Single-domain multiferroic BiFeO3 films

Modulating Lattice Oxygen Activity of Iron-Based Triple-Conducting Nanoheterostructure Air Electrode via Sc-Substitution Strategy for Protonic Ceramic Cells

Iron-based perovskite air electrodes for protonic ceramic cells (PCCs) offer broad application prospects owing to their reasonable thermomechanical compatibility and steam tolerance. However, their insufficient electrocatalytic activity has considerably limited further development. Herein, oxygen-vacancy-rich BaFe0.6Ce0.2Sc0.2O3-δ (BFCS) perovskite is rationally designed by a facile Sc-substitution strategy for BaFe0.6Ce0.4O3-δ (BFC) as efficient and stable air electrode for PCCs. The BFCS electrode with an optimized Fe 3d-eg orbital occupancy and more oxygen vacancies exhibits a polarization resistance of ≈ 0.175 Ω cm2 at 600 °C, ≈ 1/3 of the BFC electrode (≈0.64 Ω cm2). Simultaneously, BFCS shows favorable proton uptake with a low proton defect formation enthalpy (- 81 kJ mol-1). By combining soft X-ray absorption spectroscopy and electrical conductivity relaxation studies, it is revealed that the enhancement of Fe4+-O2- interactions in BFCS promotes the activation and mobility of lattice oxygen, triggering the activity of BFCS in both oxygen reduction and evolution reactions (ORR/OER). The single cell achieves encouraging output performance in both fuel cell (1.55 W cm-2) and electrolysis cell (-2.96 A cm-2 at 1.3 V) modes at 700 °C. These results highlight the importance of activating lattice oxygen in air electrodes of PCCs.

The Ferroelectric Effects of Rhombohedral and Tetragonal BiFeO3 in Photoelectrochemical Water Splitting

The phase of BiFeO3 (BFO) as well as its domain configuration can be tuned by strain engineering. Phase change may greatly influence the properties of the polarization field and hence charge separation. However, the photoelectrochemical properties of different BFO phases have rarely been addressed. Here, the photoelectrochemical study of tetragonal (T-) and rhombohedral (R-) phase BFO films was conducted under visible light illumination. The photocurrent density of R-BFO is 5 times that of T-BFO. A ferroelectric domain study shows that T-BFO features single domain structure in contrast to the polydomain structure of R-BFO. Higher charge separation efficiency is achieved in R-BFO, dominated by the domain walls as conducting pathways for efficient charge separation and transfer. This work provides a fundamental understanding of the photoelectrochemical properties of T- and R-BFO, offering valuable insights for the development of BFO-based materials for solar energy conversion.

Insights into Strain Engineering: From Ferroelectrics to Related Functional Materials and Beyond

Ferroelectrics have become indispensable components in various application fields, including information processing, energy harvesting, and electromechanical conversion, owing to their unique ability to exhibit electrically or mechanically switchable polarization. The distinct polar noncentrosymmetric lattices of ferroelectrics make them highly responsive to specific crystal structures. Even slight changes in the lattice can alter the polarization configuration and response to external fields. In this regard, strain engineering has emerged as a prevalent regulation approach that not only offers a versatile platform for structural and performance optimization within ferroelectrics but also unlocks boundless potential in various functional materials. In this review, we systematically summarize the breakthroughs in ferroelectric-based functional materials achieved through strain engineering and progress in method development. We cover research activities ranging from fundamental attributes to wide-ranging applications and novel functionalities ranging from electromechanical transformation in sensors and actuators to tunable dielectric materials and information technologies, such as transistors and nonvolatile memories. Building upon these achievements, we also explore the endeavors to uncover the unprecedented properties through strain engineering in related chemical functionalities, such as ferromagnetism, multiferroicity, and photoelectricity. Finally, through discussions on the prospects and challenges associated with strain engineering in the materials, this review aims to stimulate the development of new methods for strain regulation and performance boosting in functional materials, transcending the boundaries of ferroelectrics.

