Electric control of magnetism at the Fe/BaTiO₃ interface

Light-Controlled Magnetic Properties: An Energy-Efficient Opto-Mechanical Control over Magnetic Films by Liquid Crystalline Networks

Magnetostrictive materials are essential components in sensors, actuators, and energy-storage devices due to their ability to convert mechanical stress into changes in magnetic properties and vice-versa. However, their operation typically requires physical contact to apply stress or relies on magnetic field sources to control magnetic properties. This poses significant limitations to devices miniaturization and their integration into contactless technologies. This work reports on an approach that overcomes these limitations by using light to transfer mechanical stress to a magnetostrictive device, thereby achieving non-contact and reversible opto-mechanical control of its magnetic and electrical properties. The proposed solution combines a magnetostrictive Fe70Ga30 thin film with a photo-responsive Liquid Crystalline Network (LCN). Magnetic properties are modulated by changing the light wavelength and illumination time. Remarkably, the stable shape change of the LCN induced by ultraviolet (UV) light leads to the retention of magnetic properties even after the light is switched off, resulting in a magnetic memory effect with an energy consumption advantage over the use of conventional magnetic field applicators. The memory effect is erased by visible light, which releases the mechanical stress in the photoresponsive layer. Therefore, this new composite material creates a fully reconfigurable magnetic system controlled by light.

Light-Controlled Magnetoelastic Effects in Ni/BaTiO3 Heterostructures

Magnetoelastic and magnetoelectric coupling in the artificial multiferroic heterostructures facilitate valuable features for device applications such as magnetic field sensors and electric-write magnetic-read memory devices. In ferromagnetic/ferroelectric heterostructures, the intertwined physical properties can be manipulated by an external perturbation, such as an electric field, temperature, or a magnetic field. Here, we demonstrate the remote-controlled tunability of these effects under visible, coherent, and polarized light. The combined surface and bulk magnetic study of domain-correlated Ni/BaTiO₃ heterostructures reveals that the system shows strong sensitivity to the light illumination via the combined effect of piezoelectricity, ferroelectric polarization, spin imbalance, magnetostriction, and magnetoelectric coupling. A well-defined ferroelastic domain structure is fully transferred from a ferroelectric substrate to the magnetostrictive layer via interface strain transfer. The visible light illumination is used to manipulate the original ferromagnetic microstructure by the light-induced domain wall motion in ferroelectric substrates and consequently the domain wall motion in the ferromagnetic layer. Our findings mimic the attractive remote-controlled ferroelectric random-access memory write and magnetic random-access memory read application scenarios, hence facilitating a perspective for room temperature spintronic device applications.

Magnetization reversal of perpendicular magnetic anisotropy regulated by ferroelectric polarization in CoFe3N/BaTiO3 heterostructures: first-principles calculations

Exploring the electric-field switching of perpendicular magnetic anisotropy (PMA) in multiferroic heterostructures has important physical significance, which attracts great interest due to its promising application for energy-efficient information storage. Herewith, we investigate the effect of ferroelectric polarization on magnetic anisotropy in CoFe₃N/BaTiO₃ heterostructures using first-principles calculations. The calculations reveal that the magnetic anisotropy of CoFe₃N can be regulated by ferroelectric polarization of BaTiO₃. When the ferroelectric polarization reverses, the PMA of FeCo-TiO₂ and FeN-BaO configurations remains, but in the FeN-TiO₂ and FeCo-BaO cases, magnetic anisotropy inverses between out-of-plane and in-plane direction. Further orbital-resolved analysis indicates that the transition of magnetic anisotropy is mainly attributed to the orbital hybridization of interfacial Fe/Co atoms with O atoms induced by the magnetoelectric effect. This study may open an effective approach toward modulating PMA and lays a foundation to the development of low energy consumption memory devices.

Strain-Induced Magnetoelectric Coupling in Fe3O4/BaTiO3 Nanopillar Composites

Magnetoelectric coupling properties are limited to the substrate clamping effect in traditional ferroelectric/ferromagnetic heterostructures. Here, Fe₃O₄/BaTiO₃ nanopillar composites are successfully constructed. The well-ordered BaTiO₃ nanopillar arrays are prepared through template-assisted pulsed laser deposition. The Fe₃O₄ layer is coated on BaTiO₃ nanopillar arrays by atomic layer deposition. The nanopillar arrays and heterostructure are confirmed by scanning electron microscopy and transmission electron microscopy. A large thermally driven magnetoelectric coupling coefficient of 395 Oe °C^-1 near the phase transition of BaTiO₃ (orthorhombic to rhombohedral) is obtained, indicating a strong strain-induced magnetoelectric coupling effect. The enhanced magnetoelectric coupling effect originated from the reduced substrate clamping effect and increased the interface area in nanopillar structures. This work opens a door toward cutting-edge potential applications in spintronic devices.

Enhanced Tunneling Magnetoresistance Effect via Ferroelectric Control of Interface Electronic/Magnetic Reconstructions

Magnetic tunnel junctions (MTJs) with tunable tunneling magnetoresistances (TMR) have already been proven to have great potential for spintronics. Especially, when ferroelectric materials are used as insulating barriers, more novel functions of MTJs can be realized due to interface magnetoelectric coupling. Here, we demonstrate a very large ferroelectric modulation of TMR (as high as 570% in low-resistance state) in the ferroelectric/magnetic La_0.5Sr_0.5MnO₃/BaTiO₃ (LSMO/BTO) junctions and find robust interfacial electronic and magnetic reconstructions via ferroelectric polarization switching. Through electrical, magnetic, and optical measurements combined with X-ray absorption and magnetic circular dichroism, we reveal that the interfacial electronic and magnetic (ferromagnetic/antiferromagnetic phase transition) reconstructions originate from strong electromagnetic coupling between BTO and LSMO at the interface and are driven by the modulation of hole/electron doping at the interface of LSMO/BTO through ferroelectric polarization switching. As a result, the ferroelectrically controlled interface barrier height and width and spin filter effect enable a giant electrical modulation of TMR. Our results shed new light on the intrinsic mechanisms governing magnetoelectric coupling and offering a new route to enhance magnetoelectric coupling for spin control in spintronic devices.

