe3O4 hybrid films with reduced graphene oxide and ZnO nanofillers

Flexible and self-standing polyvinylidene fluoride (PVDF) films loaded with nanofillers, reduced graphene oxide (RGO), zinc oxide (ZnO) and magnetic iron oxide (Fe₃O₄) nanoparticles, were prepared by a solvent casting method.

Structural and micro-Raman studies of DyMnO3 with potassium substitution at the Dy site

We synthesize a new series of compounds, Dy_1−xK_xMnO₃ (x = 0.1, 0.2 & 0.3) and report the structural and spectroscopic properties of the new compounds in comparison to DyMnO₃.

Modulated multiferroic properties of MnWO4via chemical doping

Here we prepare polycrystalline Mn_1−xNi_xWO₄ ceramics with x = 0, 0.02, 0.04, 0.06 for investigating their magnetic, ferroelectric, and multiferroic properties.

Double perovskite Y2NiMnO6 nanowires: high temperature ferromagnetic–ferroelectric multiferroic

Y₂NiMnO₆ NWs exhibited room temperature ferroelectricity and ferromagnetism because of surface spins and surface polarization of the electron, respectively.

Phase evolution in sonochemically synthesized Fe3+ doped BaTiO3 nanocrystallites: structural, magnetic and ferroelectric characterisation

Sonochemically synthesized Fe³⁺ doped BaTiO₃ nanostructures exhibiting phase transition and room temperature ferromagnetic and ferroelectric behaviour.

Oxalate-bridged heterometallic chains with monocationic dabco derivatives

A series of bimetallic oxalate-bridged one-dimensional chains with monocationic dabco derivatives were synthesized, and their metamagnetic behavior of ferromagnetic Cr–Co oxalate chain and a specific paraelectronic relaxation behavior were investigated.

Electric-field control of magnetism via strain transfer across ferromagnetic/ferroelectric interfaces

By taking advantage of the coupling between magnetism and ferroelectricity, ferromagnetic (FM)/ferroelectric (FE) multiferroic interfaces play a pivotal role in manipulating magnetism by electric fields. Integrating the multiferroic heterostructures into spintronic devices significantly reduces energy dissipation from Joule heating because only an electric field is required to switch the magnetic element. New concepts of storage and processing of information thus can be envisioned when the electric-field control of magnetism is a viable alternative to the traditional current based means of controlling magnetism. This article reviews some salient aspects of the electric-field effects on magnetism, providing a short overview of the mechanisms of magneto-electric (ME) coupling at the FM/FE interfaces. A particular emphasis is placed on the ME effect via interfacial magneto-elastic coupling arising from strain transfer from the FE to FM layer. Recent results that demonstrate the electric-field control of magnetic anisotropy, magnetic order, magnetic domain wall motion, and etc are described. Obstacles that need to be overcome are also discussed for making this a reality for future device applications.

Dynamical magnetoelectric phenomena of multiferroic skyrmions

Magnetic skyrmions, vortex-like swirling spin textures characterized by a quantized topological invariant, realized in chiral-lattice magnets are currently attracting intense research interest. In particular, their dynamics under external fields is an issue of vital importance both for fundamental science and for technical application. Whereas observations of magnetic skyrmions has been limited to metallic magnets so far, their realization was also discovered in a chiral-lattice insulating magnet Cu2OSeO3 in 2012. Skyrmions in the insulator turned out to exhibit multiferroic nature with spin-induced ferroelectricity. Strong magnetoelectric coupling between noncollinear skyrmion spins and electric polarizations mediated by relativistic spin-orbit interaction enables us to drive motion and oscillation of magnetic skyrmions by application of electric fields instead of injection of electric currents. Insulating materials also provide an environment suitable for detection of pure spin dynamics through spectroscopic measurements owing to the absence of appreciable charge excitations. In this article, we review recent theoretical and experimental studies on multiferroic properties and dynamical magnetoelectric phenomena of magnetic skyrmions in insulators. We argue that multiferroic skyrmions show unique coupled oscillation modes of magnetizations and polarizations, so-called electromagnon excitations, which are both magnetically and electrically active, and interference between the electric and magnetic activation processes leads to peculiar magnetoelectric effects in a microwave frequency regime.