Highly Effective Pyroelectric Catalysis for Simultaneous Tumor-Targeted Dynamic Therapy and Gentle Photothermal Therapy by Oxygen-Vacancy-Rich CeO2-BaTiO3 Nanorods

Pyroelectric nanostructures can effectively generate temperature-mediated reactive oxygen species (ROS) through the pyroelectric effect, providing promise for treating hypoxic tumors; and therefore, the synergistic application of photothermal therapy (PTT) and pyroelectric dynamic therapy (PEDT) presents an intriguing approach for cancer therapy. However, this method still faces challenges in improving pyroelectric catalysis and achieving precise tumor localization. In this study, a nano-heterojunction based on CeO2-BaTiO3 nanorods (IR1061@PCBNR) is reported, which exhibits highly effective pyroelectric catalysis for simultaneous tumor-targeted dynamic therapy and gentle photothermal therapy through the utilization of the rich oxygen vacancies. The oxygen vacancies create active sites that facilitate the migration of pyroelectrically-induced charge carriers, improving charge separation and ROS generation. IR1061@PCBNR also demonstrates high tumor penetration; while, minimizing damage to normal cells. This precise nanomedicine strategy holds great potential for advancing dynamic cancer therapies by overcoming the limitations of conventional approaches.

Ferroics in Hybrid Organic-Inorganic Perovskites: Fundamentals, Design Strategies, and Implementation

Hybrid organic-inorganic perovskites (HOIPs) afford highly versatile structure design and lattice dimensionalities; thus, they are actively researched as material platforms for the tailoring of ferroic behaviors. Unlike single-phase organic or inorganic materials, the interlayer coupling between organic and inorganic components in HOIPs allows the modification of strain and symmetry by chirality transfer or lattice distortion, thereby enabling the coexistence of ferroic orders. This review focuses on the principles for engineering one or multiple ferroic orders in HOIPs, and the conditions for achieving multiferroicity and magnetoelectric properties. The prospects of multilevel ferroic modulation, chiral spin textures, and spin orbitronics in HOIPs are also presented.

Onset of Multiferroicity in Prototypical Single-Spin Cycloid BiFeO3 Thin Films

In the room-temperature magnetoelectric multiferroic BiFeO3, the noncollinear antiferromagnetic state is coupled to the ferroelectric order, opening applications for low-power electric-field-controlled magnetic devices. While several strategies have been explored to simplify the ferroelectric landscape, here we directly stabilize a single-domain ferroelectric and spin cycloid state in epitaxial BiFeO3 (111) thin films grown on orthorhombic DyScO3 (011). Comparing them with films grown on SrTiO3 (111), we identify anisotropic in-plane strain as a powerful handle for tailoring the single antiferromagnetic state. In this single-domain multiferroic state, we establish the thickness limit of the coexisting electric and magnetic orders and directly visualize the suppression of the spin cycloid induced by the magnetoelectric interaction below the ultrathin limit of 1.4 nm. This as-grown single-domain multiferroic configuration in BiFeO3 thin films opens an avenue both for fundamental investigations and for electrically controlled noncollinear antiferromagnetic spintronics.

Effects of Oxygen Pressure on the Microstructures and Nanomechanical Properties of Samarium-Doped BiFeO3 Thin Films

In this study, samarium (Sm-10at%)-doped BiFeO3 (SmBFO) thin films were grown on platinum-coated glass substrates using pulsed laser deposition (PLD) to unveil the correlation between the microstructures and nanomechanical properties of the films. The PLD-derived SmBFO thin films were prepared under various oxygen partial pressures (PO2) of 10, 30, and 50 mTorr at a substrate temperature of 600 °C. The scanning electron microscopy analyses revealed a surface morphology consisting of densely packed grains, although the size distribution varied with the PO2. X-ray diffraction results indicate that all SmBFO thin films are textured and preferentially oriented along the (110) crystallographic orientation. The crystallite sizes of the obtained SmBFO thin films calculated from the Scherrer and (Williamson-Hall) equations increased from 20 (33) nm to 25 (52) nm with increasing PO2. In addition, the nanomechanical properties (the hardness and Young's modulus) of the SmBFO thin films were measured by using nanoindentation. The relationship between the hardness and crystalline size of SmBFO thin films appears to closely follow the Hall-Petch equation. In addition, the PO2 dependence of the film microstructure, the crystallite size, the hardness, and Young's modulus of SmBFO thin films are discussed.