Magnetoelectric Coupling at the Ni/Hf0.5Zr0.5O2 Interface

Composite multiferroics containing ferroelectric and ferromagnetic components often have much larger magnetoelectric coupling compared to their single-phase counterparts. Doped or alloyed HfO₂-based ferroelectrics may serve as a promising component in composite multiferroic structures potentially feasible for technological applications. Recently, a strong charge-mediated magnetoelectric coupling at the Ni/HfO₂ interface has been predicted using density functional theory calculations. Here, we report on the experimental evidence of such magnetoelectric coupling at the Ni/Hf_0.5Zr_0.5O₂(HZO) interface. Using a combination of operando XAS/XMCD and HAXPES/MCDAD techniques, we probe element-selectively the local magnetic properties at the Ni/HZO interface in functional Au/Co/Ni/HZO/W capacitors and demonstrate clear evidence of the ferroelectric polarization effect on the magnetic response of a nanometer-thick Ni marker layer. The observed magnetoelectric effect and the electronic band lineup of the Ni/HZO interface are interpreted based on the results of our theoretical modeling. It elucidates the critical role of an ultrathin NiO interlayer, which controls the sign of the magnetoelectric effect as well as provides a realistic band offset at the Ni/HZO interface, in agreement with the experiment. Our results hold promise for the use of ferroelectric HfO₂-based composite multiferroics for the design of multifunctional devices compatible with modern semiconductor technology.

Enhanced strain-induced magnetoelectric coupling in polarization-free Fe/BaTiO3 heterostructures

The search for magnetoelectric materials typically revolves around the struggle to make magnetic and ferroelectric orders simultaneously coexist in the same material, using either an intrinsic or an extrinsic/composite approach. Via ab initio calculations of a prototypical Fe/BaTiO3 interface, we predict that it is possible to tune the magnitude of the individual magnetic moments even for non-polar BaTiO3. By comparing polar and non-polar Fe/BaTiO3 heterostructures, we show that the Fe, Ti and equatorial O atomic magnetic moments are induced and enhanced as a result of their local crystal field. The crystal field may be controlled solely by manipulation of the inter-atomic distances of their neighbouring atoms (which will affect their electrostatic fields and orbital hybridizations), or by the BaTiO3 electric dipole moments, working as a local polarization. When this polarization is present, it dominates the crystal field contributions, thus constraining the effects of other perturbations such as strain. We also find that, contrary to conventional expectations, the non-polar heterostructure shows higher strain induced magnetization sensitivity than its polar counterpart.

Chemical Structure and Magnetism of FeOx/Fe2O3 Interface Studied by X-ray Absorption Spectroscopy

The chemical and magnetic states of Fe/Fe2O3 thin films prepared by e-beam evaporation were investigated by using element-specific techniques, X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD). It was clearly shown that the Fe layers are oxidized to form an antiferromagnetic (AFM) FeOx<1, while the bottom oxide remained a weak ferromagnet (wFM) (α+γ)-type Fe2O3. Dependences of the peak intensities and lineshapes on the Fe thickness and measurement geometry further demonstrate that FeOx<1 layers reside mostly at the interface realizing an FM (Fe)/AFM (FeOx)/wFM (Fe2O3), whilst the spin directions lie in the sample plane for all the samples. The self-stabilized intermediate oxide can act as a physical barrier for spins to be injected into the wFM oxide, implying a substantial influence on tailoring the spin tunneling efficiency for spintronics application.

Magneto-electric multiferroics: designing new materials from first-principles calculations

In parallel with the revival of interest for magneto-electric multiferroic materials in the beginning of the century, first-principles simulations have grown incredibly in efficiency during the last two decades. Density functional theory calculations, in particular, have so become a must-have tool for physicists and chemists in the multiferroic community. While these calculations were originally used to support and explain experimental behaviour, their interest has progressively moved to the design of novel magneto-electric multiferroic materials. In this article, we mainly focus on oxide perovskites, an important class of multifunctional material, and review some significant advances to which contributed first-principles calculations. We also briefly introduce the various theoretical developments that were at the core of all these advances.

Changing the magnetic states of an Fe/BaTiO3 interface through crystal field effects controlled by strain

The search for better and inexpensive magnetoelectric materials is now commonplace in solid state physics, using electric field induced strain to change the multiferroic magnetic state.

