Comment on "Epitaxial BiFeO3 multiferroic thin film heterostructures"

Magnetoelectric coupling in multiferroics probed by optical second harmonic generation

Magnetoelectric coupling, as a fundamental physical nature and with the potential to add functionality to devices while also reducing energy consumption, has been challenging to be probed in freestanding membranes or two-dimensional materials due to their instability and fragility. In this paper, we report a magnetoelectric coupling probed by optical second harmonic generation with external magnetic field, and show the manipulation of the ferroelectric and antiferromagnetic orders by the magnetic and thermal fields in BiFeO₃ films epitaxially grown on the substrates and in the freestanding ones. Here we define an optical magnetoelectric-coupling constant, denoting the ability of controlling light-induced nonlinear polarization by the magnetic field, and found the magnetoelectric-coupling was suppressed by strain releasing but remain robust against thermal fluctuation for freestanding BiFeO₃.

Outstanding Ferroelectricity in Sol-Gel-Derived Polycrystalline BiFeO3 Films within a Wide Thickness Range

As a promising lead-free ferroelectric, BiFeO₃ has a very large intrinsic polarization of ∼100 μC/cm², enabling its great potential in electronic applications especially in a film format. In this sense, reliable ferroelectric properties are desired; however, pure-phase BiFeO₃ films are notorious for their large leakage current, especially of those processed by using the sol-gel method─a facile and industrially scalable method for film preparation. In this study, a protection layer, which can be easily integrated in the sol-gel process, is used to ensure the acquirement of remnant polarization of ∼65 μC/cm² in ∼200 nm BiFeO₃ thin films, whereas O₂ annealing can enhance that to ∼120 μC/cm² in ∼400-700 nm films. Reliable ferroelectricity of BiFeO₃ films on Si wafers within a wide thickness range was thus achieved. The obtained ferroelectricity is among the best-achieved properties to date of BiFeO₃ films for both thin and intermediate thicknesses, including both chemically and physically derived. These results are helpful to advance potential use of sol-gel-processed BiFeO₃ films in electromechanical devices with different desired thicknesses.

An antisite defect mechanism for room temperature ferroelectricity in orthoferrites

Single-phase multiferroic materials that allow the coexistence of ferroelectric and magnetic ordering above room temperature are highly desirable, motivating an ongoing search for mechanisms for unconventional ferroelectricity in magnetic oxides. Here, we report an antisite defect mechanism for room temperature ferroelectricity in epitaxial thin films of yttrium orthoferrite, YFeO₃, a perovskite-structured canted antiferromagnet. A combination of piezoresponse force microscopy, atomically resolved elemental mapping with aberration corrected scanning transmission electron microscopy and density functional theory calculations reveals that the presence of Y_Fe antisite defects facilitates a non-centrosymmetric distortion promoting ferroelectricity. This mechanism is predicted to work analogously for other rare earth orthoferrites, with a dependence of the polarization on the radius of the rare earth cation. Our work uncovers the distinctive role of antisite defects in providing a mechanism for ferroelectricity in a range of magnetic orthoferrites and further augments the functionality of this family of complex oxides for multiferroic applications.

Spinodal Decomposition-Driven Endurable Resistive Switching in Perovskite Oxides

Common pursuits of developing nanometric logic and neuromorphic applications have motivated intensive research studies into low-dimensional resistive random-access memory (RRAM) materials. However, fabricating resistive switching medium with inherent stability and homogeneity still remains a bottleneck. Herein, we report a self-assembled uniform biphasic system, comprising low-resistance 3 nm-wide (Bi_0.4,La_0.6)FeO_3-δ nanosheets coherently embedded in a high-resistance (Bi_0.2,La_0.8)FeO_3-δ matrix, which were spinodally decomposed from an overall stoichiometry of the (Bi_0.24,La_0.76)FeO_3-δ parent phase, as a promising nanocomposite to be a stable and endurable RRAM medium. The Bi-rich nanosheets accommodating high concentration of oxygen vacancies as corroborated by X-ray photoelectron spectroscopy and electron energy loss spectroscopy function as fast carrier channels, thus enabling an intrinsic electroforming-free character. Surficial electrical state and resistive switching properties are investigated using multimodal scanning probe microscopy techniques and macroscopic I-V measurements, showing high on/off ratio (∼10³) and good endurance (up to 1.6 × 10⁴ cycles). The established spinodal decomposition-driven phase-coexistence BLFO system demonstrates the merits of stability, uniformity, and endurability, which is promising for further application in RRAM devices.