The size effect of BiFeO3 nanocrystals on the spatial spin modulated structure

The spatial spin modulated structure (SSMS) of the cycloid type present in bulk BiFeO3 prevents the linear magnetoelectric effect. One way to influence this structure is to reduce the crystal size to the nanoscale. Various opinions are circulating in the literature about the effect of nanocrystal size on SSMS, and to investigate this issue, we used a number of methods, with zero-field NMR (ZF NMR) spectroscopy at the forefront. ZF NMR spectroscopy enables the direct observation of the distribution profile of local fields on iron atoms and defines the SSMS presence and its properties. We also examined the synthesized samples using XRD, TEM, and magnetometry. We conclude that SSMS persists as the nanocrystal size decreases to the cycloid period and less, becoming more harmonic. This is accompanied by the change of the anisotropy type from an "easy axis" to an "easy plane". Magnetic measurements show a significant increase in the saturation magnetization, remanent magnetization, coercivity, and exchange bias of nanocrystals with sizes close to the cycloid period, which is probably associated with incomplete spin compensation in the case of an incomplete cycloid period. Despite the fact that SSMS is retained in the samples with decreased size, the magnetic properties experience a sharp increase up to applicable values.

Boosting Photocatalytic Water Oxidation on Photocatalysts with Ferroelectric Single Domains

Ferroelectric materials are considered as promising photocatalysts due to their efficient charge separation via a polarization-induced built-in electric field. However, the polydomain structures hinder spatial charge separation and transfer due to the cancellation of polarization vectors in the domains. In this work, taking BiFeO₃ (BFO) as a prototype, single-domain BFO nanosheets with visible-light absorption are prepared, as evident by piezoresponse force microscopy (PFM), spatially resolved surface photovoltage spectroscopy (SRSPS), and photodeposition experiments. The single-domain BFO nanosheets show nine times activity in photocatalytic water oxidation reaction under visible-light irradiation, compared with that of the polydomain BFO particles. With the asymmetric driving force for charge separation in a single domain, selective deposition of cocatalysts further enhances the photocatalytic activity of single-domain ferroelectric BFO nanosheets. These results demonstrate the role of the single-domain structure in constructing the driving force of charge separation in ferroelectric photocatalysts. The fabrication of single-domain structures in ferroelectric photocatalysts to achieve enhanced photocatalytic activity offers a path to efficiently utilize the photogenerated charges in solar energy conversion.

Brief Theoretical Overview of Bi-Fe-O Based Thin Films

This paper will provide a brief overview of the unique multiferroic material Bismuth ferrite (BFO). Considering that Bismuth ferrite is a unique material which possesses both ferroelectric and magnetic properties at room temperature, the uniqueness of Bismuth ferrite material will be discussed. Fundamental properties of the material including electrical and ferromagnetic properties also will be mentioned in this paper. Electrical properties include characterization of basic parameters considering the electrical resistivity and leakage current. Ferromagnetic properties involve the description of magnetic hysteresis characterization. Bismuth ferrite can be fabricated in a different form. The common forms will be mentioned and include powder, thin films and nanostructures. The most popular method of producing thin films based on BFO materials will be described and compared. Finally, the perspectives and potential applications of the material will be highlighted.