Multifunctional magnetoelectric materials for device applications

Over the past decade magnetoelectric (ME) mutiferroic (MF) materials and their devices are one of the highest priority research topics that has been investigated by the scientific ferroics community to develop the next generation of novel multifunctional materials. These systems show the simultaneous existence of two or more ferroic orders, and cross-coupling between them, such as magnetic spin, polarisation, ferroelastic ordering, and ferrotoroidicity. Based on the type of ordering and coupling, they have drawn increasing interest for a variety of device applications, such as magnetic field sensors, nonvolatile memory elements, ferroelectric photovoltaics, nano-electronics etc. Since single-phase materials exist rarely in nature with strong cross-coupling properties, intensive research activity is being pursued towards the discovery of new single-phase multiferroic materials and the design of new engineered materials with strong magneto-electric (ME) coupling. This review article summarises the development of different kinds of multiferroic material: single-phase and composite ceramic, laminated composite and nanostructured thin films. Thin-film nanostructures have higher magnitude direct ME coupling values and clear evidence of indirect ME coupling compared with bulk materials. Promising ME coupling coefficients have been reported in laminated composite materials in which the signal to noise ratio is good for device fabrication. We describe the possible applications of these materials.

Enhanced exchange bias and improved ferromagnetic properties in Permalloy-BiFe0.95Co0.05O3 core-shell nanostructures

Hybrid core-shell nanostructures consisting of permalloy (Ni80Fe20) and multiferroic(BiFeO3, BFO/BiFe0.95Co0.05O3, BFC) materials were synthesized by a two-step method, based on wet chemical impregnation and subsequent electrodeposition within porous alumina membranes. Structural and magnetic characterizations have been done to investigate doping effect on magnetic properties and exchange bias. The magnetometry analysis revealed significant enhancements of the exchange bias and coercivity in NiFe-BFC core-shell nanostructures as compared with NiFe-BFO core-shell nanostructures. The enhancements can be attributed to the effective reduction of ferromagnet domain sizes between adjacent layers of core-shell structure. It indicates that it is possible to improve properties of multiferroic composites by site-engineering method. Our approach opens a pathway to obtain optimized nanostructured multiferroic composites exhibiting tunable magnetic properties.

Multiferroicity and Magnetoelectric Coupling in TbMnO3 Thin Films

In this work, we report the growth and functional characterizations of multiferroic TbMnO3 thin films grown on Nb-doped SrTiO3 (001) substrates using pulsed laser deposition. By performing detailed magnetic and ferroelectric properties measurements, we demonstrate that the multiferroicity of spin origin known in the bulk crystals can be successfully transferred to TbMnO3 thin films. Meanwhile, anomalous magnetic transition and unusual magnetoelectric coupling related to Tb moments are observed, suggesting a modified magnetic configuration of Tb in the films as compared to the bulk counterpart. In addition, it is found that the magnetoelectric coupling enabled by Tb moments can even be seen far above the Tb spin ordering temperature, which provides a larger temperature range for the magnetoelectric control involving Tb moments.

Functional domain walls in multiferroics

During the last decade a wide variety of novel and fascinating correlation phenomena has been discovered at domain walls in multiferroic bulk systems, ranging from unusual electronic conductance to inseparably entangled spin and charge degrees of freedom. The domain walls represent quasi-2D functional objects that can be induced, positioned, and erased on demand, bearing considerable technological potential for future nanoelectronics. Most of the challenges that remain to be solved before turning related device paradigms into reality, however, still fall in the field of fundamental condensed matter physics and materials science. In this topical review seminal experimental findings gained on electric and magnetic domain walls in multiferroic bulk materials are addressed. A special focus is put on the physical properties that emerge at so-called charged domain walls and the added functionality that arises from coexisting magnetic order. The research presented in this review highlights that we are just entering a whole new world of intriguing nanoscale physics that is yet to be explored in all its details. The goal is to draw attention to the persistent challenges and identify future key directions for the research on functional domain walls in multiferroics.

Giant Spin-Driven Ferroelectric Polarization in BiFeO₃ at Room Temperature

The spin-driven polarizations of type-I multiferroics are veiled by the preexisting ferroelectric (FE) polarization. Using first-principles calculations combined with a spin model, we uncover two hidden but huge spin-driven polarizations in the room-temperature multiferroic BiFeO(3). One is associated with the global inversion symmetry broken by a FE distortion, and the other is associated with the local inversion symmetry broken by an antiferrodistortive octahedral rotation. Comparison with recent neutron scatterings reveals tha first polarization reaches ∼3.0  μC/cm(2), which is larger than in any other multiferroic material. Our exhaustive study paves a way to uncover the various magnetoelectric couplings that generate hidden spin-driven polarizations in other type-I multiferroics.

