Full electric control of exchange bias

Confined Magnetization at the Sublattice-Matched Ruthenium Oxide Heterointerface

Creating a heterostructure by combining two magnetically and structurally distinct ruthenium oxides is a crucial approach for investigating their emergent magnetic states and interactions. Previously, research has predominantly concentrated on the intrinsic properties of the ferromagnet SrRuO3 and recently discovered altermagnet RuO2 solely. Here, the study engineers an ultrasharp sublattice-matched heterointerface using pseudo-cubic SrRuO3 and rutile RuO2, conducting an in-depth analysis of their spin interactions. Structurally, to accommodate the lattice symmetry mismatch, the inverted RuO2 layer undergoes an in-plane rotation of 18 degrees during epitaxial growth on SrRuO3 layer, resulting in an interesting and rotational interface with perfect crystallinity and negligible chemical intermixing. Performance-wise, the interfacial layer of 6 nm in RuO2 adjacent to SrRuO3 exhibits a nonzero magnetic moment, contributing to an enhanced anomalous Hall effect (AHE) at low temperatures. Furthermore, the observations indicate that in contrast to SrRuO3 single layers, the AHE of [(RuO2)15/(SrRuO3)n] heterostructures show nonlinear behavior and reaches its maximum when the SrRuO3 thickness reaches tens of nm. These results suggest that the interfacial magnetic interaction surpasses that of all-perovskite oxides (≈5-unit cells). This study underscores the significance and potential applications of magnetic interactions based on the crystallographic asymmetric interfaces in the design of spintronic devices.

Two-dimensional multiferroic RuClF/AgBiP2S6 van der Waals heterostructures with valley splitting properties and controllable magnetic anisotropy

The investigation of new properties in two-dimensional (2D) multiferroic heterostructures is significant. In this work, the electronic properties and magnetic anisotropy energies (MAEs) of 2D multiferroic RuClF/AgBiP2S6 van der Waals (vdW) heterostructures are systematically studied by first principles calculations based on density functional theory (DFT). The Hubbard on-site Coulomb parameter (U) of Ru atoms is necessary to account for the strong correlation among the three-dimensional electrons of Ru. RuClF/AgBiP2S6 heterostructures in different polarizations (RuClF/AgBiP2S6-P↑ and RuClF/AgBiP2S6-P↓) are ferromagnetic semiconductors with stable structures. Valley polarizations are present in the band structures of RuClF/AgBiP2S6 heterostructures with spin-orbit coupling (SOC), the valley splitting energies of which are 279 meV and 263 meV, respectively. The MAEs of RuClF/AgBiP2S6 heterostructures indicate perpendicular magnetic anisotropy (PMA), which are primarily attributed to the differences in matrix elements within Ru (dyz, dz2) orbitals. In addition, valley splittings and MAEs of RuClF/AgBiP2S6 heterostructures are modified at different biaxial strains. Specifically, the highest valley splittings are 283 meV and 287 meV at ε = 2%, while they disappear at ε = -6%. The PMA of RuClF/AgBiP2S6-P↑ is gradually decreased at biaxial strains of -6% to 2%, and MAE is transformed into in-plane magnetic anisotropy (IMA) at ε = 4%. RuClF/AgBiP2S6-P↓ maintains PMA at different strains. The study of non-volatile electrical control of valley splitting phenomena in multiferroic RuClF/AgBiP2S6 heterostructures is crucial in the field of valleytronic devices, which has important theoretical significance.

Large exchange bias enhancement and control of ferromagnetic energy landscape by solid-state hydrogen gating

Voltage control of exchange bias is desirable for spintronic device applications, however dynamic modulation of the unidirectional coupling energy in ferromagnet/antiferromagnet bilayers has not yet been achieved. Here we show that by solid-state hydrogen gating, perpendicular exchange bias can be enhanced by > 100% in a reversible and analog manner, in a simple Co/Co0.8Ni0.2O heterostructure at room temperature. We show that this phenomenon is an isothermal analog to conventional field-cooling and that sizable changes in average coupling energy can result from small changes in AFM grain rotatability. Using this method, we show that a bi-directionally stable ferromagnet can be made unidirectionally stable, with gate voltage alone. This work provides a means to dynamically reprogram exchange bias, with broad applicability in spintronics and neuromorphic computing, while simultaneously illuminating fundamental aspects of exchange bias in polycrystalline films.

Enhanced Exchange Bias in Epitaxial High-Entropy Oxide Heterostructures

High-entropy oxides (HEOs) have gained significant interest in recent years due to their unique structural characteristics and potential to tailor functional properties. However, the electronic structure of the HEOs currently remains vastly unknown. In this work, combining magnetometry measurements, scanning transmission electron microscopy, and element-specific X-ray absorption spectroscopy, the electronic structure and magnetic properties of the perovskite-HEO La(Cr0.2Mn0.2Fe0.2Co0.2Ni0.2)O3 epitaxial thin films are systemically studied. It is found that enhanced magnetic frustration emerges from competing exchange interactions of the five transition-metal cations with energetically favorable half-filled/full-filled electron configurations, resulting in an unprecedented large vertical exchange bias effect in the single-crystalline films. Furthermore, our findings demonstrate that the La(Cr0.2Mn0.2Fe0.2Co0.2Ni0.2)O3 layer with a thickness down to 1 nm can be used as a pinning layer and strongly coupled with a ferromagnetic La0.7Sr0.3MnO3 layer, leading to a notable exchange bias and coercivity enhancement in a cooling field as small as 5 Oe. Our studies not only provide invaluable insight into the electronic structure of HEOs but also pave the way for a new era of large bias materials for spintronics devices.