Polarization Control of the Interface Ferromagnetic to Antiferromagnetic Phase Transition in Co/Pb(Zr,Ti)O3

Based on first-principles calculations, we predict the polarization control of the interfacial magnetic phase and a giant electronically driven magnetoelectric coupling (MEC) in Co/PbZr_0.25Ti_0.75O₃ (PZT)(001). The effect of Co oxidation at the interface shared with (Zr,Ti)O₂-terminated PZT is evidenced. The magnetic phase of the oxidized Co interface layer is electrically switched from the ferromagnetic to the antiferromagnetic state by reversing the PZT polarization from upward to downward, respectively. A comparative study between oxidized and unoxidized Co/PZT interfaces shows that in oxidized Co/PZT bilayers, the variation of the interface spin moment upon polarization reversal exceeds that of unoxidized Co/PZT bilayers by about 1 order of magnitude. We define a surface MEC constant α_S taking into account the polarization dependence of both the spin and orbital moments. In unoxidized Co/PZT bilayers, we obtain α_S ≈ 2 × 10^-10 G cm² V^-1, while a giant surface coupling α_S ≈ 12 × 10^-10 G cm² V^-1 is found in the case of oxidized Co/PZT. We demonstrate that the polarization control of the magnetocrystalline anisotropy via spin-orbit coupling is not only effective at the interface but it extends to the Co film despite the interface origin of the MEC. This study shows that tailoring the nature of atomic bonding and electron occupancies allows for improving the performance of functional interfaces, enabling an efficient electric field control of spin-orbit interactions. Moreover, the nonlocal character of this effect holds promising perspectives for the application of electronically driven interface MEC in spin-orbitronic devices.

Magnetoelectricity in multiferroics: a theoretical perspective

The key physical property of multiferroic materials is the existence of coupling between magnetism and polarization, i.e. magnetoelectricity. The origin and manifestations of magnetoelectricity can be very different in the available plethora of multiferroic systems, with multiple possible mechanisms hidden behind the phenomena. In this review, we describe the fundamental physics that causes magnetoelectricity from a theoretical viewpoint. The present review will focus on mainstream physical mechanisms in both single-phase multiferroics and magnetoelectric heterostructures. The most recent tendencies addressing possible new magnetoelectric mechanisms will also be briefly outlined.

Oxygen-Valve Formed in Cobaltite-Based Heterostructures by Ionic Liquid and Ferroelectric Dual-Gating

Manipulation of oxygen vacancies via electric-field-controlled ionic liquid gating has been reported in many model systems within the emergent fields of oxide electronics and iontronics. It is then significant to investigate the oxygen vacancy formation/annihilation and migration across an additional ferroelectric layer with ionic liquid gating. Here, we report that via a combination of ionic liquid and ferroelectric gating, the remote control of oxygen vacancies and magnetic phase transition can be achieved in SrCoO_2.5 films capped with an ultrathin ferroelectric BaTiO₃ layer at room temperature. The ultrathin BaTiO₃ layer acts as an atomic oxygen valve and is semitransparent to oxygen-ion transport due to the competing interaction between vertical electron tunneling and ferroelectric polarization plus surface electrochemical changes in itself, thus resulting in the striking emergence of new mixed-phase SrCoO _x. The lateral coexistence of brownmillerite phase SrCoO_2.5 and perovskite phase SrCoO_3-δ was directly observed by transmission electron microscopy. Besides the fundamental significance of long-range interaction in ionic liquid gating, the ability to control the flow of oxygen ions across the heterointerface by the oxygen valve provides a new approach on the atomic scale for designing multistate memories, sensors, and solid-oxide fuel cells.

Recent Progress in the Voltage-Controlled Magnetic Anisotropy Effect and the Challenges Faced in Developing Voltage-Torque MRAM

The electron spin degree of freedom can provide the functionality of "nonvolatility" in electronic devices. For example, magnetoresistive random access memory (MRAM) is expected as an ideal nonvolatile working memory, with high speed response, high write endurance, and good compatibility with complementary metal-oxide-semiconductor (CMOS) technologies. However, a challenging technical issue is to reduce the operating power. With the present technology, an electrical current is required to control the direction and dynamics of the spin. This consumes high energy when compared with electric-field controlled devices, such as those that are used in the semiconductor industry. A novel approach to overcome this problem is to use the voltage-controlled magnetic anisotropy (VCMA) effect, which draws attention to the development of a new type of MRAM that is controlled by voltage (voltage-torque MRAM). This paper reviews recent progress in experimental demonstrations of the VCMA effect. First, we present an overview of the early experimental observations of the VCMA effect in all-solid state devices, and follow this with an introduction of the concept of the voltage-induced dynamic switching technique. Subsequently, we describe recent progress in understanding of physical origin of the VCMA effect. Finally, new materials research to realize a highly-efficient VCMA effect and the verification of reliable voltage-induced dynamic switching with a low write error rate are introduced, followed by a discussion of the technical challenges that will be encountered in the future development of voltage-torque MRAM.

Atomic-Scale Control of Magnetism at the Titanite-Manganite Interfaces

Complex oxide thin-film heterostructures often exhibit magnetic properties different from those known for bulk constituents. This is due to the altered local structural and electronic environment at the interfaces, which affects the exchange coupling and magnetic ordering. The emergent magnetism at oxide interfaces can be controlled by ferroelectric polarization and has a strong effect on spin-dependent transport properties of oxide heterostructures, including magnetic and ferroelectric tunnel junctions. Here, using prototype La_2/3Sr_1/3MnO₃/BaTiO₃ heterostructures, we demonstrate that ferroelectric polarization of BaTiO₃ controls the orbital hybridization and magnetism at heterointerfaces. We observe changes in the enhanced orbital occupancy and significant charge redistribution across the heterointerfaces, affecting the spin and orbital magnetic moments of the interfacial Mn and Ti atoms. Importantly, we find that the exchange coupling between Mn and Ti atoms across the interface is tuned by ferroelectric polarization from ferromagnetic to antiferromagnetic. Our findings provide a viable route to electrically control complex magnetic configurations at artificial multiferroic interfaces, taking a step toward low-power spintronics.

