E‐Field Control of Exchange Bias and Deterministic Magnetization Switching in AFM/FM/FE Multiferroic Heterostructures

The coexistence of electrical polarization and magnetization in multiferroic materials provides great opportunities for novel information storage systems. In particular, magnetoelectric (ME) effect can be realized in multi­ferroic composites consisting of both ferromagnetic and ferroelectric phases through a strain mediated interaction, which offers the possibility of electric field (E‐field) manipulation of magnetic properties or vice versa, and enables novel multiferroic devices such as magnetoelectric random access memories (MERAMs). These MERAMs combine the advantages of FeRAMs (ferroelectric random access memories) and MRAMs (magnetic random access memories), which are non‐volatile magnetic bits switchable by electric field (E‐field). However, it has been challenging to realize 180° deterministic switching of magnetization by E‐field, on which most magnetic memories are based. Here we show E‐field modulating exchange bias and for the first time realization of near 180° dynamic magnetization switching at room temperature in novel AFM (antiferromagnetic)/FM (ferromagnetic)/FE (ferroelectric) multiferroic heterostructures of FeMn/Ni₈₀Fe₂₀/FeGaB/PZN‐PT (lead zinc niobate–lead titanate). Through competition between the E‐field induced uniaxial anisotropy and unidirectional anisotropy, large E‐field‐induced exchange bias field‐shift up to $ {{{\Delta H_{ex}}}\over{{H_{ex}}}} = 218\%$ and near 180° deterministic magnetization switching were demonstrated in the exchange‐coupled multiferroic system of FeMn/Ni₈₀Fe₂₀/FeGaB/PZN‐PT. This E‐field tunable exchange bias and near 180° deterministic magnetization switching at room temperature in AFM/FM/FE multiferroic heterostructures paves a new way for MERAMs and other memory technologies.     

The Ferroelectric Field Effect within an Integrated Core/Shell Nanowire

The synthesis of cylindrical silicon‐core and ferroelectric oxide perovskite‐shell nanowires and their response characteristics as individual three‐terminal nanoscale electronic devices is reported. The co‐axial nanowire geometry facilitates large ferroelectric field‐effect modulation (>10⁴) of nanowire conductivity following sequential application and removal of an applied dc field. Source‐drain current–voltage traces collected during sweeps of ferroelectric gate potential and switching of the component of shell outward and inward polarization provide direct evidence of ferroelectric coupling on nanowire channel conductance. Despite a very small (1:20) ferroelectric‐to‐semiconductor channel thickness ratio, an unexpectedly strong electrostatic coupling of ferroelectric polarization to channel conductance is observed because of the co‐axial gate geometry and curvature‐induced strain enhancement of ferroelectric polarization.     

Ferroelectric,Photoelectric, and Photovoltaic Performance of Silver Niobate Ceramics

Ferroelectrics coupled with solar energy conversion are receiving intensive research interest. However, most ferroelectrics with a large remnant polarization can only harvest ultraviolet light in the solar spectrum. Herein, high‐quality silver niobate (AgNbO₃) ceramics prepared by spark plasma sintering is reported with a bandgap of ≈2.75 eV and a long tail absorption until 800 nm, leading to outstanding photoelectric properties featured by the visible‐light response over 550 nm. Instantaneous photoresponse measurement using a 355 nm nanosecond pulse laser shows a fast response speed in nanoseconds. Moreover, the ceramic exhibits an intriguing photovoltaic effect under either electric poling or mechanical polishing. Both approaches have switchable characteristics and produce a stable photovoltage as well as photocurrent, while temperature dependence behavior reveals distinctions between ferroelectric polarization and ferroelastic strains in determining the photovoltaic properties. Piezoelectric force microscopy characterization further confirms distinctions between the underlying mechanisms. The electric poling induced photovoltaic effect stems from the aligned polarization involving the ferroelectric component, whereas the mechanical polishing induced photovoltaic effect is associated with the flexoelectricity induced by strain gradients. These results not only show AgNbO₃ to be a promising material for photoelectric application but also deepen the understanding of the mechanism underlying ferroelectric photovoltaics.     