Polar Dynamics at the Jahn-Teller Transition in Ferroelectric GaV₄S₈

We present a dielectric spectroscopy study of the polar dynamics linked to the orbitally driven ferroelectric transition in the Skyrmion host GaV(4)S(8). By combining THz and MHz-GHz spectroscopy techniques, we succeed in detecting the relaxational dynamics arising from coupled orbital and polar fluctuations in this material and trace its temperature dependence in the paraelectric as well as in the ferroelectric phase. The relaxation time significantly increases when approaching the critical temperature from both sides of the transition. It is natural to assume that these polar fluctuations map the orbital dynamics at the Jahn-Teller transition. Because of the first-order character of the orbital-ordering transition, the relaxation time shows an enormous jump of about 5 orders of magnitude at the polar and structural phase transition.

Peculiar Magnetoelectric Coupling in BaTiO₃:Fe₁₁₃ ppm Nanoscopic Segregations

We report polycrystalline BaTiO3 with cooperative magnetization behavior associated with the scarce presence of about 113 atomic ppm of Fe ions, clearly displaying magnetoelectric coupling with significant changes in magnetization (up to ΔM/M ≈ 32%) at the ferroelectric transitions. We find that Fe ions are segregated mostly at the interfaces between grain boundaries and an Fe-rich phase, forming a self-composite with high magnetoelectric coupling above room temperature. We compare our results with ab initio calculations and other experimental results found in the literature, proposing mechanisms that could be behind the magnetoelectric coupling within the ferroelectric matrix. These findings open the way for further strategies to optimize interfacial magnetoelectric couplings.

Control of multiferroic domains by external electric fields in TbMnO₃

The control of multiferroic domains through external electric fields has been studied by dielectric measurements and by polarized neutron diffraction on single-crystalline TbMnO3. Full hysteresis cycles were recorded by varying an external field of the order of several kV mm(-1) and by recording the chiral magnetic scattering as well as the charge in a sample capacitor. Both methods yield comparable coercive fields that increase upon cooling.

Piezomagnetoelectric Effect of Spin Origin in Dysprosium Orthoferrite

The piezomagnetoelectric effect, namely, the simultaneous induction of both the ferromagnetic moment and electric polarization by an application of uniaxial stress, was demonstrated in the nonferroelectric antiferromagnetic ground state of DyFeO(3). The induced electric polarization and ferromagnetic moment are coupled with each other, and monotonically increase with increasing uniaxial stress. The present work provides a new guiding principle for designing multiferroics where its magnetic symmetry is broken by external uniaxial stress.

Multiferroicity and skyrmions carrying electric polarization in GaV4S8

Skyrmions are whirl-like topological spin objects with high potential for future magnetic data storage. A fundamental question that is relevant to both basic research and application is whether ferroelectric (FE) polarization can be associated with skyrmions' magnetic texture and whether these objects can be manipulated by electric fields. We study the interplay between magnetism and electric polarization in the lacunar spinel GaV4S8, which undergoes a structural transition associated with orbital ordering at 44 K and reveals a complex magnetic phase diagram below 13 K, including ferromagnetic, cycloidal, and Néel-type skyrmion lattice (SkL) phases. We found that the orbitally ordered phase of GaV4S8 is FE with a sizable polarization of ~1 μC/cm(2). Moreover, we observed spin-driven excess polarizations in all magnetic phases; hence, GaV4S8 hosts three different multiferroic phases with coexisting polar and magnetic order. These include the SkL phase, where we predict a strong spatial modulation of FE polarization close to the skyrmion cores. By taking into account the crystal symmetry and spin patterns of the magnetically ordered phases, we identify exchange striction as the main microscopic mechanism behind the spin-driven FE polarization in each multiferroic phase. Because GaV4S8 is unique among known SkL host materials owing to its polar crystal structure and the observed strong magnetoelectric effect, this study is an important step toward the nondissipative electric field control of skyrmions.