Observation of Skyrmions at Room Temperature in Co2FeAl Heusler Alloy Ultrathin Film Heterostructures

Magnetic skyrmions are topological spin-textures having immense potential for energy efficient spintronic devices. Here, we report the observation of stable skyrmions in unpatterned Ta/Co₂FeAl(CFA)/MgO thin film heterostructures at room temperature in remnant state employing magnetic force microscopy. It is shown that these skyrmions consisting of ultrathin ferromagnetic CFA Heusler alloy result from strong interfacial Dzyaloshinskii-Moriya interaction (i-DMI) as evidenced by Brillouin light scattering measurements, in agreement with the results of micromagnetic simulations. We also emphasize on room temperature observation of multiple skyrmions which can be stabilized for suitable combinations of CFA layer thickness, perpendicular magnetic anisotropy, and i-DMI. These results provide a significant step towards designing of room temperature spintronic devices based on skyrmions in full Heusler alloy based thin films.

Electric field control of resistive switching and magnetization in epitaxial LaBaCo2O5+δ thin films

The low operating temperature and volatile characteristics of the magnetization change are the main obstacles for the practical applications of spintronic and magnetic memories. In this work, both the resistive switching and magnetization switching are realized in Pt/LaBaCo2O5+δ (LBCO)/Nb-doped SrTiO3 (Nb-STO) devices at room temperature through an electric field. Unlike the traditional approach of an external stress inducing a volatile magnetization change, the magnetization in the Pt/LBCO/Nb-STO device is modulated by an electrical field, along with the resistive switching. The resistive and magnetization switching can be attributed to the variation of the depletion layer width at the LBCO/Nb-STO interface via oxygen vacancy migration and the increase/decrease of the Co-O-Co bond length, respectively. The present device with the synchronous manipulation of both resistance and magnetization at room temperature can be applied in nonvolatile resistive memories and novel magnetic multifunctional devices.

Structural and electronic properties of Fe monolayer on BaTiO3(0 0 1)

The structural, electronic, and phonon properties of the BaTiO₃(0 0 1) surface and the Fe/BaTiO₃(0 0 1) interface have been studied within the density functional theory. Attention is paid to the lattice instabilities (soft phonon modes) that induce ferroelectric distortions in the surface and the interface. A phonon-induced monoclinic (Cm) thin-film counterpart of the low-temperature rhombohedral (R3m) ferroelectric bulk BaTiO₃ phase is found. The changes in crystal structure, electronic density of states, atomic charges, and magnetic moments associated with the ferroelectric distortions are discussed comparing the results of the standard GGA and the hybrid DFT calculations. The magnetoelectric coupling at the Fe/BaTiO₃(0 0 1) interface is investigated by the analysis of changes in magnetic moments on Fe and Ti atoms induced by the atomic displacements perpendicular and parallel to the surface.

Giant magnetoelectric effect at the graphone/ferroelectric interface

Multiferroic heterostructures combining ferromagnetic and ferroelectric layers are promising for applications in novel spintronic devices, such as memories with electrical writing and magnetic reading, assuming their magnetoelectric coupling (MEC) is strong enough. For conventional magnetic metal/ferroelectric heterostructures, however, the change of interfacial magnetic moment upon reversal of the electric polarization is often very weak. Here, by using first principles calculations, we demonstrate a new pathway towards a strong MEC at the interface between the semi-hydrogenated graphene (also called graphone) and ferroelectric PbTiO₃. By reversing the polarization of PbTiO₃, the magnetization of graphone can be electrically switched on and off through the change of carbon-oxygen bonding at the interface. Furthermore, a ferroelectric polarization can be preserved down to ultrathin PbTiO₃ layers less than one nanometer due to an enhancement of the polarization at the interface. The predicted strong magnetoelectric effect in the ultimately thin graphone/ferroelectric layers opens a new opportunity for the electric control of magnetism in high-density devices.

Synergies of Electrochemical Metallization and Valance Change in All-Inorganic Perovskite Quantum Dots for Resistive Switching

The in-depth understanding of ions' generation and movement inside all-inorganic perovskite quantum dots (CsPbBr₃ QDs), which may lead to a paradigm to break through the conventional von Neumann bottleneck, is strictly limited. Here, it is shown that formation and annihilation of metal conductive filaments and Br^- ion vacancy filaments driven by an external electric field and light irradiation can lead to pronounced resistive-switching effects. Verified by field-emission scanning electron microscopy as well as energy-dispersive X-ray spectroscopy analysis, the resistive switching behavior of CsPbBr₃ QD-based photonic resistive random-access memory (RRAM) is initiated by the electrochemical metallization and valance change. By coupling CsPbBr₃ QD-based RRAM with a p-channel transistor, the novel application of an RRAM-gate field-effect transistor presenting analogous functions of flash memory is further demonstrated. These results may accelerate the technological deployment of all-inorganic perovskite QD-based photonic resistive memory for successful logic application.

Tunable Magnetism in Nanoporous CuNi Alloys by Reversible Voltage-Driven Element-Selective Redox Processes

Voltage-driven manipulation of magnetism in electrodeposited 200 nm thick nanoporous single-phase solid solution Cu₂₀ Ni₈₀ (at%) alloy films (with sub 10 nm pore size) is accomplished by controlled reduction-oxidation (i.e., redox) processes in a protic solvent, namely 1 m NaOH aqueous solution. Owing to the selectivity of the electrochemical processes, the oxidation of the CuNi film mainly occurs on the Cu counterpart of the solid solution, resulting in a Ni-enriched alloy. As a consequence, the magnetic moment at saturation significantly increases (up to 33% enhancement with respect to the as-prepared sample), while only slight changes in coercivity are observed. Conversely, the reduction process brings Cu back to its metallic state and, remarkably, it becomes alloyed to Ni again. The reported phenomenon is fully reversible, thus allowing for the precise adjustment of the magnetic properties of this system through the sign and amplitude of the applied voltage.