Electric field control of magnetic states in ferromagnetic-multiferroic nanostructures

Multiferroic oxides are considered as key elements of energy-consuming devices required for the development of scalable logic and information storage technologies. In this regard, understanding the mechanisms of magnetoelectric switching and finding the optimal way to switch magnetization by an electric field is of crucial importance. In this study, we develop a model for studying magnetic states in a nanoscale exchange-coupled ferromagnetic-multiferroic heterostructure subjected to the action of an electric field. Based on bias effects emerging due to the coupling between a ferromagnetic subsystem and an antiferromagnetically ordered multiferroic material, we explore the magnetic textures and the magnetization reversal processes in a ferromagnet. As the multiferroic material, we consider BiFeO3, where magnetic ordering and ferroelectric ordering are determined by the mutually perpendicular antiferromagnetic (L), weak ferromagnetic (M) and polarization (P) vectors. Application of an electric voltage removes degeneration from eight energetically equivalent positions of P|| 〈111〉, allocates the definite directions of vectors P, M, and L and as a consequence the unidirectional magnetic anisotropy axis in the reference ferromagnetic layer. Our study reveals the features of the magnetic configurations in systems of different geometries, with varying exchange and magnetic anisotropy, necessary to determine the optimal conditions for switching magnetic states in a multiferroic bi-layer by an electric field.

Ferroelectric control of band alignments and magnetic properties in the two-dimensional multiferroic VSe2/In2Se3

Our first-principles evidence shows that the two-dimensional (2D) multiferroic VSe₂/In₂Se₃experiences continuous change of electronic structures, i.e. with the change of the ferroelectric (FE) polarization of In₂Se₃, the heterostructure can possess type-I, -II, and -III band alignments. When the FE polarization points from In₂Se₃to VSe₂, the heterostructure has a type-III band alignment, and the charge transfer from In₂Se₃into VSe₂induces half-metallicity. With reversal of the FE polarization, the heterostructure enters the type-I band alignment, and the spin-polarized current is turned off. When the In₂Se₃is depolarized, the heterostructure has a type-II band alignment. In addition, influence of the FE polarization on magnetism and magnetic anisotropy energy of VSe₂was also analyzed, through which we reveal the interfacial magnetoelectric coupling effects. Our investigation about VSe₂/In₂Se₃predicts its wide applications in the fields of both 2D spintronics and multiferroics.

Cooling Field Dependence of Exchange Bias in Mn-Doped Metal- Organic Framework [NH2(CH3)2][FeIIIFeII(HCOO)6]

Herein, an intriguing exchange bias (EB) effect manifesting itself from positive to negative with an increase in the cooling field (H_FC) is reported in the single crystal of Mn-doped metal-organic framework (MOF) [NH₂(CH₃)₂][Fe^IIIFe^II(HCOO)₆] (1) by finely tuning the exchange interactions between the magnetic ions. Note that the doping ratio of Mn relative to the total metal ions is about 15%. Negative magnetization and EB below the compensation temperature were both observed in 1, and the EB field (H_E) changes its sign from positive to negative when H_FC is larger than ∼10 kOe. The abnormal H_FC dependence of EB can be interpreted explicitly by a combination of negative magnetization and couplings among the ions of Fe³⁺, Fe²⁺, and Mn²⁺ with varying the H_FC. This work demonstrates a tunable EB in MOFs, in favor of designing novel magnetic devices.

Electric Control of Exchange Bias Effect in FePS3-Fe5GeTe2 van der Waals Heterostructures

Manipulating the exchange bias (EB) effect using an electronic gate is a significant goal in spintronics. The emergence of van der Waals (vdW) magnetic heterostructures has provided improved means to study interlayer magnetic coupling, but to date, these heterostructures have not exhibited electrical gate-controlled EB effects. Here, we report electrically controllable EB effects in a vdW heterostructure, FePS₃-Fe₅GeTe₂. By applying a solid protonic gate, the EB effects were repeatably electrically tuned. The EB field reaches up to 23% of the coercivity and the blocking temperature ranges from 30 to 60 K under various gate-voltages. The proton intercalations not only tune the average magnetic exchange coupling but also change the antiferromagnetic configurations in the FePS₃ layer. These result in a dramatic modulation of the total interface exchange coupling and the resultant EB effects. The study is a significant step toward vdW heterostructure-based magnetic logic for future low-energy electronics.

Electric Control of Exchange Bias at Room Temperature by Resistive Switching via Electrochemical Metallization

Electric field control of exchange bias (EB) plays an important role in spintronics due to its attractive merit of lower energy consumption. Here, we propose a novel method for electrically tunable EB at room temperature in a device with the stack of Si/SiO2/Ta/Pt/Ag/Mn-doped ZnO (MZO)/Pt/FeMn/Co/ITO by resistive switching (RS) via electrochemical metallization (ECM). The device shows enhanced and weakened EB when set at high-resistance state (HRS) and low-resistance state (LRS), respectively. For the device at LRS, the aberration-corrected scanning transmission electron microscopy (STEM) characterizations unambiguously reveal that the Ag filaments grow initially from the Ag anode and then elongate toward the ITO cathode. It is inferred that at LRS, a small portion of Ag filaments have passed through the MZO and the intervening thin Pt layer and extended into the FeMn layer. After applying reverse voltage, these Ag filaments are electrochemically dissolved and ruptured near the MZO/Pt interface. This is considered to be the main mechanism responsible for RS and switchable EB as well. This work presents a new strategy for designing low-power, nonvolatile magnetoelectric random access memory devices.