The nature of magnetoelectric coupling in Pb(Zr,Ti)O3 -Pb(Fe,Ta)O3

The coupling between magnetization and polarization in a room temperature multiferroic (Pb(Zr,Ti)O3 -Pb(Fe,Ta)O3 ) is explored by monitoring the changes in capacitance that occur when a magnetic field is applied in each of three orthogonal directions. Magnetocapacitance effects, consistent with P(2) M(2) coupling, are strongest when fields are applied in the plane of the single crystal sheet investigated.

Magnetoelectric effects in the skyrmion host material Cu2OSeO3

Insulating helimagnetic Cu₂OSeO₃ shows sizeable magnetoelectric effects in its skyrmion phase. Using magnetization measurements, magneto-current analysis and dielectric spectroscopy, we provide a thorough investigation of magnetoelectric coupling, polarization and dielectric constants of the ordered magnetic and polar phases of single-crystalline Cu₂OSeO₃ in external magnetic fields up to 150 mT and at temperatures below 60 K. From these measurements we construct a detailed phase diagram. Especially, the skyrmion phase and the metamagnetic transition of helical to conical spin order are characterized in detail. Finally we address the question if there is any signature of polar order that can be switched by an external electric field, which would imply multiferroic behaviour of Cu₂OSeO₃.

Dielectric spectroscopy on organic charge-transfer salts

This topical review provides an overview of the dielectric properties of a variety of organic charge-transfer salts, based on both, data reported in literature and our own experimental results. Moreover, we discuss in detail the different processes that can contribute to the dielectric response of these materials. We concentrate on the family of the 1D (TMTTF)2 X systems and the 2D BEDT-TTF-based charge-transfer salts, which in recent years have attracted considerable interest due to their often intriguing dielectric properties. We will mainly focus on the occurrence of electronic ferroelectricity in these systems, which also includes examples of multiferroicity.

Optical Diode Effect at Spin-Wave Excitations of the Room-Temperature Multiferroic BiFeO_{3}

Multiferroics permit the magnetic control of the electric polarization and the electric control of the magnetization. These static magnetoelectric (ME) effects are of enormous interest: The ability to read and write a magnetic state current-free by an electric voltage would provide a huge technological advantage. Dynamic or optical ME effects are equally interesting, because they give rise to unidirectional light propagation as recently observed in low-temperature multiferroics. This phenomenon, if realized at room temperature, would allow the development of optical diodes which transmit unpolarized light in one, but not in the opposite, direction. Here, we report strong unidirectional transmission in the room-temperature multiferroic BiFeO_{3} over the gigahertz-terahertz frequency range. The supporting theory attributes the observed unidirectional transmission to the spin-current-driven dynamic ME effect. These findings are an important step toward the realization of optical diodes, supplemented by the ability to switch the transmission direction with a magnetic or electric field.

Multiferroic Vacancies at Ferroelectric PbTiO(3) Surfaces

Multiferroics in nanoscale dimensions are promising for novel functional device paradigms, such as magnetoelectric memories, due to an intriguing cross-coupling between coexisting ferroelectric and (anti)ferromagnetic order parameters. However, the ferroic order is inevitably destroyed below the critical dimension of several nanometers. Here, we demonstrate a new path towards atomic-size multiferroics while resolving the controversial origin of dilute ferromagnetism that unexpectedly emerges in nanoparticles of nonmagnetic ferroelectric PbTiO(3). Systematic exploration using predictive quantum-mechanical calculations demonstrates that oxygen vacancies formed at surfaces induce ferromagnetism due to local nonstoichiometry and orbital symmetry breaking. The localized character of the emerged magnetization allows an individual oxygen vacancy to act as an atomic-scale multiferroic element with a nonlinear magnetoelectric effect that involves rich ferromagnetic-antiferromagnetic-nonmagnetic phase transitions in response to switching of the spontaneous polarization.

Upgrade of beamline BL08B at Taiwan Light Source from a photon-BPM to a double-grating SGM beamline

During the last 20 years, beamline BL08B has been upgraded step by step from a photon beam-position monitor (BPM) to a testing beamline and a single-grating beamline that enables experiments to record X-ray photo-emission spectra (XPS) and X-ray absorption spectra (XAS) for research in solar physics, organic semiconductor materials and spinel oxides, with soft X-ray photon energies in the range 300-1000 eV. Demands for photon energy to extend to the extreme ultraviolet region for applications in nano-fabrication and topological thin films are increasing. The basic spherical-grating monochromator beamline was again upgraded by adding a second grating that delivers photons of energy from 80 to 420 eV. Four end-stations were designed for experiments with XPS, XAS, interstellar photoprocess systems (IPS) and extreme-ultraviolet lithography (EUVL) in the scheduled beam time. The data from these experiments show a large count rate in core levels probed and excellent statistics on background normalization in the L-edge adsorption spectrum.