A Strain‐Mediated Magnetoelectric‐Spin‐Torque Hybrid Structure

Magnetization dynamics induced by spin–orbit torques in a heavy‐metal/ferromagnet can potentially be used to design low‐power spintronics and logic devices. Recent computations have suggested that a strain‐mediated spin–orbit torque (SOT) switching in magnetoelectric (ME) heterostructures is fast, energy‐efficient, and permits a deterministic 180° magnetization switching. However, its experimental realization has remained elusive. Here, the coexistence of the strain‐mediated ME coupling and the SOT in a CoFeB/Pt/ferroelectric hybrid structure is shown experimentally. The voltage‐induced strain only slightly modifies the efficiency of SOT generation, but it gives rise to an effective magnetic anisotropy and rotates the magnetic easy axis which eliminates the incubation delay in current‐induced magnetization switching. The phase field simulations show that the electric‐field‐induced effective magnetic anisotropy field can reduce the switching time approximately by a factor of three for SOT in‐plane magnetization switching. It is anticipated that such strain‐mediated ME‐SOT hybrid structures may enable field‐free, ultrafast magnetization switching.     

Interfacial Strain Gradients Control Nanoscale Domain Morphology in Epitaxial BiFeO3 Multiferroic Films

Domain switching pathways fundamentally control performance in ferroelectric thin film devices. In epitaxial bismuth ferrite (BiFeO₃) films, the domain morphology is known to influence the multiferroic orders. While both striped and mosaic domains have been observed, the origins of the latter have remained unclear. Here, it is shown that domain morphology is defined by the strain profile across the film–substrate interface. In samples with mosaic domains, X‐ray diffraction analysis reveals strong strain gradients, while geometric phase analysis using scanning transmission electron microscopy finds that within 5 nm of the film–substrate interface, the out‐of‐plane strain shows an anomalous dip while the in‐plane strain is constant. Conversely, if uniform strain is maintained across the interface with zero strain gradient, striped domains are formed. Critically, an ex situ thermal treatment, which eliminates the interfacial strain gradient, converts the domains from mosaic to striped. The antiferromagnetic state of the BiFeO₃ is also influenced by the domain structure, whereby the mosaic domains disrupt the long‐range spin cycloid. This work demonstrates that atomic scale tuning of interfacial strain gradients is a powerful route to manipulate the global multiferroic orders in epitaxial films.     

Mechanical Switching of Nanoscale Multiferroic Phase Boundaries

Tuning the lattice degree of freedom in nanoscale functional crystals is critical to exploit the emerging functionalities such as piezoelectricity, shape‐memory effect, or piezomagnetism, which are attributed to the intrinsic lattice‐polar or lattice‐spin coupling. Here it is reported that a mechanical probe can be a dynamic tool to switch the ferroic orders at the nanoscale multiferroic phase boundaries in BiFeO₃ with a phase mixture, where the material can be reversibly transformed between the “soft” tetragonal‐like and the “hard” rhombohedral‐like structures. The microscopic origin of the nonvolatile mechanical switching of the multiferroic phase boundaries, coupled with a reversible 180° rotation of the in‐plane ferroelectric polarization, is the nanoscale pressure‐induced elastic deformation and reconstruction of the spontaneous strain gradient across the multiferroic phase boundaries. The reversible control of the room‐temperature multiple ferroic orders using a pure mechanical stimulus may bring us a new pathway to achieve the potential energy conversion and sensing applications.     

Relaxor/Ferroelectric Composites: A Solution in the Quest for Practically Viable Lead‐Free Incipient Piezoceramics

Recently developed lead‐free incipient piezoceramics are promising candidates for off‐resonance actuator applications with their exceptionally large electromechanical strains. Their commercialization currently faces two major challenges: high electric field required for activating the large strains and large strain hysteresis. It is demonstrated that design of a relaxor/ferroelectric composite provides a highly effective way to resolve both challenges. Experimental results in conjunction with numerical simulations provide key parameters for the development of viable incipient piezoceramics.     