Charge screening-controlled Verwey phase transition in Fe3O4/SrTiO3 heterostructure

Despite intensive investigations into the Verwey phase transition of Fe₃O₄ over half a century, the mechanism of this phase transition remains controversial and needs further research. In this work, we build the Fe₃O₄/SrTiO₃ multiferroic heterostructure and investigate the temperature dependence of its saturation magnetization under various electric fields. It is found that the charge-screening effect not only influences the magnetization but also induces the temperature of the Verwey phase transition shifting ~13 K. It suggests that the Verwey phase transition has certain correlations with the electron distribution and the change of the number of minority spin electrons in the trimerons plays a dominant role in the temperature shift of the phase transition.

Giant magneto-electric coupling in 100 nm thick Co capped by ZnO nanorods

Here we report a giant, completely reversible magneto-electric coupling of 100 nm polycrystalline Co layer in contact with ZnO nanorods. When the sample is under an applied bias of ±2 V, the Co magnetic coercivity is reduced by a factor 5 from the un-poled case, with additionally a reduction of total magnetic moment in Co. Taking into account the chemical properties of ZnO nanorods measured by X-rays absorption near edge spectroscopy under bias, we conclude that these macroscopic effects on the magnetic response of the Co layer are due to the microstructure and the strong strain-driven magneto-electric coupling induced by the ZnO nanorods, whose nanostructuration maximizes the piezoelectric response under bias.

Strain and electric-field control of magnetism in supercrystalline iron oxide nanoparticle-BaTiO3 composites

The manipulation of the magnetism of self-assembled iron oxide nanoparticle (NP) monolayers on top of BaTiO₃ (BTO) single crystals is reported. We observe strain induced magnetoelectric coupling (MEC) as shown by measurements of both the magnetization and magneto-electric AC susceptibility (MEACS). The magnetization, coercivity, remanent magnetization and MEACS signal as a function of temperature show abrupt jumps at the BTO phase transition temperatures. Hereby the jump values are opposite for in-plane and out-of-plane measurements. Grazing incidence small angle X-ray scattering (GISAXS) and scanning electron microscopy (SEM) confirm a hexagonal close-packed supercrystalline order of the NP monolayers. Cross-sectional scanning transmission electron microscopy (STEM) experiments provide information about the layer structure of the sample. This work opens up viable possibilities for fabricating energy-efficient electronic devices by self-assembly techniques.

Voltage controlled interfacial magnetism through platinum orbits

Electric fields at interfaces exhibit useful phenomena, such as switching functions in transistors, through electron accumulations and/or electric dipole inductions. We find one potentially unique situation in a metal–dielectric interface in which the electric field is atomically inhomogeneous because of the strong electrostatic screening effect in metals. Such electric fields enable us to access electric quadrupoles of the electron shell. Here we show, by synchrotron X-ray absorption spectroscopy, electric field induction of magnetic dipole moments in a platinum monatomic layer placed on ferromagnetic iron. Our theoretical analysis indicates that electric quadrupole induction produces magnetic dipole moments and provides a large magnetic anisotropy change. In contrast with the inability of current designs to offer ultrahigh-density memory devices using electric-field-induced spin control, our findings enable a material design showing more than ten times larger anisotropy energy change for such a use and highlight a path in electric-field control of condensed matter.

Electric field control of magnetization is usually weak and this hampers its application for the ultralow-power-consumption spintronic devices. Here, the authors demonstrate a mechanism to enhance the control of magnetic anisotropy by voltage-induced electric quadrupole in a metal–dielectric interface.

Strain-Mediated Coexistence of Volatile and Nonvolatile Converse Magnetoelectric Effects in Fe/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 Heterostructure

Strain-mediated ferromagnetic/ferroelectric (FE) heterostructures have played an important role in multiferroic materials to investigate the electric-field control of magnetism in the past decade, due to their excellent performances, such as room-temperature operation and large magnetoelectric (ME) coupling effect. Because of the different FE-switching-originated strain behaviors and varied interfacial coupling effect, both loop-like (nonvolatile) and butterfly-like (volatile) converse ME effects have been reported. Here, we investigate the electric-field control of magnetism in a multiferroic heterostructure composed of a polycrystalline Fe thin film and a Pb(Mg_1/3Nb_2/3)_0.7Ti_0.3O₃ single crystal, and the experimental results exhibit complex behaviors, suggesting the coexistence of volatile and nonvolatile converse ME effects. By separating the symmetrical and antisymmetrical parts of the electrical modulation of magnetization, we distinguished the loop-like hysteresis and butterfly-like magnetization changes tuned by electric fields, corresponding to the strain effects related to the FE 109° switching and 71/180° switching, respectively. Further magnetic-field-dependent as well as angular-dependent investigation of the converse ME effect confirmed the strain-mediated magnetism involving competition among the Zeeman energy, magnetocrystalline anisotropy energy, and strain-generated magnetoelastic energy. This study is helpful for understanding the electric-field control of magnetism in multiferroic heterostructures as well as its relevant applications.