Twisted oxide lateral homostructures with conjunction tunability

Epitaxial growth is of significant importance over the past decades, given it has been the key process of modern technology for delivering high-quality thin films. For conventional heteroepitaxy, the selection of proper single crystal substrates not only facilitates the integration of different materials but also fulfills interface and strain engineering upon a wide spectrum of functionalities. Nevertheless, the lattice structure, regularity and crystalline orientation are determined once a specific substrate is chosen. Here, we reveal the growth of twisted oxide lateral homostructure with controllable in-plane conjunctions. The twisted lateral homostructures with atomically sharp interfaces can be composed of epitaxial “blocks” with different crystalline orientations, ferroic orders and phases. We further demonstrate that this approach is universal for fabricating various complex systems, in which the unconventional physical properties can be artificially manipulated. Our results establish an efficient pathway towards twisted lateral homostructures, adding additional degrees of freedom to design epitaxial films.

It is challenging to construct lateral homostructures with controllable geometry and repeated alternating configurations. Here the authors develop a generic approach for fabricating twisted lateral homostructures with tunable crystal orientation, epitaxial constrain, and phase stability.

Photonic-Crafting of Non-Volatile and Rewritable Antiferromagnetic Spin Textures with Drastic Difference in Electrical Conductivity

Antiferromagnetic spintronics is an emerging field of non-volatile data storage and information processing. The zero net magnetization and zero stray fields of antiferromagnetic materials eliminate interference between neighbor units, leading to high-density memory integrations. However, this invisible magnetic character at the same time also poses a great challenge in controlling and detecting magnetic states in antiferromagnets. Here, two antiferromagnetic spin states close in energy in strained BiFeO₃ thin films at room temperature are discovered. It can be reversibly switched between these two non-volatile antiferromagnetic states by a moderate magnetic field and a non-contact optical approach. Importantly, the conductivity of the areas with each antiferromagnetic textures is drastically different. It is conclusively demonstrated the capability of optical writing and electrical reading of these newly discovered bistable antiferromagnetic states in the BiFeO₃ thin films.

The Study of Magnetic Properties for Non-Magnetic Ions Doped BiFeO3

BiFeO₃ is considered as a single phase multiferroic. However, its magnetism is very weak. We study the magnetic properties of BiFeO₃ by Cu and (Cu, Zn). Polycrystalline samples Bi(Fe_0.95Cu_0.05)O₃ and BiFe_0.95(Zn_0.025Cu_0.025)O₃ are prepared by the sol-gel method. The magnetic properties of BiFe_0.95(Zn_0.025Cu_0.025)O₃ are greater than that of BiFeO₃ and Bi(Fe_0.95Cu_0.05)O₃. The analyses of X-ray absorption fine structure data show that the doped Cu atoms well occupy the sites of the Fe atoms. X-ray absorption near edge spectra data confirm that the valence state of Fe ions does not change. Cu and Zn metal ion co-doping has no impact on the local structure of the Fe and Bi atoms. The modification of magnetism by doping Zn can be understood by the view of the occupation site of non-magnetically active Zn²⁺.

Room-Temperature Magnetoelectric Coupling in Electronic Ferroelectric Film based on [(n-C3H7)4N][FeIIIFeII(dto)3] (dto = C2O2S2)

Great importance has been attached to magnetoelectric coupling in multiferroic thin films owing to their extremely practical use in a new generation of devices. Here, a film of [(n-C₃H₇)₄N][Fe^IIIFe^II(dto)₃] (1; dto = C₂O₂S₂) was fabricated using a simple stamping process. As was revealed by our experimental results, in-plane ferroelectricity over a wide temperature range from 50 to 300 K was induced by electron hopping between Fe^II and Fe^III sites. This mechanism was further confirmed by the ferroelectric observation of the compound [(n-C₃H₇)₄N][Fe^IIIZn^II(dto)₃] (2; dto = C₂O₂S₂), in which Fe^II ions were replaced by nonmagnetic metal Zn^II ions, resulting in no obvious ferroelectric polarization. However, both ferroelectricity and magnetism are related to the magnetic Fe ions, implying a strong magnetoelectric coupling in 1. Through piezoresponse force microscopy (PFM), the observation of magnetoelectric coupling was achieved by manipulating ferroelectric domains under an in-plane magnetic field. The present work not only provides new insight into the design of molecular-based electronic ferroelectric/magnetoelectric materials but also paves the way for practical applications in a new generation of electronic devices.