Ultrafast Nanoscale Polymer Coating on Porous 3D Structures Using Microwave Irradiation

Thin polymer coatings are very popular, but the coatings on uneven surfaces or porous 3D structures are difficult to obtain with traditional methods. The pores are easily clogged due to nonuniform polymer curing processes caused by inevitable macroscale temperature gradients from their hotter outer to colder inner sides. Here an ultrafast and simple fabrication method is developed to obtain nanoscale coating layers on the inner and outer surfaces of a porous 3D sponge‐like carbon nanotube (CNT). Microwave irradiation rapidly and selectively heats the CNT immersed in a mixture solution of an uncured polymer and a diluent solvent, solidifying the polymer only adjacent to the CNT with five repeated 3 s microwave irradiation. The coating layers can be controlled by adjusting the concentration of the uncured polymer in the solution and controlling the CNT temperature via microwave power and irradiation time. The nanoscale coating strongly ties the junction between CNTs without filling the pores with the polymer, resulting in excellent resilience to compressive stress with large strains (≈180 kPa at 60%), which is maintained throughout repeated 8000 cycles of 0–60% strain. The unfilled pores allow for maintaining the low thermal conductivity, ≈26 mW m^?1 K^?1, and the electrical resistance is varied with strain. This facile selective polymer curing methodology can be utilized in processing various materials with uneven surfaces or pores.     

Ultrahigh Tunability of Room Temperature Electronic Transport and Ferromagnetism in Dilute Magnetic Semiconductor and PMN‐PT Single‐Crystal‐Based Field Effect Transistors via Electric Charge Mediation

Multiferroic heterostructures composed of complex oxide thin films and ferroelectric single crystals have aroused considerable interest due to the electrically switchable strain and charge elements of oxide films by the polarization reversal of ferroelectrics. Previous studies have demonstrated that the electric‐field‐control of physical properties of such heterostructures is exclusively due to the ferroelectric domain switching‐induced lattice strain effects. Here, the first successful integration of the hexagonal ZnO:Mn dilute magnetic semiconductor thin films with high performance (111)‐oriented perovskite Pb(Mg_1/3Nb_2/3)O₃‐PbTiO₃ (PMN‐PT) single crystals is reported, and unprecedented charge‐mediated electric‐field control of both electronic transport and ferromagnetism at room temperature for PMN‐PT single crystal‐based oxide heterostructures is realized. A significant carrier concentration‐tunability of resistance and magnetization by ≈400% and ≈257% is achieved at room temperature. The electric‐field controlled bistable resistance and ferromagnetism switching at room temperature via interfacial electric charge presents a potential strategy for designing prototype devices for information storage. The results also disclose that the relative importance of the strain effect and interfacial charge effect in oxide film/ferroelectric crystal heterostructures can be tuned by appropriately adjusting the charge carrier density of oxide films.     

Nonvolatile Electrical Control and Heterointerface‐Induced Half‐Metallicity of 2D Ferromagnets

Electrical control of atom‐thick van der Waals (vdW) ferromagnets is a key toward future magnetoelectric nanodevices; however, state‐of‐the‐art control approaches are volatile. In this work, introducing ferroelectric switching as an aided layer is demonstrated to be an effective approach toward achieving nonvolatile electrical control of 2D ferromagnets. For example, when a ferromagnetic monolayer CrI₃ and ferroelectric MXene Sc₂CO₂ come together into multiferroic heterostructures, CrI₃ is controlled by polarized states P↑ and P↓ of Sc₂CO₂. P↑ Sc₂CO₂ does not change the semiconducting nature of CrI₃, but surprisingly P↓ Sc₂CO₂ makes CrI₃ half‐metallic. Nonvolatility of the electrical switching between two oppositely ferroelectric polarized states, therefore, indirectly enables nonvolatile electrical control of CrI₃ between ferromagnetic semiconductor and half‐metal. The heterointerface‐induced half‐metallicity in CrI₃ is intrinsic without resorting to any chemical functionalization or external physical modification, which is rather beneficial to the practical application. This work paves the way for nonvolatile electrical control of 2D vdW ferromagnets and applications of CrI₃ in half‐metal‐based nanospintronics.     