Probing a Device's Active Atoms

Materials science and device studies have, when implemented jointly as "operando" studies, better revealed the causal link between the properties of the device's materials and its operation, with applications ranging from gas sensing to information and energy technologies. Here, as a further step that maximizes this causal link, the paper focuses on the electronic properties of those atoms that drive a device's operation by using it to read out the materials property. It is demonstrated how this method can reveal insight into the operation of a macroscale, industrial-grade microelectronic device on the atomic level. A magnetic tunnel junction's (MTJ's) current, which involves charge transport across different atomic species and interfaces, is measured while these atoms absorb soft X-rays with synchrotron-grade brilliance. X-ray absorption is found to affect magnetotransport when the photon energy and linear polarization are tuned to excite FeO bonds parallel to the MTJ's interfaces. This explicit link between the device's spintronic performance and these FeO bonds, although predicted, challenges conventional wisdom on their detrimental spintronic impact. The technique opens interdisciplinary possibilities to directly probe the role of different atomic species on device operation, and shall considerably simplify the materials science iterations within device research.

Voltage-induced Interface Reconstruction and Electrical Instability of the Ferromagnet-Semiconductor Device

Using x-ray magnetic spectroscopy with in-situ electrical characterizations, we investigated the effects of external voltage on the spin-electronic and transport properties at the interface of a Fe/ZnO device. Layer-, element-, and spin-resolved information of the device was obtained by cross-tuning of the x-ray mode and photon energy, when voltage was applied. At the early stage of the operation, the device exhibited a low-resistance state featuring robust Fe-O bonds. However, the Fe-O bonds were broken with increasing voltage. Breaking of the Fe-O bonds caused the formation of oxygen vacancies and resulted in a high-resistance state. Such interface reconstruction was coupled to a charge-transfer effect via Fe-O hybridization, which suppressed/enhanced the magnetization/coercivity of Fe electronically. Nevertheless, the interface became stabilized with the metallic phase if the device was continuously polarized. During this stage, the spin-polarization of Fe was enhanced whereas the coercivity was lowered by voltage, but changes of both characteristics were reversible. This stage is desirable for spintronic device applications, owing to a different voltage-induced electronic transition compared to the first stage. The study enabled a straightforward detection of the spin-electronic state at the ferromagnet-semiconductor interface in relation to the transport and reversal properties during operation process of the device.

3D Visualization of the Iron Oxidation State in FeO/Fe3O4 Core-Shell Nanocubes from Electron Energy Loss Tomography

The physicochemical properties used in numerous advanced nanostructured devices are directly controlled by the oxidation states of their constituents. In this work we combine electron energy-loss spectroscopy, blind source separation, and computed tomography to reconstruct in three dimensions the distribution of Fe(2+) and Fe(3+) ions in a FeO/Fe3O4 core/shell cube-shaped nanoparticle with nanometric resolution. The results highlight the sharpness of the interface between both oxides and provide an average shell thickness, core volume, and average cube edge length measurements in agreement with the magnetic characterization of the sample.

Interfacial Ion Intermixing Effect on Four-Resistance States in La0.7Sr0.3MnO3/BaTiO3/La0.7Sr0.3MnO3 Multiferroic Tunnel Junctions

A multiferroic tunnel junction (MFTJ), employing a ferroelectric barrier layer sandwiched between two ferromagnetic layers, presents at least four resistance states in a single memory cell and therefore opens an avenue for the development of the next generation of high-density nonvolatile memory devices. Here, using the all-perovskite-oxide La0.7Sr0.3MnO3/BaTiO3/La0.7Sr0.3MnO3 as a model MFTJ system, we demonstrate asymmetrical Mn-Ti sublattice intermixing at the La0.7Sr0.3MnO3/BaTiO3 interfaces by direct local measurements of the structure and valence, which reveals the relationship between ferroelectric polarization directions and four-resistance states, and the low temperature anomalous tunneling behavior in the MFTJ. These findings emphasize the crucial role of the interfaces in MFTJs and are quite important for understanding the electric transport of MFTJs as well as designing high-density multistates storage devices.

Interface Magnetoelectric Coupling in Co/Pb(Zr,Ti)O3

Magnetoelectric coupling at multiferroic interfaces is a promising route toward the nonvolatile electric-field control of magnetization. Here, we use optical measurements to study the static and dynamic variations of the interface magnetization induced by an electric field in Co/PbZr0.2Ti0.8O3 (Co/PZT) bilayers at room temperature. The measurements allow us to identify different coupling mechanisms. We further investigate the local electronic and magnetic structure of the interface by means of transmission electron microscopy, soft X-ray magnetic circular dichroism, and density functional theory to corroborate the coupling mechanism. The measurements demonstrate a mixed linear and quadratic optical response to the electric field, which results from a magneto-electro-optical effect. We propose a decomposition method of the optical signal to discriminate between different components involved in the electric field-induced polarization rotation of the reflected light. This allows us to extract a signal that we can ascribe to interface magnetoelectric coupling. The associated surface magnetization exhibits a clear hysteretic variation of odd symmetry with respect to the electric field and nonzero remanence. The interface coupling is remarkably stable over a wide frequency range (1-50 kHz), and the application of a bias magnetic field is not necessary for the coupling to occur. These results show the potential of exploiting interface coupling with the prospect of optimizing the performance of magnetoelectric memory devices in terms of stability, as well as fast and dissipationless operation.

Full Electroresistance Modulation in a Mixed-Phase Metallic Alloy

We report a giant, ∼22%, electroresistance modulation for a metallic alloy above room temperature. It is achieved by a small electric field of 2  kV/cm via piezoelectric strain-mediated magnetoelectric coupling and the resulting magnetic phase transition in epitaxial FeRh/BaTiO_{3} heterostructures. This work presents detailed experimental evidence for an isothermal magnetic phase transition driven by tetragonality modulation in FeRh thin films, which is in contrast to the large volume expansion in the conventional temperature-driven magnetic phase transition in FeRh. Moreover, all the experimental results in this work illustrate FeRh as a mixed-phase model system well similar to phase-separated colossal magnetoresistance systems with phase instability therein.