Extremely Fast Optical and Nonvolatile Control of Mixed-Phase Multiferroic BiFeO3 via Instantaneous Strain Perturbation

Multiferroics-materials that exhibit coupled ferroic orders-are considered to be one of the most promising candidate material systems for next-generation spintronics, memory, low-power nanoelectronics and so on. To advance potential applications, approaches that lead to persistent and extremely fast functional property changes are in demand. Herein, it is revealed that the phase transition and the correlated ferroic orders in multiferroic BiFeO₃ (BFO) can be modulated via illumination of single short/ultrashort light pulses. Heat transport simulations and ultrafast optical pump-probe spectroscopy reveal that the transient strain induced by light pulses plays a key role in determining the persistent final states. Having identified the diffusionless phase transformation features via scanning transmission electron microscopy, sequential laser pulse illumination is further demonstrated to perform large-area phase and domain manipulation in a deterministic way. The work contributes to all-optical and rapid nonvolatile control of multiferroicity, offering different routes while designing novel optoelectronics.

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.

Giant Thermal Enhancement of the Electric Polarization in Ferrimagnetic BiFe_{1-x}Co_{x}O_{3} Solid Solutions near Room Temperature

Thermal excitations typically reduce the electric polarization in ferroelectric materials. Here, we show by means of first-principles calculations that multiferroic BiFe_{1-x}Co_{x}O_{3} solid solutions with 0.25≤x≤0.50 (BFCO) represent a noteworthy exception to this behavior. In particular, we find that, at room temperature and for moderate pressures of 0.1-1.0 GPa, depending on the composition, the electric polarization of bulk BFCO increases by ∼150%. The origin of such an exceptional behavior is a phase transformation involving a low-T rhombohedral (R) phase and a high-T supertetragonal (T) phase. Both R and T phases are ferrimagnetic near room temperature with an approximate net magnetization of 0.13 μ_{B} per formula unit. Contrary to what occurs in either bulk BiFeO_{3} or BiCoO_{3}, the T phase is stabilized over the R by increasing temperature due to its higher vibrational entropy. This extraordinary T-induced R→T phase transition is originated by polar phonon modes that involve concerted displacements of transition-metal and oxygen ions.

Manipulating magnetoelectric energy landscape in multiferroics

Magnetoelectric coupling at room temperature in multiferroic materials, such as BiFeO₃, is one of the leading candidates to develop low-power spintronics and emerging memory technologies. Although extensive research activity has been devoted recently to exploring the physical properties, especially focusing on ferroelectricity and antiferromagnetism in chemically modified BiFeO₃, a concrete understanding of the magnetoelectric coupling is yet to be fulfilled. We have discovered that La substitutions at the Bi-site lead to a progressive increase in the degeneracy of the potential energy landscape of the BiFeO₃ system exemplified by a rotation of the polar axis away from the 〈111〉_pc towards the 〈112〉_pc discretion. This is accompanied by corresponding rotation of the antiferromagnetic axis as well, thus maintaining the right-handed vectorial relationship between ferroelectric polarization, antiferromagnetic vector and the Dzyaloshinskii-Moriya vector. As a consequence, La-BiFeO₃ films exhibit a magnetoelectric coupling that is distinctly different from the undoped BiFeO₃ films.

BiFeO₃ has a wide application but the impact of rare-earth substitution for the evolution of the coupling mechanism is unknown. Here, the authors reveal the correlation between ferroelectricity, antiferromagnetism, a weak ferromagnetic moment, and their switching pathways in La-substituted BiFeO₃.