Nanoscale Insight Into Lead‐Free BNT‐BT‐xKNN

Piezoresponse force microscopy (PFM) is used to afford insight into the nanoscale electromechanical behavior of lead‐free piezoceramics. Materials based on Bi_1/2Na_1/2TiO₃ exhibit high strains mediated by a field‐induced phase transition. Using the band excitation technique the initial domain morphology, the poling behavior, the switching behavior, and the time‐dependent phase stability in the pseudo‐ternary system (1–x)(0.94Bi_1/2Na_1/2TiO₃‐0.06BaTiO₃)‐xK_0.5Na_0.5NbO₃ (0 <= x <= 18 mol%) are revealed. In the base material (x = 0 mol%), macroscopic domains and ferroelectric switching can be induced from the initial relaxor state with sufficiently high electric field, yielding large macroscopic remanent strain and polarization. The addition of KNN increases the threshold field required to induce long range order and decreases the stability thereof. For x = 3 mol% the field‐induced domains relax completely, which is also reflected in zero macroscopic remanence. Eventually, no long range order can be induced for x >= 3 mol%. This PFM study provides a novel perspective on the interplay between macroscopic and nanoscopic material properties in bulk lead‐free piezoceramics.     

铁电/铁磁复合材料的磁性能和介电性能研究

铁电/铁磁复合材料是一类在材料内部同时共存铁电相和铁磁相的重要功能材料。在外加磁场和电场的作用下,其磁化和极化状态易于调控。笔者以铁电性的钛酸钡和铁磁性镍铜锌铁氧体纳米粉为原料,采用固相反应法,合成了一系列铁电/铁磁复合材料。研究表明:在一定温度下烧结所得的铁电/铁磁复合材料,由铁电相和铁磁相两相所组成,对外表现出铁电性和铁磁性。该复合材料同时具有电感和电容两种特性,而且频率稳定性好,有望替代现有的分立式无源滤波器,做到真正的集成而广泛应用在集成电路中。     

Observation of a Negative Thermal Hysteresis in Relaxor Ferroelectric Polymers

Hysteresis phenomena, including both electrical and thermal types, are essential to ferroelectric materials. The former, known as polarization‐electric field hysteresis, has been intensively studied in a wide range of ferroelectric materials. However, relevant experimental evidence on thermal hysteresis remains limited, especially in ferroelectric polymers, even though thermal hysteresis is crucial to the caloric effect, which is usually the largest near the phase transition. Here, the thermal hysteresis behavior in ferroelectric polymers is studied in terms of temperature‐dependent polarization upon heating and cooling. In contrast to common belief, a negative thermal hysteresis is observed in relaxor ferroelectric polymers, which is probably due to local stabilization of ferroelectric distortion induced by electric field. Using the polymer blend as a platform, it is further shown that the negative thermal hysteresis arises at the disappearance of long‐range ferroelectric distortion and the thermal hysteresis behavior may be effectively controlled through the blend approach. This study not only provides deeper insights into electrocaloric effect in ferroelectric polymers but also offers an approach to study the critical phenomenon in a ferroelectric system.     

Manipulation of Magnetic Properties by Oxygen Vacancies in Multiferroic YMnO3

Generating a single material with multiple ferroic properties has been the hotspot of research interest for decades. The existing studies mostly focus on the intrinsic properties of multiferroic materials, overlooking the importance of the widely distributed defects in the materials. Here, the strong influence of oxygen vacancies (V _O) on the magnetic properties of YMnO₃ is demonstrated. The first‐principles calculations reveal that the V _O at axial positions can induce a nonzero net magnetization along the c‐axis. By structural characterization and magnetic measurement, this theoretically predicted ferromagnetic property is experimentally confirmed in the YMnO₃ film grown on a c‐Al₂O₃ substrate. The large in‐plane compressive strain provided by the Al₂O₃ substrate allows to create the axial V _O of YMnO₃ film in this system. The ferroelectricity of YMnO₃ is also preserved even under large in‐plane compressive strain. Therefore, the coexistence of the ferroelectric and ferromagnetic properties can be realized in the YMnO₃ film, which is of practical interest for technological applications.     