The Experimentalist's Guide to the Cycloid, or Noncollinear Antiferromagnetism in Epitaxial BiFeO3

Bismuth ferrite (BiFeO₃ ) is one of the most widely studied multiferroics. The coexistence of ferroelectricity and antiferromagnetism in this compound has driven an intense search for electric-field control of the magnetic order. Such efforts require a complete understanding of the various exchange interactions that underpin the magnetic behavior. An important characteristic of BiFeO₃ is its noncollinear magnetic order; namely, a long-period incommensurate spin cycloid. Here, the progress in understanding this fascinating aspect of BiFeO₃ is reviewed, with a focus on epitaxial films. The advances made in developing the theory used to capture the complexities of the cycloid are first chronicled, followed by a description of the various experimental techniques employed to probe the magnetic order. To help the reader fully grasp the nuances associated with thin films, a detailed description of the spin cycloid in the bulk is provided. The effects of various perturbations on the cycloid are then described: magnetic and electric fields, doping, epitaxial strain, finite size effects, and temperature. To conclude, an outlook on possible device applications exploiting noncollinear magnetism in BiFeO₃ films is presented. It is hoped that this work will act as a comprehensive experimentalist's guide to the spin cycloid in BiFeO₃ thin films.

Piezoelectric Materials for Controlling Electro-Chemical Processes

Piezoelectric materials have been analyzed for over 100 years, due to their ability to convert mechanical vibrations into electric charge or electric fields into a mechanical strain for sensor, energy harvesting, and actuator applications. A more recent development is the coupling of piezoelectricity and electro-chemistry, termed piezo-electro-chemistry, whereby the piezoelectrically induced electric charge or voltage under a mechanical stress can influence electro-chemical reactions. There is growing interest in such coupled systems, with a corresponding growth in the number of associated publications and patents. This review focuses on recent development of the piezo-electro-chemical coupling multiple systems based on various piezoelectric materials. It provides an overview of the basic characteristics of piezoelectric materials and comparison of operating conditions and their overall electro-chemical performance. The reported piezo-electro-chemical mechanisms are examined in detail. Comparisons are made between the ranges of material morphologies employed, and typical operating conditions are discussed. In addition, potential future directions and applications for the development of piezo-electro-chemical hybrid systems are described. This review provides a comprehensive overview of recent studies on how piezoelectric materials and devices have been applied to control electro-chemical processes, with an aim to inspire and direct future efforts in this emerging research field.

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.

Lead palladium titanate: A room temperature nanoscale multiferroic thin film

The discovery of single-phase multiferroic materials and the understanding of coupling mechanisms between their spin and polarization is important from the point of view of next generation logic and memory devices. Herein we report the fabrication, dielectric, ferroelectric, piezo-response force microscopy, and magnetization measurements of Pd-substituted room-temperature magnetoelectric multiferroic PbPd_0.3Ti_0.7O₃ (PbPdT) thin films. Highly oriented PbPdT thin films were deposited on {(LaAlO₃)_0.3(Sr₂AlTaO₆)_0.7} (LSAT) substrates in oxygen atmosphere using pulsed laser deposition technique. X-ray diffraction studies revealed that the films had tetragonal phase with (001) orientation. Surface morphology studies using atomic force and scanning electron microscopy suggest a smooth and homogeneous distribution of grains on the film surface with roughness ~2 nm. A large dielectric constant of ~1700 and a low-loss tangent value of ~0.3 at 10 kHz were obtained at room temperature. Temperature dependent dielectric measurements carried out on Pt/PbPdT/La_0.7Sr_0.3MnO₃ (LSMO) metal-dielectric-metal capacitors suggest a ferroelectric to paraelectric transition above 670 K. The measured polarization hysteresis loops at room temperature were attributed to its ferroelectric behavior. From a Tauc plot of (αhν)² versus energy, the direct band gap E_g of PbPdT thin films was calculated as 3 eV. Ferroelectric piezoelectric nature of the films was confirmed from a strong domain switching response revealed from piezo-response force microscopy. A well-saturated magnetization M-H loop with remanent magnetization of 3.5 emu/cm³ was observed at room temperature, and it retains ferromagnetic ordering in the temperature range 5-395 K. Origin of the magnetization could be traced to the mixed oxidation states of Pd²⁺/Pd⁴⁺ dispersed in polar PbTiO₃ matrix, as revealed by our x-ray photoelectron spectroscopic results. These results suggest that PbPdT thin films are multiferroic (ferroelectric-ferromagnetic) at room temperature.