Synthesis of Organic-Inorganic Hybrids Based on Perovskite-like Bismuth Titanate H2K0.5Bi2.5Ti4O13·H2O and n-Alkylamines

New organic-inorganic hybrids have been synthesized by the intercalation of n-alkylamines (methylamine, ethylamine, n-propylamine, n-butylamine, n-hexylamine, and n-octylamine) into the structure of the protonated and hydrated form of the perovskite-like layered titanate H₂K_0.5Bi_2.5Ti₄O₁₃·H₂O (HKBT₄·H₂O). The possibility of the synthesis of the hybrid materials was studied in a wide range of conditions. It was found that interlayer water plays a crucial role in the formation of intercalated hybrids. The obtained compounds were characterized with powder X-ray diffraction analysis; Raman, IR, and NMR spectroscopies; thermogravimetry (TG), TG coupled with mass spectrometry, and CHN analyses; and scanning electron microscopy. It was suggested that the intercalated n-alkylamines exist in the form of alkylammonium ions forming a paraffin-like bilayer with an average tilting angle of ∼77.5°. The obtained HKBT₄×RNH₂ compounds contain 0.4-0.7 n-alkylamine molecules per formula unit as well as the varied amount of intercalated water. By gentle heating, they can be obtained as dehydrated forms, which are thermally stable up to 250 °C.

Structure, Performance, and Application of BiFeO3 Nanomaterials

Multiferroic nanomaterials have attracted great interest due to simultaneous two or more properties such as ferroelectricity, ferromagnetism, and ferroelasticity, which can promise a broad application in multifunctional, low-power consumption, environmentally friendly devices. Bismuth ferrite (BiFeO3, BFO) exhibits both (anti)ferromagnetic and ferroelectric properties at room temperature. Thus, it has played an increasingly important role in multiferroic system. In this review, we systematically discussed the developments of BFO nanomaterials including morphology, structures, properties, and potential applications in multiferroic devices with novel functions. Even the opportunities and challenges were all analyzed and summarized. We hope this review can act as an updating and encourage more researchers to push on the development of BFO nanomaterials in the future.

Effects of pillar size modulation on the magneto-structural coupling in self-assembled BiFeO3–CoFe2O4 heteroepitaxy

The magneto-structural coupling of BiFeO₃ (BFO)–CoFe₂O₄ (CFO)/LaAlO₃ (LAO) heteroepitaxy with various lateral sizes of CFO pillars embedded in a BFO matrix was investigated.

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

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

Uncovering the role of Ag in layer-alternating Ni3S2/Ag/Ni3S2 as an electrocatalyst with enhanced OER performance

It is increasingly important to develop an efficient OER catalyst that can provide high current density at low overpotentials to improve water splitting efficiency.

Strain effect on orbital and magnetic structures of Mn ions in epitaxial Nd0.35Sr0.65MnO3/SrTiO3 films using X-ray diffraction and absorption

This study probes the temperature-dependent strain that is strongly correlated with the orbital and magnetic structures of epitaxial films of Nd_0.35Sr_0.65MnO₃ (NSMO) that are fabricated by pulsed laser deposition with two thicknesses, 17 (NS17) and 103 nm (NS103) on SrTiO₃ (STO) substrate. This investigation is probed using X-ray diffraction (XRD) and absorption-based techniques, X-ray linear dichroism (XLD) and the X-ray magnetic circular dichroism (XMCD). XRD indicates a significant shift in the (004) peak position that is associated with larger strain in NS17 relative to that of NS103 at both 30 and 300 K. Experimental and atomic multiplet simulated temperature-dependent Mn L_3,2-edge XLD results reveal that the stronger strain in a thinner NS17 film causes less splitting of Mn 3d e_g state at low temperature, indicating an enhancement of orbital fluctuations in the band above the Fermi level. This greater Mn 3d orbital fluctuation can be the cause of both the enhanced ferromagnetism (FM) as a result of spin moments and the reduced Néel temperature of C-type antiferromagnetism (AFM) in NS17, leading to the FM coupling of the canted-antiferromagnetism (FM-cAFM) state in NSMO/STO epitaxial films at low temperature (T = 30 K). These findings are also confirmed by Mn L_3,2-edge XMCD measurements.

Magnetic properties and oxygen migration induced resistive switching effect in Y substituted multiferroic bismuth ferrite

Low field magnetic saturation and forming-free resistive switching behavior in non-magnetic modified room temperature multiferroic BiFeO₃ thin film.