Long decay length of magnon-polarons in BiFeO3/La0.67Sr0.33MnO3 heterostructures

Magnons can transfer information in metals and insulators without Joule heating, and therefore are promising for low-power computation. The on-chip magnonics however suffers from high losses due to limited magnon decay length. In metallic thin films, it is typically on the tens of micrometre length scale. Here, we demonstrate an ultra-long magnon decay length of up to one millimetre in multiferroic/ferromagnetic BiFeO₃(BFO)/La_0.67Sr_0.33MnO₃(LSMO) heterostructures at room temperature. This decay length is attributed to a magnon-phonon hybridization and is more than two orders of magnitude longer than that of bare metallic LSMO. The long-distance modes have high group velocities of 2.5 km s^-1 as detected by time-resolved Brillouin light scattering. Numerical simulations suggest that magnetoelastic coupling via the BFO/LSMO interface hybridizes phonons in BFO with magnons in LSMO to form magnon-polarons. Our results provide a solution to the long-standing issue on magnon decay lengths in metallic magnets and advance the bourgeoning field of hybrid magnonics.

Progress and Perspectives on Aurivillius-Type Layered Ferroelectric Oxides in Binary Bi4Ti3O12-BiFeO3 System for Multifunctional Applications

Driven by potentially photo-electro-magnetic functionality, Bi-containing Aurivillius-type oxides of binary Bi4Ti3O12-BiFeO3 system with a general formula of Bin+1Fen−3Ti3O3n+3, typically in a naturally layered perovskite-related structure, have attracted increasing research interest, especially in the last twenty years. Benefiting from highly structural tolerance and simultaneous electric dipole and magnetic ordering at room temperature, these Aurivillius-phase oxides as potentially single-phase and room-temperature multiferroic materials can accommodate many different cations and exhibit a rich spectrum of properties. In this review, firstly, we discussed the characteristics of Aurivillius-phase layered structure and recent progress in the field of synthesis of such materials with various architectures. Secondly, we summarized recent strategies to improve ferroelectric and magnetic properties, consisting of chemical modification, interface engineering, oxyhalide derivatives and morphology controlling. Thirdly, we highlighted some research hotspots on magnetoelectric effect, catalytic activity, microwave absorption, and photovoltaic effect for promising applications. Finally, we provided an updated overview on the understanding and also highlighting of the existing issues that hinder further development of the multifunctional Bin+1Fen−3Ti3O3n+3 materials.

Multiferroic heterostructures for spintronics

For next-generation technology, magnetic systems are of interest due to the natural ability to store information and, through spin transport, propagate this information for logic functions. Controlling the magnetization state through currents has proven energy inefficient. Multiferroic thin-film heterostructures, combining ferroelectric and ferromagnetic orders, hold promise for energy efficient electronics. The electric field control of magnetic order is expected to reduce energy dissipation by 2–3 orders of magnitude relative to the current state-of-the-art. The coupling between electrical and magnetic orders in multiferroic and magnetoelectric thin-film heterostructures relies on interfacial coupling though magnetic exchange or mechanical strain and the correlation between domains in adjacent functional ferroic layers. We review the recent developments in electrical control of magnetism through artificial magnetoelectric heterostructures, domain imprint, emergent physics and device paradigms for magnetoelectric logic, neuromorphic devices, and hybrid magnetoelectric/spin-current-based applications. Finally, we conclude with a discussion of experiments that probe the crucial dynamics of the magnetoelectric switching and optical tuning of ferroelectric states towards all-optical control of magnetoelectric switching events.

Modulating Magnetism in Ferroelectric Polymer-Gated Perovskite Manganite Films with Moderate Gate Pulse Chains

Most previous attempts on achieving electric-field manipulation of ferromagnetism in complex oxides, such as La_0.66Sr_0.33MnO₃ (LSMO), are based on electrostatically induced charge carrier changes through high-k dielectrics or ferroelectrics. Here, the use of a ferroelectric copolymer, polyvinylidene fluoride with trifluoroethylene [P(VDF-TrFE)], as a gate dielectric to successfully modulate the ferromagnetism of the LSMO thin film in a field-effect device geometry is demonstrated. Specifically, through the application of low-voltage pulse chains inadequate to switch the electric dipoles of the copolymer, enhanced tunability of the oxide magnetic response is obtained, compared to that induced by ferroelectric polarization. Such observations have been attributed to electric field-induced oxygen vacancy accumulation/depletion in the LSMO layer upon the application of pulse chains, which is supported by surface-sensitive-characterization techniques, including X-ray photoelectron spectroscopy and X-ray magnetic circular dichroism. These techniques not only unveil the electrochemical nature of the mechanism but also establish a direct correlation between the oxygen vacancies created and subsequent changes to the valence states of Mn ions in LSMO. These demonstrations based on the pulsing strategy can be a viable route equally applicable to other functional oxides for the construction of electric field-controlled magnetic devices.

Ferroelectric Switching of Pure Spin Polarization in Two-Dimensional Electron Gas

Two-dimensional electron gas (2DEG) created at compound interfaces can exhibit a broad range of exotic physical phenomena, including quantum Hall phase, emergent ferromagnetism, and superconductivity. Although electron spin plays key roles in these phenomena, the fundamental understanding and application prospects of such emergent interfacial states have been largely impeded by the lack of purely spin-polarized 2DEG. In this work, by first-principles calculations of the multiferroic superlattice GeTe/MnTe, we find the ferroelectric polarization of GeTe is concurrent with the half-metallic 2DEG at interfaces. Remarkably, the pure spin polarization of the 2DEG can be created and annihilated by polarizing and depolarizing the ferroelectrics and can be switched (between pure spin-up and pure spin-down) by flipping the ferroelectric polarization. Given the electric-field amplification effect of ferroelectric electronics, we envision multiferroic superlattices could open up new opportunities for low-power, high-efficiency spintronic devices such as spin field-effect transistors.