Tunable valley and spin splitting in 2H-VSe2/BiFeO3(111) triferroic heterostructures

The spin and valley degrees of freedom in monolayer transition metal dichalcogenides have potential applications in spintronics and valleytronics. However, nonvolatile control on the valley and spin degrees of freedom of two-dimensional ferromagnetic materials by multiferroic materials has been rarely reported. Here, the electronic structure of monolayer 2H-VSe2/BiFeO3(111) triferroic heterostructures has been investigated by first-principles calculations. It is found that the V magnetic moment, spin and valley splitting of monolayer VSe2 can be affected by the BiFeO3(111) substrate with ferroelectric polarization and G-type antiferromagnetic order. Particularly, the reversed orientation of ferroelectric polarization and magnetic order of the BiFeO3(111) substrate can modulate the magnitude of spin and valley splitting, and change the spin splitting direction and the spin-dependent valley state in the valence band of monolayer VSe2. The coupling among ferroelectrics, magnetism and ferrovalley is realized in 2H-VSe2/BiFeO3(111) triferroic heterostructures. These results provide a new platform for multiferroic regulation in spintronics and valleytronics, which can enrich the diversity for high-performance devices based on two dimensional multiferroic heterostructures.

Biocatalyst and Colorimetric/Fluorescent Dual Biosensors of H2O2 Constructed via Hemoglobin-Cu3(PO4)2 Organic/Inorganic Hybrid Nanoflowers

In this article, the three-dimensional hemoglobin (Hb)-Cu₃(PO₄)₂ organic/inorganic hybrid nanoflowers (Hb-Cu₃(PO₄)₂ HNFs) self-assembled by nanopetals were synthesized via a facile one-pot green synthetic method. The compositions and microstructure of the Hb-Cu₃(PO₄)₂ HNFs were well-characterized with X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and UV-vis spectrometry, respectively. The as-prepared Hb-Cu₃(PO₄)₂ HNFs were to be used as a biocatalyst to construct colorimetric/fluorescent dual biosensors. The experimental results show that the colorimetric/fluorescent dual biosensors exhibited two linear responses in the range of 2-10 ppb and 20-100 ppb for H₂O₂. The colorimetric and fluorescent detection limits were 0.1 and 0.01 ppb, respectively. Compared with the free Hb, the biocatalytic activity of the Hb-Cu₃(PO₄)₂ HNFs can be improved for 3-4 times under optimal conditions. The sensing performance of these Hb-Cu₃(PO₄)₂ HNF-based dual biosensors can be contributed such that the active sites of Hb molecules were more exposed on the surface of the Cu₃(PO₄)₂ nanopetals. Second, the unique nanopetal-assembled hybrid flowerlike structure was favorable to contact the detected substance with the biosensors. The dual biosensors were successfully applied for the determination of H₂O₂ in rainwater, tap water, and waste water samples. These results show that the dual biosensors had a potential application in the field of medical analysis, environmental monitoring, and food engineering.

Local Enhancement of Polarization at PbTiO3/BiFeO3 Interfaces Mediated by Charge Transfer

Ferroelectrics hold promise for sensors, transducers, and telecommunications. With the demand of electronic devices scaling down, they take the form of nanoscale films. However, the polarizations in ultrathin ferroelectric films are usually reduced dramatically due to the depolarization field caused by incomplete charge screening at interfaces, hampering the integrations of ferroelectrics into electric devices. Here, we design and fabricate a ferroelectric/multiferroic PbTiO₃/BiFeO₃ system, which exhibits discontinuities in both chemical valence and ferroelectric polarization across the interface. Aberration-corrected scanning transmission electron microscopic study reveals an 8% elongation of out-of-plane lattice spacing associated with 104%, 107%, and 39% increments of δ_Ti, δ_O1, and δ_O2 in the PbTiO₃ layer near the head-to-tail polarized interface, suggesting an over ∼70% enhancement of polarization compared with that of bulk PbTiO₃. Besides that in PbTiO₃, polarization in the BiFeO₃ is also remarkably enhanced. Electron energy loss spectrum and X-ray photoelectron spectroscopy investigations demonstrate the oxygen vacancy accumulation as well as the transfer of Fe³⁺ to Fe²⁺ at the interface. On the basis of the polar catastrophe model, FeO₂/PbO interface is determined. First-principles calculation manifests that the oxygen vacancy at the interface plays a predominate role in inducing the local polarization enhancement. We propose a charge transfer mechanism that leads to the remarkable polarization increment at the PbTiO₃/BiFeO₃ interface. This study may facilitate the development of nanoscale ferroelectric devices by tailing the coupling of charge and lattice in oxide heteroepitaxy.