Electric-Field Control of Magnetism in Co40Fe40B20/(1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 Multiferroic Heterostructures with Different Ferroelectric Phases

Electric-field control of magnetism in multiferroic heterostructures composed of Co40Fe40B20 (CoFeB) and (1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-xPT) with different ferroelectric phases via changing composition and temperature is explored. It is demonstrated that the nonvolatile looplike bipolar-electric-field-controlled magnetization, previously found in the CoFeB/PMN-xPT heterostructures with PMN-xPT in the rhombohedral (R) phase around the morphotropic phase boundary (MPB), also occurs for PMN-xPTs with both R phase (far away from MPB) and monoclinic (M) phase, suggesting that the phenomenon is the common feature of CoFeB/PMN-xPT multiferroic heterostructures for PMN-xPT with different phases. The magnitude of the effect changes with increasing temperature and volatile bipolar-electric-field-controlled magnetization with a butterflylike behavior occurs when the ferroelectric phase changes to the tetragonal phase (T). Moreover, for the R-phase sample with x = 0.18, an abrupt and giant increase of magnetization is observed at a characteristic temperature in the temperature dependence of magnetization curve. These results are discussed in terms of coupling between magnetism and ferroelectric domains including macro- and microdomains for different ferroelectric phases. This work is helpful for understanding the phenomena of electric-field control of magnetism in FM/FE multiferroic heterostructures and is also important for applications.

Multiferroic Heterostructures Integrating Ferroelectric and Magnetic Materials

Multiferroic heterostructures can be synthesized by integrating monolithic ferroelectric and magnetic materials, with interfacial coupling between electric polarization and magnetization, through the exchange of elastic, electric, and magnetic energy. Although the nature of the interfaces remains to be unraveled, such cross coupling can be utilized to manipulate the magnetization (or polarization) with an electric (or magnetic) field, known as a converse (or direct) magnetoelectric effect. It can be exploited to significantly improve the performance of or/and add new functionalities to many existing or emerging devices such as memory devices, tunable microwave devices, sensors, etc. The exciting technological potential, along with the rich physical phenomena at the interface, has sparked intensive research on multiferroic heterostructures for more than a decade. Here, we summarize the most recent progresses in the fundamental principles and potential applications of the interface-based magnetoelectric effect in multiferroic heterostructures, and present our perspectives on some key issues that require further study in order to realize their practical device applications.

Grain size dependent magnetoelectric coupling of BaTiO3 nanoparticles

We report the magnetoelectric (ME) coupling properties of BaTiO₃ nanoparticles of different grain sizes ranging from 16–26 nm synthesized using a modified Pechini method.

Prediction of large magnetoelectric coupling in Fe4N/BaTiO3 and MnFe3N/BaTiO3 junctions from a first-principles study

The large magnetoelectric effects are predicted in Fe₄N/BaTiO₃ and MnFe₃N/BaTiO₃ junctions.

Electric field control of magnetic properties and electron transport in BaTiO₃-based multiferroic heterostructures

In this paper, we report on a purely electric mechanism for achieving the electric control of the interfacial spin polarization and magnetoresistance in multiferroic tunneling junctions. We investigate micrometric devices based on the Co/Fe/BaTiO3/La0.7Sr0.3MnO3 heterostructure, where Co/Fe and La0.7Sr0.3MnO3 are the magnetic electrodes and BaTiO3 acts both as a ferroelectric element and tunneling barrier. We show that, at 20 K, devices with a 2 nm thick BaTiO3 barrier present both tunneling electroresistance (TER = 12   ±   0.1%) and tunneling magnetoresistance (TMR). The latter depends on the direction of the BaTiO3 polarization, displaying a sizable change of the TMR from  -0.32   ±   0.05% for the polarization pointing towards Fe, to  -0.12   ±   0.05% for the opposite direction. This is consistent with the on-off switching of the Fe magnetization at the Fe/BaTiO3 interface, driven by the BaTiO3 polarization, we have previously demonstrated in x-ray magnetic circular dichroism experiments.

Polarization-Mediated Thermal Stability of Metal/Oxide Heterointerface

A polarization-mediated heterointerface is designed to research the thermal stability of magnetic metal/oxide interfaces. Using polarization engineering, the thermal stability of the interface between BiFeO3 and CoFeB can be improved by about 100°C. This finding provides new insight into the chemistry of the metal/oxide heterointerface.

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.

Magnetoelectric Coupling Induced by Interfacial Orbital Reconstruction

Reversible orbital reconstruction driven by ferroelectric polarization modulates the magnetic performance of model ferroelectric/ferromagnetic heterostructures without onerous limitations. Mn-d(x2-y2) orbital occupancy and related interfacial exotic magnetic states are enhanced and weakened by negative and positive electric fields, respectively, filling the missing member-orbital in the mechanism of magnetoelectric coupling and advancing the application of orbitals to microelectronics.