The ferroelectric polarization of Y2CoMnO6 aligns along the b-axis: the first-principles calculations

Double-perovskite A2BB'O6 oxides with magnetic B and B' ions and E*-type antiferromagnetic order (E*-AFM, i.e. the ↑↑↓↓ structure) are believed to exhibit promising multiferroic properties, and Y2CoMnO6 (YCMO) is one candidate in this category. However, the microscopic origins for magnetically induced ferroelectricity in YCMO remain unclear. In this study, we perform detailed symmetry analysis on the exchange striction effect and lattice distortion, plus the first-principles calculations on YCMO. The E*-AFM state as the ground state with other competing states such as ferromagnetic and A-antiferromagnetic orders is confirmed. It is observed that the ferroelectricity is generated by the exchange striction associated with the E*-AFM order and chemically ordered Mn/Co occupation. Both the lattice symmetry consideration and first-principles calculations predict that the electric polarization aligns along the b-axis. The calculated polarization reaches up to 0.4682 μC cm(-2), mainly from the ionic displacement contribution. The present study presents a comprehensive understanding of the multiferroic mechanisms in YCMO and is of general significance for predicting emergent multiferroicity in other double-perovskite magnetic oxides.

Design of a Mott Multiferroic from a Nonmagnetic Polar Metal

We examine the electronic properties of the newly discovered "ferroelectric metal" LiOsO3 combining density-functional and dynamical mean-field theories. We show that the material is close to a Mott transition and that electronic correlations can be tuned to engineer a Mott multiferroic state in the 1/1 superlattice of LiOsO3 and LiNbO3. We use electronic structure calculations to predict that the (LiOsO3)1/(LiNbO3)1 superlattice exhibits strong coupling between magnetic and ferroelectric degrees of freedom with a ferroelectric polarization of 41.2  μC cm(-2), Curie temperature of 927 K, and Néel temperature of 379 K. Our results support a route towards high-temperature multiferroics, i.e., driving nonmagnetic polar metals into correlated insulating magnetic states.

Observation of Magnetoelectric Multiferroicity in a Cubic Perovskite System: LaMn(3)Cr(4)O(12)

Magnetoelectric multiferroicity is not expected to occur in a cubic perovskite system because of the high structural symmetry. By versatile measurements in magnetization, dielectric constant, electric polarization, neutron and x-ray diffraction, Raman scattering, as well as theoretical calculations, we reveal that the A-site ordered perovskite LaMn(3)Cr(4)O(12) with cubic symmetry is a novel spin-driven multiferroic system with strong magnetoelectric coupling effects. When a magnetic field is applied in parallel (perpendicular) to an electric field, the ferroelectric polarization can be enhanced (suppressed) significantly. The unique multiferroic phenomenon observed in this cubic perovskite cannot be understood by conventional spin-driven microscopic mechanisms. Instead, a nontrivial effect involving the interactions between two magnetic sublattices is likely to play a crucial role.

Ferroelectric-like metallic state in electron doped BaTiO3

We report that a ferroelectric-like metallic state with reduced anisotropy of polarization is created by the doping of conduction electrons into BaTiO3, on the bases of x-ray/electron diffraction and infrared spectroscopic experiments. The crystal structure is heterogeneous in nanometer-scale, as enabled by the reduced polarization anisotropy. The enhanced infrared intensity of soft phonon along with the resistivity reduction suggests the presence of unusual electron-phonon coupling, which may be responsible for the emergent ferroelectric structure compatible with metallic state.