Interesting Evidence for Template‐Induced Ferroelectric Behavior in Ultra‐Thin Titanium Dioxide Films Grown on (110) Neodymium Gallium Oxide Substrates

The first evidence for room‐temperature ferroelectric behavior in anatase‐phase titanium dioxide (a‐TiO₂) is reported. Behavior strongly indicative of ferroelectric characteristics is induced in ultra‐thin (20 nm to 80 nm) biaxially‐strained epitaxial films of a‐TiO₂ deposited by liquid injection chemical vapor deposition onto (110) neodymium gallium oxide (NGO) substrates. The films exhibit significant orthorhombic strain, as analyzed by X‐ray diffraction and high‐resolution transmission electron microscopy. The films on NGO show a switchable dielectric spontaneous polarization when probed by piezoresponse force microscopy (PFM), the ability to retain polarization information written into the film using the PFM tip for extended periods (several hours) and at elevated temperatures (up to 100 °C) without significant loss, and the disappearance of the polarization at a temperature between 180 and 200 °C, indicative of a Curie temperature within this range. This combination of effects constitutes strong experimental evidence for ferroelectric behavior, which has not hitherto been reported in a‐TiO₂ and opens up the possibility for a range of new applications. A model is presented for the effects of large in‐plane strains on the crystal structure of anatase which provides a possible explanation for the experimental observations.     

Unleashing Strain Induced Ferroelectricity in Complex Oxide Thin Films via Precise Stoichiometry Control

Strain tuning has emerged as a powerful means to enhance properties and to induce otherwise unattainable phenomena in complex oxide films. However, by employing strain alone, the predicted properties sometimes fail to emerge. In this work, the critical role of precise stoichiometry control for realizing strain‐induced ferroelectricity in CaTiO₃ films is demonstrated. An adsorption controlled growth window is discovered for CaTiO₃ films grown by hybrid molecular beam epitaxy, which ensures an excellent control over the Ti:Ca atomic percent ratio of <0.8% in the films. Superior ferroelectric and dielectric properties are found for films grown inside the stoichiometric growth window, yielding maximum polarization, dielectric constant, and paraelectric‐to‐ferroelectric transition temperatures. Outside this growth window, properties are severely deteriorated and ultimately suppressed by defects in the films. This study exemplifies the important role of precise compositional control for achieving strain‐induced properties. Untangling the effects of strain and stoichiometry on functional properties will accelerate both fundamental discoveries yet to be made in the vast materials design space of strained complex oxide films, as well as utilization of strain‐stabilized phenomena in future devices.     

Spatially Resolved Electric‐Field Manipulation of Magnetism for CoFeB Mesoscopic Discs on Ferroelectrics

Electric‐field control of magnetism in ferromagnetic/ferroelectric multiferroic heterostructures is a promising way to realize fast and nonvolatile random‐access memory with high density and low‐power consumption. An important issue that has not been solved is the magnetic responses to different types of ferroelectric‐domain switching. Here, for the first time three types of magnetic responses are reported induced by different types of ferroelectric domain switching with in situ electric fields in the CoFeB mesoscopic discs grown on PMN‐PT(001), including type I and type II attributed to 109°, 71°/180° ferroelectric domain switching, respectively, and type III attributed to a combined behavior of multiferroelectric domain switching. Rotation of the magnetic easy axis by 90° induced by 109° ferroelectric domain switching is also found. In addition, the unique variations of effective magnetic anisotropy field with electric field are explained by the different ferroelectric domain switching paths. The spatially resolved study of electric‐field control of magnetism on the mesoscale not only enhances the understanding of the distinct magnetic responses to different ferroelectric domain switching and sheds light on the path of ferroelectric domain switching, but is also important for the realization of low‐power consumption and high‐speed magnetic random‐access memory utilizing these materials.     

