Electric Field‐Induced Oxidation of Ferromagnetic/Ferroelectric Interfaces

Composite multiferroics are a new class of material where magneto‐electric coupling is achieved by creating an interface between a ferromagnetic and a ferroelectric compound. The challenge of understanding the chemical and magnetic properties of such interface is a key to achieve good magneto‐electric coupling. The unique possibilities offered by isotope sensitive techniques are used to selectively investigate the interface's chemistry and magnetism in Fe/BaTiO₃ and Fe/LiNbO₃ systems during the application of an electric field. With a large enough electric field, a strong oxidation of Fe is triggered, which creates a magnetically dead interface. This leads to an irreversible decrease of the magneto‐electric coupling properties. Material parameters are identified that determine under which electric field the interface may be modified. The results are confirmed on the two systems and are expected to be widespread in this new class of hybrid material.     

Domain Wall Displacement is the Origin of Superior Permittivity and Piezoelectricity in BaTiO3 at Intermediate Grain Sizes

The dielectric and piezoelectric properties of ferroelectric polycrystalline materials have long been known to be strong functions of grain size and extrinsic effects such as domain wall motion. In BaTiO₃, for example, it has been observed for several decades that the piezoelectric and dielectric properties are maximized at intermediate grain sizes (≈1 μm) and different theoretical models have been introduced to describe the physical origin of this effect. Here, using in situ, high‐energy X‐ray diffraction during application of electric fields, it is shown that 90° domain wall motion during both strong (above coercive) and weak (below coercive) electric fields is greatest at these intermediate grain sizes, correlating with the enhanced permittivity and piezoelectric properties observed in BaTiO₃. This result validates the long‐standing theory in attributing the size effects in polycrystalline BaTiO₃ to domain wall displacement. It is now empirically established that a doubling or more in the piezoelectric and dielectric properties of polycrystalline ferroelectric materials can be achieved through domain wall displacement effects; such mechanisms are suggested for use in the design of new ferroelectric materials with enhanced properties.     

Multiferroic Polymer Laminate Composites Exhibiting High Magnetoelectric Response Induced by Hydrogen‐Bonding Interactions

The coupling of the magnetic, electric, and elastic properties in multiferroics creates new collective phenomena and enables next‐generation device paradigms. In this work, the hydrogen bonding interaction between hydrate salts and ferroelectric polymers is exploited in the development of high‐performance magnetoelectric (ME) polymer laminate composites. The microstructures and crystallite structures of the Al(NO₃)₃·9H₂O doped poly(vinylidene fluoride‐co‐hexafluoropropylene), P(VDF‐HFP), are carefully studied. The effect of hydrogen bonding interaction on the polarization ordering of the ferroelectric polymers is investigated by 2D wide‐angle X‐ray diffraction, polarized Fourier transform infrared spectra, and dielectric spectra at varied frequencies and temperatures. It is found that hydrogen bond not only promotes the formation of the polar crystallite phase but also improves the polarization ordering in the ferroelectric polymer, which subsequently increases the remnant polarization of the polymers as verified in the polarization‐electric field loop measurements. These entail marked improvement in the ME voltage coefficients (α_ME) of the resulting polymer laminate composites based on ferromagnetic Metglas relative to analogous composites. The composite exhibits a state‐of‐the‐art α_ME value of 20 V cm^‐1 Oe under a dc magnetic field of ≈4 Oe and a colossal α_ME of 320 V cm^‐1 Oe at a frequency of 68 kHz.     

Ferroelectric and Ferromagnetic Behavior of Pb(Zr0.52Ti0.48)O3–Co0.9Zn0.1Fe2O4 Multilayered Thin Films Prepared via Solution Processing

Multilayered multiferroic nanocomposite films of Pb(Zr_0.52Ti_0.48)O₃ (PZT) and Co_0.9Zn_0.1Fe₂O₄ (CZFO) are prepared on general Pt/Ti/SiO₂/Si substrates via a simple solution‐processing method. Structural characterization by X‐ray diffraction and electron microscopy techniques reveals good surface and cross‐sectional morphologies of these multilayered thin films. In particular, at room temperature strong ferroelectric and ferromagnetic responses are simultaneously observed in the multilayered thin films, depending on the deposited sequences and volume fractions of ferroelectric PZT phase and magnetic CZFO phase.     