Tuning ferroelectricity and ferromagnetism in BiFeO3/BiMnO3 superlattices

Multiferroic materials with multifunctional characteristics play a critical role in the field of microelectronics. In a perovskite oxide, ferroelectric polarization and ferromagnetism usually cannot coexist in a single-phase material at the same time. In this work, we design a superlattice structure composed of alternating BiFeO₃ and BiMnO₃ layers and illustrate how tuning the supercell size of epitaxial BiFeO₃/BiMnO₃ superlattices facilitates ferroelectric polarization while maintaining relatively strong ferromagnetism. A comprehensive investigation reveals that the enhanced ferroelectric polarization of BiMnO₃ layers originates from the induction effect induced by a strong polarization field generated by the adjacent ferroelectric BiFeO₃ layers. For the magnetic behavior, we consider the existence of interfacial antiferromagnetic superexchange interaction of Fe-O-Mn between BiFeO₃ and BiMnO₃ layers in our superlattices. This modulation effect of artificial superlattices provides a platform to accurately control the multiple order parameters in a multiferroic oxide system.

Ferroelectric Control of Magnetism in Ultrathin HfO2\Co\Pt Layers

The recent demonstration of ferroelectricity in ultrathin HfO₂ has kickstarted a new wave of research into this material. HfO₂ in the orthorhombic phase can be considered the first and only truly nanoscale ferroelectric material that is compatible with silicon-based nanoelectronics applications. In this article, we demonstrate the ferroelectric control of the magnetic properties of cobalt deposited on ultrathin aluminum-doped, atomic layer deposition-grown HfO₂ (t_HfO2 = 6.5 nm). The ferroelectric effect is shown to control the shape of the magnetic hysteresis, quantified here by the magnetic switching energy. Furthermore, the magnetic properties such as the remanence are modulated by up to 41%. We show that this modulation does not only correlate with the charge accumulation at the interface but also shows an additional component associated with the ferroelectric polarization switching. An in-depth analysis using first order reversal curves shows that the coercive and interaction field distributions of cobalt can be modulated up to, respectively, 5.8% and 10.5% with the ferroelectric polarization reversal.

Voltage-Control of Magnetism in All-Solid-State and Solid/Liquid Magnetoelectric Composites

The control of magnetism by means of low-power electric fields, rather than dissipative flowing currents, has the potential to revolutionize conventional methods of data storage and processing, sensing, and actuation. A promising strategy relies on the utilization of magnetoelectric composites to finely tune the interplay between electric and magnetic degrees of freedom at the interface of two functional materials. Albeit early works predominantly focused on the magnetoelectric coupling at solid/solid interfaces; however, recently there has been an increased interest related to the opportunities offered by liquid-gating techniques. Here, a comparative overview on voltage control of magnetism in all-solid-state and solid/liquid composites is presented within the context of the principal coupling mediators, i.e., strain, charge carrier doping, and ionic intercalation. Further, an exhaustive and critical discussion is carried out, concerning the suitability of using the common definition of coupling coefficient α C = Δ M Δ E   to compare the strength of the interaction between electricity and magnetism among different magnetoelectric systems.

Tailoring Magnetoelectric Coupling in BiFeO3 /La0.7 Sr0.3 MnO3 Heterostructure through the Interface Engineering

Electric field control of magnetism ultimately opens up the possibility of reducing energy consumption of memory and logic devices. Electric control of magnetization and exchange bias are demonstrated in all-oxide heterostructures of BiFeO₃ (BFO) and La_0.7 Sr_0.3 MnO₃ (LSMO). However, the role of the polar heterointerface on magnetoelectric (ME) coupling is not fully explored. Here, the ME coupling in BFO/LSMO heterostructures with two types of interfaces, achieved by exploiting the interface engineering at the atomic scale, is investigated. It is shown that both magnetization and exchange bias are reversibly controlled by switching the ferroelectric polarization of BFO. Intriguingly, distinctly different modulation behaviors that depend on the interfacial atomic sequence are observed. These results provide new insights into the underlying physics of ME coupling in the model system. This study highlights that designing interface at the atomic scale is of general importance for functional spintronic devices.

Electric Field Control of Phase Transition and Tunable Resistive Switching in SrFeO2.5

SrFeO _x (SFO _x) compounds exhibit ionic conduction and oxygen-related phase transformation, having potential applications in solid oxide fuel cells, smart windows, and memristive devices. The phase transformation in SFO _x typically requires a thermal annealing process under various pressure conditions, hindering their practical applications. Here, we have achieved a reversible phase transition from brownmillerite (BM) to perovskite (PV) in SrFeO_2.5 (SFO_2.5) films through ionic liquid (IL) gating. The real-time phase transformation is imaged using in situ high-resolution transmission electron microscopy. The magnetic transition in SFO_2.5 is identified by fabricating an assisted La_0.7Sr_0.3MnO₃ (LSMO) bottom layer. The IL-gating-converted PV phase of a SrFeO_3-δ (SFO_3-δ) layer shows a ferromagnetic-like behavior but applies a huge pinning effect on LSMO magnetic moments, which consequently leads to a prominent exchange bias phenomenon, suggesting an uncompensated helical magnetic structure of SFO_3-δ. On the other hand, the suppression of both magnetic and exchange coupling signals for a BM-phased SFO_2.5 layer elucidates its fully compensated G-type antiferromagnetic nature. We also demonstrated that the phase transition by IL gating is an effective pathway to tune the resistive switching parameters, such as set, reset, and high/low-resistance ratio in SFO_2.5-based resistive random-access memory devices.

Full Electric Control of Exchange Bias at Room Temperature by Resistive Switching

Electric control of exchange bias (EB) is of vital importance in energy-efficient spintronics. Although many attempts have been made during the past decade, each has its own limitations for operation and thus falls short of full direct and reversible electrical control of EB at room temperature. Here, a novel approach is proposed by virtue of unipolar resistive switching to accomplish this task in a Si/SiO₂ /Pt/Co/NiO/Pt device. By applying certain voltages, the device displays obvious EB in the high-resistance-state while negligible EB in the low-resistance state. Conductive filaments forming in the NiO layer and rupturing near the Co-NiO interface are considered to play dominant roles in determining the combined resistive switching and EB phenomena. This work paves a new way for designing multifunctional and nonvolatile magnetoelectric random access memory devices.