Resistive switching and related magnetization switching in Pt/BiFeO3/Nb:SrTiO3 heterostructures

We report the coexistence of nonvolatile resistive and magnetization switching in Pt/BFO/Nb:SrTiO₃ heterostructures.

Ferroelectric BiFeO3 as an Oxide Dye in Highly Tunable Mesoporous All-Oxide Photovoltaic Heterojunctions

As potential photovoltaic materials, transition-metal oxides such as BiFeO₃ (BFO) are capable of absorbing a substantial portion of solar light and incorporating ferroic orders into solar cells with enhanced performance. But the photovoltaic application of BFO has been hindered by low energy-conversion efficiency due to poor carrier transport and collection. In this work, a new approach of utilizing BFO as a light-absorbing sensitizer is developed to interface with charge-transporting TiO₂ nanoparticles. This mesoporous all-oxide architecture, similar to that of dye-sensitized solar cells, can effectively facilitate the extraction of photocarriers. Under the standard AM1.5 (100 mW cm^-2 ) irradiation, the optimized cell shows an open-circuit voltage of 0.67 V, which can be enhanced to 1.0 V by tailoring the bias history. A fill factor of 55% is achieved, which is much higher than those in previous reports on BFO-based photovoltaic devices. The results provide here a new viable approach toward developing highly tunable and stable photovoltaic devices based on ferroelectric transition-metal oxides.

Hydrogen-treated BiFeO3 nanoparticles with enhanced photoelectrochemical performance

Compared with pristine BiFeO₃, hydrogen-treated BiFeO₃ nanoparticles exhibit higher photoelectrochemical performance.

Multifunctional magnetoelectric materials for device applications

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

Visible-light driven water splitting over BiFeO₃ photoanodes grown via the LPCVD reaction of [Bi(OtBu)₃] and [Fe(OtBu)₃]₂ and enhanced with a surface nickel oxygen evolution catalyst

Phase-pure BiFeO3 films were grown directly via dual-source low-pressure CVD (LPCVD) from the ligand-matched precursors [Bi(O(t)Bu)3] and [Fe(O(t)Bu)3]2, without the requirement for oxidising gas or post deposition annealing. Photocatalytic testing for water oxidation revealed extremely high activity for PEC water splitting and photocatalytic water oxidation under visible light irradiation (λ > 420 nm) with a benchmark IPCE for BiFeO3 of 23% at 400 nm. The high activity is ascribed to the ultrafine morphology achieved via the LPCVD process. The performance was enhanced by over four times when the BiFeO3 photoanode is coupled to a Ni-B surface OEC.

Stabilization of weak ferromagnetism by strong magnetic response to epitaxial strain in multiferroic BiFeO3

Multiferroic BiFeO₃ exhibits excellent magnetoelectric coupling critical for magnetic information processing with minimal power consumption. However, the degenerate nature of the easy spin axis in the (111) plane presents roadblocks for real world applications. Here, we explore the stabilization and switchability of the weak ferromagnetic moments under applied epitaxial strain using a combination of first-principles calculations and group-theoretic analyses. We demonstrate that the antiferromagnetic moment vector can be stabilized along unique crystallographic directions ([110] and [–110]) under compressive and tensile strains. A direct coupling between the anisotropic antiferrodistortive rotations and the Dzyaloshinskii-Moria interactions drives the stabilization of the weak ferromagnetism. Furthermore, energetically competing C- and G-type magnetic orderings are observed at high compressive strains, suggesting that it may be possible to switch the weak ferromagnetism “on” and “off” under the application of strain. These findings emphasize the importance of strain and antiferrodistortive rotations as routes to enhancing induced weak ferromagnetism in multiferroic oxides.