Probing the Effects of Nanoscale Architecture on the Mechanical Properties of Hexagonal Silica/Polymer Composite Thin Films

We examine the effects of controlling nanoscale architecture on the tensile properties of honeycomb‐structured silica/polymer composite films. The hexagonal films are produced using evaporation‐induced self‐assembly and uniaxially strained using a home‐built tensile testing apparatus. Significant differences in the yield strain, failure strain, and tensile moduli between the axes parallel and perpendicular to the film‐deposition direction are observed for the thinnest films examined and are attributed to anisotropy in the film nanostructure that is further characterized with transmission electron microscopy and atomic force microscopy. For properly oriented composites, these films have tensile moduli comparable to the Young's modulus of bulk silica but exhibit failure strains that are about an order of magnitude larger than those seen in typical bulk‐silica systems. The yielding and failure processes are explored using X‐ray diffraction and optical microscopy and are characterized by irreversible changes in the nanoscale architecture. We show that tuning the nanoscale architecture can provide control over the tensile properties of composites, allowing for materials with combinations of stiffness and elasticity unachievable in the analogous bulk systems.     

Intrinsic Half‐Metallicity in 2D Ternary Chalcogenides with High Critical Temperature and Controllable Magnetization Direction

Searching for 2D ferromagnetic materials with a high critical temperature, large spin polarization, and controllable magnetization direction is a key challenge for their broad applications in spintronics. Here, through a systematic study on a series of 2D ternary chalcogenides with first‐principles calculations, it is demonstrated that a family of experimentally available 2D CoGa₂X₄ (X = S, Se, or Te) are half‐metallic ferromagnets, and they exhibit high critical temperature, fully polarized spin state, and strain‐dependent magnetization direction simultaneously. Following the Goodenough–Kanamori rules, the half‐metallic ferromagnetism of CoGa₂X₄ family is caused by superexchange interaction mediated by Co? X? Co bonds. The half‐metal gaps are large enough (>0.5 eV) to ensure that the half‐metallicity is stable against the spin flipping at room temperature. Magnetocrystalline anisotropy energy calculations indicate that CoGa₂X₄ favor easy plane magnetization. Under achievable biaxial tensile strain (2–6%), the magnetization directions of CoGa₂X₄ can change from in‐plane to out‐of‐plane, providing a route to control the efficiency of spin injection/detection. Further, the critical temperatures T_c of ferromagnetic phase transition for CoGa₂X₄ are close to room temperature. Belonging to the big family of layered AB₂X₄ compounds, the proposed CoGa₂X₄ systems will enrich the available 2D candidates and their heterojunctions for various applications.     

Giant Flexoelectric Polarization in a Micromachined Ferroelectric Diaphragm

The coupling between dielectric polarization and strain gradient, known as flexoelectricity, becomes significantly large on the micro‐ and nanoscale. Here, it is shown that giant flexoelectric polarization can reverse remnant ferroelectric polarization in a bent Pb(Zr_0.52Ti_0.48)O₃ (PZT) diaphragm fabricated by micromachining. The polarization induced by the strain gradient and the switching behaviors of the polarization in response to an external electric field are investigated by observing the electromechanical coupling of the diaphragm. The method allows determination of the absolute zero polarization state in a PZT film, which is impossible using other existing methods. Based on the observation of the absolute zero polarization state and the assumption that bending of the diaphragm is the only source of the self‐polarization, the upper bound of flexoelectric coefficient of PZT film is calculated to be as large as 2.0 × 10^?4 C m^?1. The strain gradient induced by bending the diaphragm is measured to be on the order of 10² m^?1, three orders of magnitude larger than that obtained in the bulk material. Because of this large strain gradient, the estimated giant flexoelectric polarization in the bent diaphragm is on the same order of magnitude as the normal remnant ferroelectric polarization of PZT film.