Engineering magnetism at functional oxides interfaces: manganites and beyond

The family of transition metal oxides (TMOs) is a large class of magnetic materials that has been intensively studied due to the rich physics involved as well as the promising potential applications in next generation electronic devices. In TMOs, the spin, charge, orbital and lattice are strongly coupled, and significant advances have been achieved to engineer the magnetism by different routes that manipulate these degrees of freedom. The family of manganites is a model system of strongly correlated magnetic TMOs. In this review, using manganites thin films and the heterostructures in conjunction with other TMOs as model systems, we review the recent progress of engineering magnetism in TMOs. We first discuss the role of the lattice that includes the epitaxial strain and the interface structural coupling. Then we look into the role of charge, focusing on the interface charge modulation. Having demonstrated the static effects, we continue to review the research on dynamical control of magnetism by electric field. Next, we review recent advances in heterostructures comprised of high T _c cuprate superconductors and manganites. Following that, we discuss the emergent magnetic phenomena at interfaces between 3d TMOs and 5d TMOs with strong spin-orbit coupling. Finally, we provide our outlook for prospective future directions.

Reversible Control of Interfacial Magnetism through Ionic-Liquid-Assisted Polarization Switching

The ability to control magnetism of materials via electric field enables a myriad of technological innovations in information storage, sensing, and computing. We use ionic-liquid-assisted ferroelectric switching to demonstrate reversible modulation of interfacial magnetism in a multiferroic heterostructure composed of ferromagnetic (FM) La_0.8Sr_0.2MnO₃ and ferroelectric (FE) PbZr_0.2Ti_0.8O₃. It is shown that ionic liquids can be used to persistently and reversibly switch a large area of a FE film. This is a prerequisite for polarized neutron reflectometry (PNR) studies that are conducted to directly probe magnetoelectric coupling of the FE polarization to the interfacial magnetization.

Four-state memory based on a giant and non-volatile converse magnetoelectric effect in FeAl/PIN-PMN-PT structure

We report a stable, tunable and non-volatile converse magnetoelectric effect (ME) in a new type of FeAl/PIN-PMN-PT heterostructure at room temperature, with a giant electrical modulation of magnetization for which the maximum relative magnetization change (ΔM/M) is up to 66%. The 109° ferroelastic domain switching in the PIN-PMN-PT and coupling with the ferromagnetic (FM) film via uniaxial anisotropy originating from the PIN-PMN-PT (011) surface are the key roles in converse ME effect. We also propose here a new, four-state memory through which it is possible to modify the remanent magnetism state by adjusting the electric field. This work represents a helpful approach to securing electric-writing magnetic-reading with low energy consumption for future high-density information storage applications.

Emerging ferroelectric transistors with nanoscale channel materials: the possibilities, the limitations

Combining the nonvolatile, locally switchable polarization field of a ferroelectric thin film with a nanoscale electronic material in a field effect transistor structure offers the opportunity to examine and control a rich variety of mesoscopic phenomena and interface coupling. It is also possible to introduce new phases and functionalities into these hybrid systems through rational design. This paper reviews two rapidly progressing branches in the field of ferroelectric transistors, which employ two distinct classes of nanoscale electronic materials as the conducting channel, the two-dimensional (2D) electron gas graphene and the strongly correlated transition metal oxide thin films. The topics covered include the basic device physics, novel phenomena emerging in the hybrid systems, critical mechanisms that control the magnitude and stability of the field effect modulation and the mobility of the channel material, potential device applications, and the performance limitations of these devices due to the complex interface interactions and challenges in achieving controlled materials properties. Possible future directions for this field are also outlined, including local ferroelectric gate control via nanoscale domain patterning and incorporating other emergent materials in this device concept, such as the simple binary ferroelectrics, layered 2D transition metal dichalcogenides, and the 4d and 5d heavy metal compounds with strong spin-orbit coupling.

Antiferromagnetic spintronics

Antiferromagnetic materials are internally magnetic, but the direction of their ordered microscopic moments alternates between individual atomic sites. The resulting zero net magnetic moment makes magnetism in antiferromagnets externally invisible. This implies that information stored in antiferromagnetic moments would be invisible to common magnetic probes, insensitive to disturbing magnetic fields, and the antiferromagnetic element would not magnetically affect its neighbours, regardless of how densely the elements are arranged in the device. The intrinsic high frequencies of antiferromagnetic dynamics represent another property that makes antiferromagnets distinct from ferromagnets. Among the outstanding questions is how to manipulate and detect the magnetic state of an antiferromagnet efficiently. In this Review we focus on recent works that have addressed this question. The field of antiferromagnetic spintronics can also be viewed from the general perspectives of spin transport, magnetic textures and dynamics, and materials research. We briefly mention this broader context, together with an outlook of future research and applications of antiferromagnetic spintronics.

Magnetization and Resistance Switchings Induced by Electric Field in Epitaxial Mn:ZnO/BiFeO3 Multiferroic Heterostructures at Room Temperature

Electric field induced reversible switchings of the magnetization and resistance were achieved at room temperature in epitaxial Mn:ZnO(110)/BiFeO3(001) heterostructures. The observed modulation of magnetic moment is ∼500% accompanying with a coercive field varying from 43 to 300 Oe and a resistive switching ratio up to ∼10(4)% with the applied voltages of ±4 V. The switching mechanisms in magnetization and resistance are attributed to the ferroelectric polarization reversal of the BiFeO3 layer under applied electric fields, combined with the reversible change of oxygen vacancy concentration at the Mn:ZnO/BiFeO3 interface.