Electric Field Control of Interfacial Ferromagnetism in CaMnO_{3}/CaRuO_{3} Heterostructures

New mechanisms for achieving direct electric field control of ferromagnetism are highly desirable in the development of functional magnetic interfaces. To that end, we have probed the electric field dependence of the emergent ferromagnetic layer at CaRuO_{3}/CaMnO_{3} interfaces in bilayers fabricated on SrTiO_{3}. Using polarized neutron reflectometry, we are able to detect the ferromagnetic signal arising from a single atomic monolayer of CaMnO_{3}, manifested as a spin asymmetry in the reflectivity. We find that the application of an electric field of 600 kV/m across the bilayer induces a significant increase in this spin asymmetry. Modeling of the reflectivity suggests that this increase corresponds to a transition from canted antiferromagnetism to full ferromagnetic alignment of the Mn^{4+} ions at the interface. This increase from 1 μ_{B} to 2.5-3.0 μ_{B} per Mn is indicative of a strong magnetoelectric coupling effect, and such direct electric field control of the magnetization at an interface has significant potential for spintronic applications.

Unveiling hidden ferrimagnetism and giant magnetoelectricity in polar magnet Fe2Mo3O8

Magnetoelectric (ME) effect is recognized for its utility for low-power electronic devices. Largest ME coefficients are often associated with phase transitions in which ferroelectricity is induced by magnetic order. Unfortunately, in these systems, large ME response is revealed only upon elaborate poling procedures. These procedures may become unnecessary in single-polar-domain crystals of polar magnets. Here we report giant ME effects in a polar magnet Fe₂Mo₃O₈ at temperatures as high as 60 K. Polarization jumps of 0.3 μC/cm², and repeated mutual control of ferroelectric and magnetic moments with differential ME coefficients on the order of 10⁴ ps/m are achieved. Importantly, no electric or magnetic poling is needed, as necessary for applications. The sign of the ME coefficients can be switched by changing the applied “bias” magnetic field. The observed effects are associated with a hidden ferrimagnetic order unveiled by application of a magnetic field.

Damped spin waves in the intermediate ordered phases in Ni3V2O8

Spin dynamics in the intermediate ordered phases (between 4 and 9 K) in Ni3V2O8 have been studied with inelastic neutron scattering. It is found that the spin waves are very diffuse, indicative of short lived correlations and the coexistence of paramagnetic moments with the long-range ordered state.

An organic approach for nanostructured multiferroics

Multiferroics are materials that simultaneously exhibit more than one ferroic order parameters, such as ferroelectricity, ferroelasticity and ferromagnetism. Recently, multiferroicity has received a significant revival of interest due to the colossal magnetoelectric coupling effect for the development of nano-ferronics. In this mini-review, we focus on a recent study of ferroelectricity, magnetism and magnetoelectric coupling within the newly discovered organic charge-transfer complexes. A systemic understanding of the origin of organic ferroelectricity and magnetism is provided. Furthermore, based on the recent mechanism of the magnetoelectric coupling effect: spin-ordering-induced electric polarization and ferroelectricity-induced spin alignment, we further present the recent progress in organic charge-transfer multiferroics and metal-organic framework multiferroics. The coexistence of polarization and magnetism at room temperature of organic charge-transfer complexes will be critical for the development of all-organic multiferroics.

First principles prediction of interfacial magnetoelectric coupling in tetragonal La2/3Sr1/3MnO3/BiFeO3 multiferroic superlattices

The electronic structure and magnetic properties of the tetragonal La2/3Sr1/3MnO3/BiFeO3 multiferroic superlattices with different interfacial terminations have been studied by first-principles calculations. Our results for all the models of the tetragonal La2/3Sr1/3MnO3/BiFeO3 superlattices exhibit a metallic electronic structure. More importantly, we find that the magnetoelectric coupling can be realized in the tetragonal La2/3Sr1/3MnO3/BiFeO3 heterostructures by means of exchange bias, which can be attributed to the interfacial exchange coupling. These findings are useful for magnetoelectrically controlled spintronic devices.

Fluorescent Ferroelectrics of Hydrogen-Bonded Pyrene Derivatives

Organic materials with diverse molecular designs show multifunctional properties such as coupled ferroelectric, optical, ferromagnetic, and transport properties. We report the design of an alkylamide-substituted pyrene derivative displaying fluorescent ferroelectric properties coupled with electron transport properties. In solution phase, this compound displayed concentration-dependent fluorescence, whereas in xerogels, a fluorescent green organogel (>0.1 mM) and entangled nanofibers were observed. A discotic hexagonal columnar liquid crystalline phase was observed above 295 K due to intermolecular hydrogen bonding and π-stacking interactions. The direction of the hydrogen-bonded chains could be inverted by the application of an external electric field along the π-stacked column, resulting in ferroelectric polarization-electric field (P-E) hysteresis. The local electric field arising from the ferroelectric macrodipole moment arrangement along the π-stacking direction affected the electron transport properties on the π-stack of pyrenes, thus confirming the current-switching phenomena according to P-E hysteresis. We report that multifunctional properties such as ferroelectricity, fluorescence, and electron transport switching were successfully achieved in hydrogen-bonded dynamic π-molecular assemblies.