Predicting a ferrimagnetic phase of Zn(2)FeOsO(6) with strong magnetoelectric coupling

Multiferroic materials, in which ferroelectric and magnetic ordering coexist, are of practical interest for the development of novel memory devices that allow for electrical writing and nondestructive magnetic readout operation. The great challenge is to create room temperature multiferroic materials with strongly coupled ferroelectric and ferromagnetic (or ferrimagnetic) orderings. BiFeO_{3} is the most heavily investigated single-phase multiferroic to date due to the coexistence of its magnetic order and ferroelectric order at room temperature. However, there is no net magnetic moment in the cycloidal (antiferromagneticlike) magnetic state of bulk BiFeO_{3}, which severely limits its realistic applications in electric field controlled memory devices. Here, we predict that LiNbO_{3}-type Zn_{2}FeOsO_{6} is a new multiferroic with properties superior to BiFeO_{3}. First, there are strong ferroelectricity and strong ferrimagnetism at room temperature in Zn_{2}FeOsO_{6}. Second, the easy plane of the spontaneous magnetization can be switched by an external electric field, evidencing the strong magnetoelectric coupling existing in this system. Our results suggest that ferrimagnetic 3d-5d LiNbO_{3}-type material may therefore be used to achieve voltage control of magnetism in future memory devices.

Templated assembly of BiFeO₃ nanocrystals into 3D mesoporous networks for catalytic applications

The self-assembly of uniform nanocrystals into large porous architectures is currently of immense interest for nanochemistry and nanotechnology. These materials combine the respective advantages of discrete nanoparticles and mesoporous structures. In this article, we demonstrate a facile nanoparticle templating process to synthesize a three-dimensional mesoporous BiFeO₃ material. This approach involves the polymer-assisted aggregating assembly of 3-aminopropanoic acid-stabilized bismuth ferrite (BiFeO₃) nanocrystals followed by thermal decomposition of the surfactant. The resulting material consists of a network of tightly connected BiFeO₃ nanoparticles (∼6-7 nm in diameter) and has a moderately high surface area (62 m(2) g(-1)) and uniform pores (ca. 6.3 nm). As a result of the unique mesostructure, the porous assemblies of BiFeO₃ nanoparticles show an excellent catalytic activity and chemical stability for the reduction of p-nitrophenol to p-aminophenol with NaBH4.

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.

Multiferroic properties of Bi0.5K0.5TiO3–BiFe1−xCoxO3 (0 ≤ x ≤ 0.2) solid solution

We have successfully prepared a binary lead-free solid-solution of Bi_0.5K_0.5TiO₃–BiFe_1−xCo_xO₃ using a modified Pechini method and investigated the magnetic and ferroelectric properties of Bi_0.5K_0.5TiO₃–BiFe_1−xCo_xO₃ (0 ≤ x ≤ 0.2).

Gas-sensing and electrical properties of perovskite structure p-type barium-substituted bismuth ferrite

Schematic diagram of the proposed gas-sensing mechanism for the p-type BiFeO₃ based gas sensor: (a) and (c) in air, (b) and (d) in reducing gas, (e) simplified equivalent circuit.

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.

Structure and spin dynamics of multiferroic BiFeO3

Multiferroic materials have attracted much interest due to the unusual coexistence of ferroelectric and (anti-)ferromagnetic ground states in a single compound. They offer an exciting platform for new physics and potentially novel devices. BiFeO3 is one of the most celebrated multiferroic materials and has highly desirable properties. It is the only known room-temperature multiferroic with TC ≈ 1100 K and TN ≈ 650 K, and exhibits one of the largest spontaneous electric polarisations, P ≈ 80 µC cm(-2). At the same time, it has a magnetic cycloid structure with an extremely long period of 620 Å, which arises from competition between the usual symmetric exchange interaction and the antisymmetric Dzyaloshinskii-Moriya (DM) interaction. There is also an intriguing interplay between the DM interaction and single ion anisotropy K. In this review, we have attempted to paint a complete picture of bulk BiFeO3 by summarising the structural and dynamic properties of both the spin and lattice parts and their magneto-electric coupling.