Anomalous Electronic Anisotropy Triggered by Ferroelastic Coupling in Multiferroic Heterostructures

The ferroelastic strain coupling in multiferroic heterostructures is explored aiming at novel physical effects and fascinating functionality. Ferroelastic domain walls in manganites induced by a stripe BiFeO3 template can modulate the electronic transfer and sufficiently block the magnetic ordering, creating a vast anisotropy. The findings suggest the great importance of ferroelastic strain engineering in material modifications.

Low energy consumption spintronics using multiferroic heterostructures

We review the recent progress in the field of multiferroic magnetoelectric heterostructures. The lack of single phase multiferroic candidates exhibiting simultaneously strong and coupled magnetic and ferroelectric orders led to an increased effort into the development of artificial multiferroic heterostructures in which these orders are combined by assembling different materials. The magnetoelectric coupling emerging from the created interface between the ferroelectric and ferromagnetic layers can result in electrically tunable magnetic transition temperature, magnetic anisotropy or magnetization reversal. The full potential of low energy consumption magnetic based devices for spintronics lies in our understanding of the magnetoelectric coupling at the scale of the ferroic domains. Although the thin film synthesis progresses resulted into the complete control of ferroic domain ordering using epitaxial strain, the local observation of magnetoelectric coupling remains challenging. The ability to imprint ferroelectric domains into ferromagnets and to manipulate those solely using electric fields suggests new technological advances for spintronics such as magnetoelectric memories or memristors.

Magnetoelectric Coupling in Well-Ordered Epitaxial BiFeO3/CoFe2O4/SrRuO3 Heterostructured Nanodot Array

Multiferroic magnetoelectric (ME) composites exhibit sizable ME coupling at room temperature, promising applications in a wide range of novel devices. For high density integrated devices, it is indispensable to achieve a well-ordered nanostructured array with reasonable ME coupling. For this purpose, we explored the well-ordered array of isolated epitaxial BiFeO3/CoFe2O4/SrRuO3 heterostructured nanodots fabricated by nanoporous anodic alumina (AAO) template method. The arrayed heterostructured nanodots demonstrate well-established epitaxial structures and coexistence of piezoelectric and ferromagnetic properties, as revealed by transmission electron microscopy (TEM) and peizoeresponse/magnetic force microscopy (PFM/MFM). It was found that the heterostructured nanodots yield apparent ME coupling, likely due to the effective transfer of interface couplings along with the substantial release of substrate clamping. A noticeable change in piezoelectric response of the nanodots can be triggered by magnetic field, indicating a substantial enhancement of ME coupling. Moreover, an electric field induced magnetization switching in these nanodots can be observed, showing a large reverse ME effect. These results offer good opportunities of the nanodots for applications in high-density ME devices, e.g., high density recording (>100 Gbit/in.(2)) or logic devices.

Angular Dependence of Exchange Bias and Magnetization Reversal Controlled by Electric-Field-Induced Competing Anisotropies

The combination of exchange-biased systems and ferroelectric materials offers a simple and effective way to investigate the angular dependence of exchange bias using one sample with electric-field-induced competing anisotropies. A reversible electric-field-controlled magnetization reversal at zero magnetic field is also realized through optimizing the anisotropy configuration, holding promising applications for ultralow power magnetoelectric devices.

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.

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.

Interfacial magnetism in complex oxide heterostructures probed by neutrons and x-rays

Magnetic complex-oxide heterostructures are of keen interest because a wealth of phenomena at the interface of dissimilar materials can give rise to fundamentally new physics and potentially valuable functionalities. Altered magnetization, novel magnetic coupling and emergent interfacial magnetism at the epitaxial layered-oxide interfaces are under intensive investigation, which shapes our understanding on how to utilize those materials, particularly for spintronics. Neutron and x-ray based techniques have played a decisive role in characterizing interfacial magnetic structures and clarifying the underlying physics in this rapidly developing field. Here we review some recent experimental results, with an emphasis on those studied via polarized neutron reflectometery and polarized x-ray absorption spectroscopy. We conclude with some perspectives.

Crystal-Orientation-Modulated Exchange Bias in Orthorhombic-YMnO3/La0.6Sr0.4MnO3 Multiferroic Heterostructures

The magnetic properties of the all-oxide multiferroic heterostructures composed of orthorhombic YMnO3 (YMO) with E-type antiferromagnetic and double-exchange ferromagnetic (FM) La0.6Sr0.4MnO3 (LSMO) were studied. An orientation-modulated exchange bias effect, which is related to the interfacial Mn-O-Mn bond angle, was discovered. Because of the large bond angle in YMO/LSMO(100) heterostructures, a strong exchange coupling at the interface is formed. This strong exchange coupling sustains an FM phase in YMO at the interface region. The FM phase with strong magnetocrystalline anisotropy contributes to the vertical shift and exchange bias effect in (100) orientation heterostructures. When LSMO (110) and (111) were layered with YMO, the Mn-O-Mn bond angle was reduced, leading to a weakened exchange coupling at the interface, and only a relatively small exchange bias at low temperatures was visible.

Electrical control of the exchange spring in antiferromagnetic metals

Electrical control of the exchange spring in antiferromagnetic metals is obtained in [Co/Pt]/IrMn Hall devices by using an ionic liquid, where the exchange spring could transfer the "force" and enable a deeper modulation depth in the IrMn. This work provides a new approach toward electrical modulation of the spin structures in metallic antiferromagnets, which should be significant in advancing the development of low-power-consumption antiferromagnet (AFM) spintronics.