Complex strain evolution of polar and magnetic order in multiferroic BiFeO3 thin films

Electric-field control of magnetism requires deterministic control of the magnetic order and understanding of the magnetoelectric coupling in multiferroics like BiFeO₃ and EuTiO₃. Despite this critical need, there are few studies on the strain evolution of magnetic order in BiFeO₃ films. Here, in (110)-oriented BiFeO₃ films, we reveal that while the polarization structure remains relatively unaffected, strain can continuously tune the orientation of the antiferromagnetic-spin axis across a wide angular space, resulting in an unexpected deviation of the classical perpendicular relationship between the antiferromagnetic axis and the polarization. Calculations suggest that this evolution arises from a competition between the Dzyaloshinskii-Moriya interaction and single-ion anisotropy wherein the former dominates at small strains and the two are comparable at large strains. Finally, strong coupling between the BiFeO₃ and the ferromagnet Co_0.9Fe_0.1 exists such that the magnetic anisotropy of the ferromagnet can be effectively controlled by engineering the orientation of the antiferromagnetic-spin axis.

Electrically Driven Reversible Magnetic Rotation in Nanoscale Multiferroic Heterostructures

Electrically driven magnetic switching (EDMS) is highly demanded for next-generation advanced memories or spintronic devices. The key challenge is to achieve repeatable and reversible EDMS at sufficiently small scale. In this work, we reported an experimental realization of room-temperature, electrically driven, reversible, and robust 120° magnetic state rotation in nanoscale multiferroic heterostructures consisting of a triangular Co nanomagnet array on tetragonal BiFeO₃ films, which can be directly monitored by magnetic force microscope (MFM) imaging. The observed reversible magnetic switching in an individual nanomagnet can be triggered by a small electric pulse within 10 V with an ultrashort time of ∼10 ns, which also demonstrates sufficient switching cycling and months-long retention lifetime. A mechanism based on synergic effects of interfacial strain and exchange coupling plus shape anisotropy was also proposed, which was also verified by micromagnetic simulations. Our results create an avenue to engineer the nanoscale EDMS for low-power-consumption, high-density, nonvolatile magnetoelectric memories and beyond.

Magnetoelectricity of CoFe2O4 and tetragonal phase BiFeO3 nanocomposites prepared by pulsed laser deposition

The coupling between the tetragonal phase (T-phase) of BiFeO₃ (BFO) and CoFe₂O₄ (CFO) in magnetoelectric heterostructures has been studied. Bilayers of CFO and BFO were deposited on (001) LaAlO₃ single crystal substrates by pulsed laser deposition. After 30 min of annealing, the CFO top layer exhibited a T-phase-like structure, developing a platform-like morphology with BFO. Magnetic hysteresis loops exhibited a strong thickness effect of the CFO layer on the coercive field, in particular along the out-of-plane direction. Magnetic force microscopy images revealed that the T-phase CFO platform contained multiple magnetic domains, which could be tuned by applying a tip bias. A combination of shape, strain, and exchange coupling effects are used to explain the observations.

Piezoelectric Effects of Materials on Bio-Interfaces

Electrical stimulation of cells and tissues is an important approach of interaction with living matter, which has been traditionally exploited in the clinical practice for a wide range of pathological conditions, in particular, related to excitable tissues. Standard methods of stimulation are, however, often invasive, being based on electrodes and wires used to carry current to the intended site. The possibility to achieve an indirect electrical stimulation, by means of piezoelectric materials, is therefore of outstanding interest for all the biomedical research, and it emerged in the latest decade as a most promising tool in many bioapplications. In this paper, we summarize the most recent achievements obtained by our group and by others in the exploitation of piezoelectric nanoparticles and nanocomposites for cell stimulation, describing the important implications that these studies present in nanomedicine and tissue engineering. A particular attention will be also dedicated to the physical modeling, which can be extremely useful in the description of the complex mechanisms involved in the mechanical/electrical transduction, yet also to gain new insights at the base of the observed phenomena.