Studies of the Room-Temperature Multiferroic Pb(Fe0.5Ta0.5)0.4(Zr0.53Ti0.47)0.6O3: Resonant Ultrasound Spectroscopy, Dielectric, and Magnetic Phenomena

Recently, lead iron tantalate/lead zirconium titanate (PZTFT) was demonstrated to possess large, but unreliable, magnetoelectric coupling at room temperature. Such large coupling would be desirable for device applications but reproducibility would also be critical. To better understand the coupling, the properties of all 3 ferroic order parameters, elastic, electric, and magnetic, believed to be present in the material across a range of temperatures, are investigated. In high temperature elastic data, an anomaly is observed at the orthorhombic mm2 to tetragonal 4mm transition, T_ot = 475 K, and a softening trend is observed as the temperature is increased toward 1300 K, where the material is known to become cubic. Thermal degradation makes it impossible to measure elastic behavior up to this temperature, however. In the low temperature region, there are elastic anomalies near ≈40 K and in the range 160-245 K. The former is interpreted as being due to a magnetic ordering transition and the latter is interpreted as a hysteretic regime of mixed rhombohedral and orthorhombic structures. Electrical and magnetic data collected below room temperature show anomalies at remarkably similar temperature ranges to the elastic data. These observations are used to suggest that the three order parameters in PZTFT are strongly coupled.

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 field control of magnetism in multiferroic heterostructures

We review the recent developments in the electric field control of magnetism in multiferroic heterostructures, which consist of heterogeneous materials systems where a magnetoelectric coupling is engineered between magnetic and ferroelectric components. The magnetoelectric coupling in these composite systems is interfacial in origin, and can arise from elastic strain, charge, and exchange bias interactions, with different characteristic responses and functionalities. Moreover, charge transport phenomena in multiferroic heterostructures, where both magnetic and ferroelectric order parameters are used to control charge transport, suggest new possibilities to control the conduction paths of the electron spin, with potential for device applications.

Influences of oxygen pressure on optical properties and interband electronic transitions in multiferroic bismuth ferrite nanocrystalline films grown by pulsed laser deposition

Bismuth ferrite (BiFeO(3)) nanocrystalline films with the crystalline size of 27-40 nm have been grown on c-sapphire substrates under various oxygen pressures of 1 × 10(-4) to 1 Pa by pulsed laser deposition. The X-ray diffraction spectra show that the films are polycrystalline and present the pure rhombohedral phase. It was found that the Raman-active phonon mode E(TO1) shifts towards a higher energy side from 74 to 76 cm(-1) with increasing oxygen pressure, indicating a larger tensile stress in the films deposited at higher oxygen pressure. The X-ray photoelectron spectroscopy analysis suggests that the concentrations of both Fe(2+) ions and oxygen vacancies in the BiFeO(3) films increase with decreasing oxygen pressure. Moreover, the dielectric functions in the photon energy range of 0.47-6.5 eV have been extracted by fitting the transmittance spectra with the Tauc-Lorentz dispersion model. From the transmittance spectra, the fundamental absorption edge is observed to present a redshift trend with increasing the temperature from 8 to 300 K. Note that the optical band gap (E(g)) decreases with increasing the temperature due to the electron-phonon interactions associated with the interatomic distance in the BiFeO(3) films. However, the E(g) decreases from 2.88 to 2.78 eV with decreasing oxygen pressure at 8 K, which can be attributed to the increment of oxygen vacancies leading to the formation of some impurity states between the valence and conduction band. It can be concluded that the oxygen pressure during the film fabrication has the significant effects on microstructure, optical properties, and electronic band structure modification of the BiFeO(3) films.

Local weak ferromagnetism in single-crystalline ferroelectric BiFeO3

Polarized small-angle neutron scattering studies of single-crystalline multiferroic BiFeO(3) reveal a long-wavelength spin density wave generated by ∼1° spin canting of the spins out of the rotation plane of the antiferromagnetic cycloidal order. This signifies weak ferromagnetism within mesoscopic regions of dimension 0.03 microns along [110], to several microns along [111], confirming a long-standing theoretical prediction. The average local magnetization is 0.06 μ(B)/Fe. Our results provide an indication of the intrinsic macroscopic magnetization to be expected in ferroelectric BiFeO(3) thin films under strain, where the magnetic cycloid is suppressed.