Superior Properties of Energetically Stable La(2/3)Sr(1/3)MnO(3)/Tetragonal BiFeO3 Multiferroic Superlattices

The superlattice of energetically stable La2/3Sr1/3MnO3 and tetragonal BiFeO3 is investigated by means of density functional theory. The superlattice as a whole exhibits a half-metallic character, as is desired for spintronic devices. The interfacial electronic states and exchange coupling are analyzed in details. We demonstrate that the interfacial O atoms play a key role in controlling the coupling. The higher ferroelectricity of tetragonal BiFeO3 and stronger response to the magnetic moments in the La2/3Sr1/3MnO3/BiFeO3 superlattice show a strongly enhanced electric control of the magnetism as compared to the rhombohedral one. Therefore, it is particularly practical interest in the magnetoelectrically controlled spintronic devices.

Electric field control of magnetic properties and magneto-transport in composite multiferroics

We study magnetic state and electron transport properties of composite multiferroic system consisting of a granular ferromagnetic thin film placed above the ferroelectric substrate. Ferroelectricity and magnetism in this case are coupled by the long-range Coulomb interaction. We show that magnetic state and magneto-transport strongly depend on temperature, external electric field and electric polarization of the substrate. Ferromagnetic order exists at finite temperature range around ferroelectric Curie point. Outside the region the film is in the superparamagnetic state. We demonstrate that magnetic phase transition can be driven by an electric field and magneto-resistance effect has two maxima associated with two magnetic phase transitions appearing in the vicinity of the ferroelectric phase transition. We show that positions of these maxima can be shifted by the external electric field and that the magnitude of the magneto-resistance effect depends on the mutual orientation of external electric field and polarization of the substrate.

Intrinsic interfacial phenomena in manganite heterostructures

We review recent advances in our understanding of interfacial phenomena that emerge when dissimilar materials are brought together at atomically sharp and coherent interfaces. In particular, we focus on phenomena that are intrinsic to the interface and review recent work carried out on perovskite manganites interfaces, a class of complex oxides whose rich electronic properties have proven to be a useful playground for the discovery and prediction of novel phenomena.

Synthetic magnetoelectric coupling in a nanocomposite multiferroic

Given the paucity of single phase multiferroic materials (with large ferromagnetic moment), composite systems seem an attractive solution to realize magnetoelectric coupling between ferromagnetic and ferroelectric order parameters. Despite having antiferromagnetic order, BiFeO₃ (BFO) has nevertheless been a key material due to excellent ferroelectric properties at room temperature. We studied a superlattice composed of 8 repetitions of 6 unit cells of La_0.7Sr_0.3MnO₃ (LSMO) grown on 5 unit cells of BFO. Significant net uncompensated magnetization in BFO, an insulating superlattice, is demonstrated using polarized neutron reflectometry. Remarkably, the magnetization enables magnetic field to change the dielectric properties of the superlattice, which we cite as an example of synthetic magnetoelectric coupling. Importantly, controlled creation of magnetic moment in BFO is a much needed path toward design and implementation of integrated oxide devices for next generation magnetoelectric data storage platforms.

Intriguing photo-control of exchange bias in BiFeO3/La2/3Sr1/3MnO3 thin films on SrTiO3 substrates

To date, electric fields have been widely used to control the magnetic properties of BiFeO3-based antiferromagnet/ferromagnet heterostructures through application of an exchange bias. To extend the applicability of exchange bias, however, an alternative mechanism to electric fields is required. Here, we report the photo-control of exchange bias in BiFeO3/La2/3Sr1/3MnO3 thin films on an SrTiO3 substrate. Through an ex situ pulsed laser deposition technique, we successfully synthesized epitaxial BiFeO3/La2/3Sr1/3MnO3 thin films on SrTiO3 substrates. By measuring magnetoresistance under light illumination, we investigated the effect of light illumination on resistance, exchange bias, and coercive field in BiFeO3/La2/3Sr1/3MnO3 thin films. After illumination of red and blue lights, the exchange bias was sharply reduced compared to that measured in the dark. With increasing light intensity, the exchange bias under red and blue lights initially decreased to zero and then appeared again. It is possible to reasonably explain these behaviors by considering photo-injection from SrTiO3 and the photo-conductivity of La2/3Sr1/3MnO3. This study may provide a fundamental understanding of the mechanism underlying photo-controlled exchange bias, which is significant for the development of new functional spintronic devices.

Strain-mediated electric-field control of exchange bias in a Co90Fe10/BiFeO3/SrRuO3/PMN-PT heterostructure

The electric-field (E-field) controlled exchange bias (EB) in a Co₉₀Fe₁₀/BiFeO₃ (BFO)/SrRuO₃/PMN-PT heterostructure has been investigated under different tensile strain states. The in-plane tensile strain of the BFO film is changed from +0.52% to +0.43% as a result of external E-field applied to the PMN-PT substrate. An obvious change of EB by the control of non-volatile strain has been observed. A magnetization reversal driven by E-field has been observed in the absence of magnetic field. Our results indicate that a reversible non-volatile E-field control of a ferromagnetic layer through strain modulated multiferroic BFO could be achieved at room temperature.

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.

Purely electric-field-driven perpendicular magnetization reversal

If achieved, magnetization reversal purely with an electric field has the potential to revolutionize the spintronic devices that currently utilize power-dissipating currents. However, all existing proposals involve the use of a magnetic field. Here we use phase-field simulations to study the piezoelectric and magnetoelectric responses in a three-dimensional multiferroic nanostructure consisting of a perpendicularly magnetized nanomagnet with an in-plane long axis and a juxtaposed ferroelectric nanoisland. For the first time, we demonstrate a full reversal of perpendicular magnetization via successive precession and damping, driven purely by a perpendicular electric-field pulse of certain pulse duration across the nanoferroelectric. We discuss the materials selection and size dependence of both nanoferroelctrics and nanomagnets for experimental verification. These results offer new inspiration to the design of spintronic devices that simultaneously possess high density, high thermal stability, and high reliability.