Microwave Magnetochiral Dichroism in the Chiral-Lattice Magnet Cu_{2}OSeO_{3}

Through broadband microwave spectroscopy in Faraday geometry, we observe distinct absorption spectra accompanying magnetoelectric (ME) resonance for oppositely propagating microwaves, i.e., directional dichroism, in the multiferroic chiral-lattice magnet Cu_{2}OSeO_{3}. The magnitude of the directional dichroism critically depends on the magnetic-field direction. Such behavior is well accounted for by considering the relative direction of the oscillating electric polarizations induced via the ME effect with respect to microwave electric fields. Directional dichroism in a system with an arbitrary form of ME coupling can be also discussed in the same manner.

Microwave Magnetochiral Effect in Cu_{2}OSeO_{3}

We theoretically find that in the multiferroic chiral magnet Cu_{2}OSeO_{3} resonant magnetic excitations are coupled to the collective oscillation of the electric polarization, and thereby attain simultaneous activity to the ac magnetic field and ac electric field. Because of the interference between these magnetic and electric activation processes, this material hosts a gigantic magnetochiral dichroism for microwaves, that is, a directional dichroism at gigahertz frequencies in the Faraday geometry. The absorption intensity of a microwave differs by as much as ~30% depending on whether its propagation direction is parallel or antiparallel to the external magnetic field.

Thermally mediated mechanism to enhance magnetoelectric coupling in multiferroics

The main roadblock on the way to practical realization of magnetoelectric devices is the lack of multiferroics with strong magnetoelectric coupling. We propose an unusual route to dramatically enhance this coupling through a thermally mediated mechanism. Such a thermally mediated magnetoelectric effect is quantified by an isentropic rather than isothermal magnetoelectric response and is computed here from first principles. A robust enhancement of the magnetoelectric coupling is predicted for both naturally occurring and heterostructured materials.

100-nm-sized magnetic domain reversal by the magneto-electric effect in self-assembled BiFeO3/CoFe2O4 bilayer films

A (001)-epitaxial-BiFeO₃/CoFe₂O₄ bilayer was grown by self-assembly on SrTiO₃ (100) substrates by just coating a mixture precursor solution. The thickness ratio of the bilayer could be controlled by adjusting the composition ratio. For example, a BiFeO_x:CoFe₂O_x = 4:1 (namely Bi₄CoFe₆O_x) mixture solution could make a total thickness of 110nm divided into 85-nm-thick BiFeO₃ and 25-nm-thick CoFe₂O₄. Self-assembly of the bilayer occurred because the perovskite BiFeO₃ better matched the lattice constant (misfit approximately 1%) and crystal symmetry of the perovskite SrTiO₃ than the spinel CoFe₂O₄ (misfit approximately 7%). The magnetic domains of the hard magnet CoFe₂O₄ were switched by the polarization change of BiFeO₃ due to an applied vertical voltage, and the switched magnetic domain size was approximately 100nm in diameter. These results suggest that self-assembled BiFeO₃/CoFe₂O₄ bilayers are interesting in voltage driven nonvolatile memory with a low manufacturing cost.

Polarization control at spin-driven ferroelectric domain walls

Unusual electronic states arise at ferroelectric domain walls due to the local symmetry reduction, strain gradients and electrostatics. This particularly applies to improper ferroelectrics, where the polarization is induced by a structural or magnetic order parameter. Because of the subordinate nature of the polarization, the rigid mechanical and electrostatic boundary conditions that constrain domain walls in proper ferroics are lifted. Here we show that spin-driven ferroelectricity promotes the emergence of charged domain walls. This provides new degrees of flexibility for controlling domain-wall charges in a deterministic and reversible process. We create and position a domain wall by an electric field in Mn0.95Co0.05WO4. With a magnetic field we then rotate the polarization and convert neutral into charged domain walls, while its magnetic properties peg the wall to its location. Using atomistic Landau-Lifshitz-Gilbert simulations we quantify the polarization changes across the two wall types and highlight their general occurrence.