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。     

The Polar/Antipolar Phase Boundary of BiMnO3–BiFeO3–PbTiO3: Interplay among Crystal Structure,Point Defects,and Multiferroism

The ferromagnetic perovskite oxide BiMnO₃ is a highly topical material, and the solid solutions it forms with antiferromagnetic/ferroelectric BiFeO₃ and with ferroelectric PbTiO₃ result in distinctive polar/nonpolar morphotropic phase boundaries (MPBs). The exploitation of such a type of MPBs could be a novel approach to engineer novel multiferroics with phase‐change magnetoelectric responses, in addition to ferroelectrics with enhanced electromechanical performance. Here, the interplay among crystal structure, point defects, and multiferroic properties of the BiMnO₃–BiFeO₃–PbTiO₃ ternary system at its line of MPBs between polymorphs of tetragonal P4mm (polar) and orthorhombic Pnma (antipolar) symmetries is reported. A strong dependence of the phase coexistence on thermal history is found: phase percentage significantly changes whether the material is quenched or slowly cooled from high temperature. The origin of this phenomenon is investigated with temperature‐dependent structural and physical property characterizations. A major role of the complex defect chemistry, where a Bi/Pb‐deficiency allows Mn and Fe ions to have a mixed‐valence state, in the delicate balance between polymorphs is proposed, and its influence in the magnetic and electric ferroic orders is defined.     

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.     

Symmetry Modulation and Enhanced Multiferroic Characteristics in Bi1‐xNdxFeO3 Ceramics

BiFeO₃ is recognized as the most important room temperature single phase multiferroic material. However, the weak magnetoelectric (ME) coupling remains as a key issue, which obstructs its applications. Since the magnetoelectric coupling in BiFeO₃ is essentially hindered by the cycloidal spin structure, here efforts to improve the magnetoelectric coupling by destroying the cycloidal state and switching to the weak ferromagnetic state through symmetry modulation are reported. The structure is tuned from polar R3c to polar Pna2₁, and finally to nonpolar Pbnm by forming Bi_1‐xNd_xFeO₃ solid solutions, where two morphotropic phase boundaries (MPBs) are detected. Greatly enhanced ferroelectric polarization is obtained together with the desired weak ferromagnetic characteristics in Bi_1‐xNd_xFeO₃ ceramics at the compositions near MPBs. The change of magnetic state from antiferromagnetic (cycloidal state) to ferromagnetic (canted antiferromagnetic) is confirmed by the observation of magnetic domains using magnetic force microscopy. More interestingly, combining experiments and first‐principles‐based simulations, an electric field‐induced structural and magnetic transition from Pna2₁ back to R3c is demonstrated, providing a great opportunity for electric field‐controlled magnetism, and this transition is shown to be reversible with additional thermal treatment.     

Inside Front Cover: Perfect Bi4Ti3O12 Single‐Crystal Films via Flux‐Mediated Epitaxy (Adv. Funct. Mater. 4/2006)

Flux‐mediated epitaxy has been developed for ferroelectric Bi₄Ti₃O₁₂ single‐crystal film growth, as shown on the inside cover. The key point is the selection of an appropriate flux material. A combinatorial high‐throughput screening technique reported by Matsumoto and co‐workers on p. 485 has led to the successful discovery of the novel flux composition, Bi–Cu–O, for Bi₄Ti₃O₁₂ single‐crystal film growth. This flux‐mediated epitaxy is not limited to oxide epitaxy, but is also widely applicable to various promising materials for the realization of non‐Si‐based electronics, such as nitrides, carbides, and halides. Excellent crystallinity of material films and atomic control of their surface/interface, sufficient for the realization of their optimal physical properties, are technological premises for modern functional‐device applications. Bi₄Ti₃O₁₂ and related compounds attract much interest as highly insulating, ferroelectric materials for use in ferroelectric random‐access memories. However, it has been difficult thus far for Bi₄Ti₃O₁₂ films to satisfy such requirements when formed using vapor‐phase epitaxy, owing to the high volatility of Bi in a vacuum. Here, we demonstrate that flux‐mediated epitaxy is one of the most promising and widely applicable concepts to overcome this inevitable problem. The key point of this process is the appropriate selection of a multi‐component flux system. A combinatorial approach has led to the successful discovery of the novel flux composition of Bi–Cu–O for Bi₄Ti₃O₁₂ single‐crystal film growth. The perfect single‐crystal nature of the stoichiometric Bi₄Ti₃O₁₂ film formed has been verified through its giant grain size and electric properties, equivalent to those of bulk single crystals. This demonstration has broad implications, opening up the possibility of preparing stoichiometric single‐crystal oxide films via vapor‐phase epitaxy, even if volatile constituents are required.     