BiFeO3 epitaxial thin films and devices: past, present and future

The celebrated renaissance of the multiferroics family over the past ten years has also been that of its most paradigmatic member, bismuth ferrite (BiFeO3). Known since the 1960s to be a high temperature antiferromagnet and since the 1970s to be ferroelectric, BiFeO3 only had its bulk ferroic properties clarified in the mid-2000s. It is however the fabrication of BiFeO3 thin films and their integration into epitaxial oxide heterostructures that have fully revealed its extraordinarily broad palette of functionalities. Here we review the first decade of research on BiFeO3 films, restricting ourselves to epitaxial structures. We discuss how thickness and epitaxial strain influence not only the unit cell parameters, but also the crystal structure, illustrated for instance by the discovery of the so-called T-like phase of BiFeO3. We then present its ferroelectric and piezoelectric properties and their evolution near morphotropic phase boundaries. Magnetic properties and their modification by thickness and strain effects, as well as optical parameters, are covered. Finally, we highlight various types of devices based on BiFeO3 in electronics, spintronics, and optics, and provide perspectives for the development of further multifunctional devices for information technology and energy harvesting.

Magnetoelectric assisted 180° magnetization switching for electric field addressable writing in magnetoresistive random-access memory

Magnetization-based memories, e.g., hard drive and magnetoresistive random-access memory (MRAM), use bistable magnetic domains in patterned nanomagnets for information recording. Electric field (E) tunable magnetic anisotropy can lower the energy barrier between two distinct magnetic states, promising reduced power consumption and increased recording density. However, integration of magnetoelectric heterostructure into MRAM is a highly challenging task owing to the particular architecture requirements of each component. Here, we show an epitaxial growth of self-assembled CoFe2O4 nanostripes with bistable in-plane magnetizations on Pb(Mg,Nb)O3-PbTiO3 (PMN-PT) substrates, where the magnetic switching can be triggered by E-induced elastic strain effect. An unprecedented magnetic coercive field change of up to 600 Oe was observed with increasing E. A near 180° magnetization rotation can be activated by E in the vicinity of the magnetic coercive field. These findings might help to solve the 1/2-selection problem in traditional MRAM by providing reduced magnetic coercive field in E field selected memory cells.

Electric field for tuning quantum entanglement in supported clusters

We show that quantum entanglement, nowadays so widely observed and used in a multitude of systems, can be traced in the atomic spins of metal clusters supported on metal surfaces. Most importantly, we show that it can be voluntarily altered with external electric fields. We use a combination of ab initio and model Heisenberg-Dirac-Van Vleck quantum spin Hamiltonian calculations to show, with the example of a prototype system (Mn dimers on Ag(0 0 1) surface), that, in an inherently unentangled system an electric field can 'switch on' the entanglement and significantly change its critical temperature parameter. The physical mechanism allowing such rigorous control of entanglement by an electric field is the field-induced change in the internal magnetic coupling of the supported nanostructure.

Effect of strain on voltage-controlled magnetism in BiFeO₃-based heterostructures

Voltage-modulated magnetism in magnetic/BiFeO₃ heterostructures can be driven by a combination of the intrinsic ferroelectric-antiferromagnetic coupling in BiFeO₃ and the antiferromagnetic-ferromagnetic exchange interaction across the heterointerface. However, ferroelectric BiFeO₃ film is also ferroelastic, thus it is possible to generate voltage-induced strain in BiFeO₃ that could be applied onto the magnetic layer across the heterointerface and modulate magnetism through magnetoelastic coupling. Here, we investigated, using phase-field simulations, the role of strain in voltage-controlled magnetism for these BiFeO₃-based heterostructures. It is predicted, under certain condition, coexistence of strain and exchange interaction will result in a pure voltage-driven 180° magnetization reversal in BiFeO₃-based heterostructures.

Interface magnetism in epitaxial BiFeO3-La0.7Sr0.3MnO3 heterostructures integrated on Si(100)

We report on the heteroepitaxial growth of ferroelectric (FE)-antiferromagnetic (AFM) BiFeO3 (BFO) on ferromagnetic La0.7Sr0.3MnO3 (LSMO), integrated on Si(100) using pulsed laser deposition via the domain matching epitaxy paradigm. The BFO/LSMO films were epitaxially grown on Si(100) by introducing epitaxial layers of SrTiO3/MgO/TiN. X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, X-ray photo absorption spectroscopy, and atomic force microscopy were employed to fully characterize the samples. Furthermore, we have investigated the magnetic behavior of this five layer heterostructure, in which a d(5) system (Fe(3+)) manifested in FE-AFM BFO is epitaxially conjoined at the interface to a multivalent transition metal ion such as Mn(3+)/Mn(4+) in LSMO. The temperature- and magnetic field-dependent magnetization measurements reveal an unexpected enhancement in magnetic moment and improved magnetic hysteresis squareness originating from the BFO/LSMO interface. We observe a stronger temperature dependence of HEB when the polarity of field cooling is negative as compared to positive field cooling. We believe such an enhancement in magnetic moment and magnetic coupling is likely directly related to an electronic orbital reconstruction at the interface and complex interplay between orbital and spin degrees of freedom, similar to what has previously been reported in the literature. Future work will involve the linearly polarized X-ray absorption measurements to prove this hypothesis. This work represents a starting step toward the realization of magneto-electronic devices integrated with Si(100).

Electric control of exchange bias training

Voltage-controlled exchange bias training and tunability are introduced. Isothermal voltage pulses are used to reverse the antiferromagnetic order parameter of magnetoelectric Cr(2)O(3), and thus continuously tune the exchange bias of an adjacent CoPd film. Voltage-controlled exchange bias training is initialized by tuning the antiferromagnetic interface into a nonequilibrium state incommensurate with the underlying bulk. Interpretation of these hitherto unreported effects contributes to new understanding in electrically controlled magnetism.