Electric field modification of magnetism in Au/La2/3Ba1/3MnO3/Pt device

The La_2/3Ba_1/3MnO₃ film is deposited in a CMOS-compatible Pt/Ti/SiO₂/Si substrate with the oxygen pressure of 10 Pa for investigating magnetoelectric effect. Bipolar resistive switching effect with excellent endurance and retention is observed in this Au/La_2/3Ba_1/3MnO₃/Pt device. Through this effect, a significant nonvolatile change of magnetization is obtained in this device as well. The change of magnetization can be understood by the break and repair of the -Mn³⁺-O²⁻-Mn⁴⁺- chains induced by the electric field through the oxygen vacancies migration. The resistance and magnetization of the Au/La_2/3Ba_1/3MnO₃/Pt device can be simultaneously manipulated by the electric field, which makes it to be a promising candidate for the multifunctional memory devices.

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.

Four-state ferroelectric spin-valve

Spin-valves had empowered the giant magnetoresistance (GMR) devices to have memory. The insertion of thin antiferromagnetic (AFM) films allowed two stable magnetic field-induced switchable resistance states persisting in remanence. In this letter, we show that, without the deliberate introduction of such an AFM layer, this functionality is transferred to multiferroic tunnel junctions (MFTJ) allowing us to create a four-state resistive memory device. We observed that the ferroelectric/ferromagnetic interface plays a crucial role in the stabilization of the exchange bias, which ultimately leads to four robust electro tunnel electro resistance (TER) and tunnel magneto resistance (TMR) states in the junction.

Nanoscale magnetization reversal caused by electric field-induced ion migration and redistribution in cobalt ferrite thin films

Reversible nanoscale magnetization reversal controlled merely by electric fields is still challenging at the moment. In this report, first-principles calculation indicates that electric field-induced magnetization reversal can be achieved by the appearance of unidirectional magnetic anisotropy along the (110) direction in Fe-deficient cobalt ferrite (CoFe(2-x)O4, CFO), as a result of the migration and local redistribution of the Co(2+) ions adjacent to the B-site Fe vacancies. In good agreement with the theoretical model, we experimentally observed that in the CFO thin films the nanoscale magnetization can be reversibly and nonvolatilely reversed at room temperature via an electrical ion-manipulation approach, wherein the application of electric fields with appropriate polarity and amplitude can modulate the size of magnetic domains with different magnetizations up to 70%. With the low power consumption (subpicojoule) characteristics and the elimination of external magnetic field, the observed electric field-induced magnetization reversal can be used for the construction of energy-efficient spintronic devices, e.g., low-power electric-write and magnetic-read memories.

Electric field mediated non-volatile tuning magnetism at the single-crystalline Fe/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 interface

We report non-volatile electric-field control of magnetism modulation in Fe/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 (PMN-PT) heterostructure by fabricating an epitaxial Fe layer on a PMN-PT substrate using a molecular beam epitaxy technique. The remnant magnetization with a different electric field shows a non-symmetric loop-like shape, which demonstrates a change of interfacial chemistry and a large magnetoelectric coupling in Fe/PMN-PT at room temperature to realize low loss multistate memory under an electric field. Fitting with the angular-dependence of the in-plane magnetization reveals that the magnetoelectric effect is dominated by the direct electric-field effect rather than the strain effect at the interface. The magnetoelectric effect and the induced surface anisotropy are found to be dependent on the Fe film thickness and are linear with respect to the applied electric field.

Control of magnetism by electric fields

The electrical manipulation of magnetism and magnetic properties has been achieved across a number of different material systems. For example, applying an electric field to a ferromagnetic material through an insulator alters its charge-carrier population. In the case of thin films of ferromagnetic semiconductors, this change in carrier density in turn affects the magnetic exchange interaction and magnetic anisotropy; in ferromagnetic metals, it instead changes the Fermi level position at the interface that governs the magnetic anisotropy of the metal. In multiferroics, an applied electric field couples with the magnetization through electrical polarization. This Review summarizes the experimental progress made in the electrical manipulation of magnetization in such materials, discusses our current understanding of the mechanisms, and finally presents the future prospects of the field.

Electric field control of the magnetocaloric effect

Through strain-mediated magnetoelectric coupling, it is demonstrated that the magnetocaloric effect of a ferromagnetic shape-memory alloy can be controlled by an electric field. Large hysteresis and the limited operating temperature region are effectively overcome by applying an electric field on a laminate comprising a piezoelectric and the alloy. Accordingly, a model for an active magnetic refrigerator with high efficiency is proposed in principle.

Deterministic switching of ferromagnetism at room temperature using an electric field

The technological appeal of multiferroics is the ability to control magnetism with electric field. For devices to be useful, such control must be achieved at room temperature. The only single-phase multiferroic material exhibiting unambiguous magnetoelectric coupling at room temperature is BiFeO3 (refs 4 and 5). Its weak ferromagnetism arises from the canting of the antiferromagnetically aligned spins by the Dzyaloshinskii-Moriya (DM) interaction. Prior theory considered the symmetry of the thermodynamic ground state and concluded that direct 180-degree switching of the DM vector by the ferroelectric polarization was forbidden. Instead, we examined the kinetics of the switching process, something not considered previously in theoretical work. Here we show a deterministic reversal of the DM vector and canted moment using an electric field at room temperature. First-principles calculations reveal that the switching kinetics favours a two-step switching process. In each step the DM vector and polarization are coupled and 180-degree deterministic switching of magnetization hence becomes possible, in agreement with experimental observation. We exploit this switching to demonstrate energy-efficient control of a spin-valve device at room temperature. The energy per unit area required is approximately an order of magnitude less than that needed for spin-transfer torque switching. Given that the DM interaction is fundamental to single-phase multiferroics and magnetoelectrics, our results suggest ways to engineer magnetoelectric switching and tailor technologically pertinent functionality for nanometre-scale, low-energy-consumption, non-volatile magnetoelectronics.