Chemical Bonding Assembly of Multifunctional Oxide Nanocomposites

The synthesis, functionalization and assembly of metal oxide nanoparticles BaTiO₃ and CoFe₂O₄ is presented. The ferroelectric (BaTiO₃) and ferromagnetic (CoFe₂O₄) oxide nanoparticle surfaces are directly functionalized via the anchoring of phosphonic acid and aminosilane molecules that engender the nanoparticles with terminal carboxylic acid and amine functional groups, respectively. These promote the electrostatic self‐assembly of the particles in non‐polar solvents and permit the synthesis of more chemically robust assemblies linked by the covalent amide bond via the addition of the chemical coupling agent N‐N′‐dicyclohexylcarbodiimide. This functionalization and assembly procedure is applied to two systems: the first comprised of 50 nm BaTiO₃ and 10 nm CoFe₂O₄ particles and the second of 200 nm BaTiO₃ and 12.5 nm CoFe₂O₄ particles. The latter composites possess magnetoelectric properties when processed into dense ceramics and, as a direct result of the assembly performed in solution, have a high degree of homogeneity between the ferroelectric and ferromagnetic phases. The developed functionalization and assembly procedure is considered to be adaptable to the preparation of other hybrid oxide nanomaterials with different property combinations.     

Defect‐Driven Structural Distortions at the Surface of Relaxor Ferroelectrics

Relaxor‐ferroelectric materials find application in a broad range of technological devices, including ultrasonic imaging transducers, nanopositioning, and high‐performance capacitors. They generally exhibit occupationally disordered structures creating local polar fluctuations that are highly sensitive to applied electric or stress fields. The sensitivity of the material structure to external field and stress conditions also makes them likely to develop skin or surface phases that are unique from the bulk. Surface layers can adjust the material response and also lead to ambiguity in structural characterization. Here, using a combination of X‐ray diffraction methods, it is shown that a ≈20 µm skin structure commonly exists in the lead‐free relaxor‐ferroelectric ceramic (Na_1/2Bi_1/2)TiO₃–BaTiO₃. Using experiments and density functional theory calculations, it is shown that the combined action of oxygen vacancies providing internal chemical pressure and the surface plane stress state dictates the stability and structure of the skin layer. This work can be extended to all perovskite relaxor ferroelectrics and provides new insights into the origin of skin layers in these materials. The opportunity exists to further enhance the functionality of these materials through engineering of surface structures using the methods outlined here.     

Perfect Bi4Ti3O12 Single‐Crystal Films via Flux‐Mediated Epitaxy

Excellent crystallinity of material films and atomic control of their surface/interface, sufficient for the realization of their optimal physical properties, are technological premises for modern functional‐device applications. Bi₄Ti₃O₁₂ and related compounds attract much interest as highly insulating, ferroelectric materials for use in ferroelectric random‐access memories. However, it has been difficult thus far for Bi₄Ti₃O₁₂ films to satisfy such requirements when formed using vapor‐phase epitaxy, owing to the high volatility of Bi in a vacuum. Here, we demonstrate that flux‐mediated epitaxy is one of the most promising and widely applicable concepts to overcome this inevitable problem. The key point of this process is the appropriate selection of a multi‐component flux system. A combinatorial approach has led to the successful discovery of the novel flux composition of Bi–Cu–O for Bi₄Ti₃O₁₂ single‐crystal film growth. The perfect single‐crystal nature of the stoichiometric Bi₄Ti₃O₁₂ film formed has been verified through its giant grain size and electric properties, equivalent to those of bulk single crystals. This demonstration has broad implications, opening up the possibility of preparing stoichiometric single‐crystal oxide films via vapor‐phase epitaxy, even if volatile constituents are required.     

Constructing Polymorphic Nanodomains in BaTiO3 Films via Epitaxial Symmetry Engineering

Ferroelectric materials owning a polymorphic nanodomain structure usually exhibit colossal susceptibilities to external mechanical, electrical, and thermal stimuli, thus holding huge potential for relevant applications. Despite the success of traditional strategies by means of complex composition design, alternative simple methods such as strain engineering have been intensively sought to achieve a polymorphic nanodomain state in lead‐free, simple‐composition ferroelectric oxides in recent years. Here, a nanodomain configuration with morphed structural phases is realized in an epitaxial BaTiO₃ film grown on a (111)‐oriented SrTiO₃ substrate. Using a combination of experimental and theoretical approaches, it is revealed that a threefold rotational symmetry element enforced by the epitaxial constraint along the [111] direction of BaTiO₃ introduces considerable instability among intrinsic tetragonal, orthorhombic, and rhombohedral phases. Such phase degeneracy induces ultrafine ferroelectric nanodomains (1–10 nm) with low‐angle domain walls, which exhibit significantly enhanced dielectric and piezoelectric responses compared to the (001)‐oriented BaTiO₃ film with uniaxial ferroelectricity. Therefore, the finding highlights the important role of epitaxial symmetry in domain engineering of oxide ferroelectrics and facilitates the development of dielectric capacitors and piezoelectric devices.     

Long‐Range Nonvolatile Electric Field Effect in Epitaxial Fe/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 Heterostructures

One of the ideal candidates of using electric field to manipulate magnetism is the recently developed multiferroics with emergent coupling of magnetism and electricity, particularly in synthesizing artificial nanoscale ferroelectric and ferromagnetic materials. Here, a long‐range nonvolatile electric field effect is investigated in Fe/Pb(Mg_1/3Nb_2/3)_0.7Ti_0.3O₃ heterostructure using the dependence of the magnon‐driven magnetoelectric coupling on the epitaxial Fe thin film (4–30 nm) thickness at room temperature using measurements based on the ferromagnetic resonance. The magnon‐driven magnetoelectric coupling tuning of the ferromagnetic resonance field shows a linear response to the electric field, with a resonance field shift that occurs under both positive and negative remanent polarizations, and demonstrates nonvolatile behavior. Moreover, the spin diffusion length of the epitaxial Fe thin film of ≈9 nm is obtained from the results that the change of the cubic magnetocrystalline anisotropy field under different electric fields varies with Fe thickness. These results are promising for the design of future multiferroic devices.     

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.     

Room‐Temperature Nonvolatile Memory Based on a Single‐Phase Multiferroic Hexaferrite

The cross‐coupling between electric polarization and magnetization in multiferroic materials provides a great potential for creating next‐generation memory devices. Current studies on magnetoelectric (ME) applications mainly focus on ferromagnetic/ferroelectric heterostructures because single‐phase multiferroics with strong magnetoelectric coupling at room temperature are still very rare. Here a type of nonvolatile memory device is presented solely based on a single‐phase multiferroic hexaferrite Sr₃Co₂Fe₂₄O₄₁ which exhibits nonlinear magnetoelectric effects at room temperature. The principle is to store binary information by employing the states (magnitude and sign) of the first‐order and the second‐order magnetoelectric coefficients (α and β), instead of using magnetization, electric polarization, and resistance. The experiments demonstrate repeatable nonvolatile switch of α and β by applying pulsed electric fields at room temperature, respectively. Such kind of memory device using single‐phase multiferroics paves a pathway toward practical applications of spin‐driven multiferroics.     

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 Effects of Multiphase Formation on Strain Relaxation and Magnetization in Multiferroic BiFeO3 Thin Films

Multiferroic epitaxial Bi‐Fe‐O thin films of different thicknesses (15–500 nm) were grown on SrTiO₃ (001) substrates by pulsed laser deposition under various oxygen partial pressures to investigate the microstructural evolution in the Bi‐Fe‐O system and its effect on misfit strain relaxation and on the magnetic properties of the films. Films grown at low oxygen partial pressure show the canted antiferromagnetic phase α‐Fe₂O₃ embedded in a matrix of BiFeO₃. The ferromagnetic phase, γ‐Fe₂O₃ is found to precipitate inside the α‐Fe₂O₃ grains. The formation of these phases changes the magnetic properties of the films and the misfit strain relaxation mechanism. The multiphase films exhibit both highly strained and fully relaxed BiFeO₃ regions in the same film. The magnetization in the multiphase Bi‐Fe‐O films is controlled by the presence of the γ‐Fe₂O₃ phase rather than heteroepitaxial strain as it is the case in pure single phase BiFeO₃. Also, our results show that this unique accommodation of misfit strain by the formation of α‐Fe₂O₃ gives rise to significant enhancement of the piezo electric properties of BiFeO_3.     

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.     

Epitaxial TiOx Surface in Ferroelectric BaTiO3: Native Structure and Dynamic Patterning at the Atomic Scale

Surfaces and interfaces of ferroelectric oxides exhibit enhanced functionality, and therefore serve as a platform for novel nano and quantum technologies. Experimental and theoretical challenges associated with examining the subtle electro‐chemo‐mechanical balance at metal‐oxide surfaces have hindered the understanding and control of their structure and behavior. Here, combined are advanced electron‐microscopy and first‐principles thermodynamics methods to reveal the atomic‐scale chemical and crystallographic structure of the surface of the seminal ferroelectric BaTiO₃. It is shown that the surface is composed of a native <2 nm thick TiO_x rock‐salt layer in epitaxial registry with the BaTiO₃. Using electron‐beam irradiation, artificial TiO_x sites with sub‐nanometer resolution are successfully patterned, by inducing Ba escape. Therefore, this work offers electro‐chemo‐mechanical insights into ferroelectric surface behavior in addition to a method for scalable high‐resolution beam‐induced chemical lithography for selectively driving surface phase transitions, and thereby functionalizing metal‐oxide surfaces.