Multiferroic BiFeO3 Thin Films Research Articles

Electric-field-induced multiferroic topological solitons.

Topologically protected spin whirls in ferromagnets are foreseen as the cart-horse of solitonic information technologies. Nevertheless, the future of skyrmionics may rely on antiferromagnets due to their immunity to dipolar fields, straight motion along the driving force and ultrafast dynamics. While complex topological objects were recently discovered in intrinsic antiferromagnets, mastering their nucleation, stabilization and manipulation with energy-efficient means remains an outstanding challenge. Designing topological polar states in magnetoelectric antiferromagnetic multiferroics would allow one to electrically write, detect and erase topological antiferromagnetic entities. Here we stabilize ferroelectric centre states using a radial electric field in multiferroic BiFeO3 thin films. We show that such polar textures contain flux closures of antiferromagnetic spin cycloids, with distinct antiferromagnetic entities at their cores depending on the electric field polarity. By tuning the epitaxial strain, quadrants of canted antiferromagnetic domains can also be electrically designed. These results open the path to reconfigurable topological states in multiferroic antiferromagnets.

Ultrafast Optically Induced Perturbation of Oxygen Octahedral Rotations in Multiferroic BiFeO3 Thin Films.

The functional properties of complex oxides, including magnetism and ferroelectricity, are closely linked to subtle structural distortions. Ultrafast optical excitations provide the means to manipulate structural features and ultimately to affect the functional properties of complex oxides with picosecond-scale precision. We report that the lattice expansion of multiferroic BiFeO3 following above-bandgap optical excitation leads to distortion of the oxygen octahedral rotation (OOR) pattern. The continuous coupling between OOR and strain was probed using time-resolved X-ray free-electron laser diffraction with femtosecond time resolution. Density functional theory calculations predict a relationship between the OOR and the elastic strain consistent with the experiment, demonstrating a route to employing this approach in a wider range of systems. Ultrafast control of the functional properties of BiFeO3 thin films is enabled by this approach because the OOR phenomena are related to ferroelectricity, and via the Fe-O-Fe bond angles, the superexchange interaction between Fe atoms.

Observation of Magnon Spin Transport in BiFeO3 Thin Films

AbstractMagnon, a quanta of spin waves, can transport in magnetically ordered insulators without substantial heat dissipation. The recently demonstrated magnon‐mediated spin torque through antiferromagnetic insulators is regarded as a highly efficient approach for electrical manipulation of magnetization. To control the magnon transport in antiferromagnets, a very large magnetic field is required, since antiferromagnets show immunity to external magnetic disturbance. The antiferromagnetic order can be electrically controlled in multiferroic materials via the magnetoelectric coupling between the antiferromagnetic and ferroelectric orders. Here, we report the experimental observation of magnon spin transport through multiferroic BiFeO3 thin films in SrRuO3/BiFeO3/NiFe tri‐layers at room temperature. We find highly efficient magnon current transmission through BiFeO3 thin film with a thickness exceeding 80 nm by the spin pumping measurement. The magnon spin‐torque efficiency in the tri‐layers measured by the spin‐torque ferromagnetic resonance is comparable with that in the SrRuO3/NiFe bilayer. This results suggest multiferroic BiFeO3 to be an excellent platform for investigating the antiferromagnetic magnon transport and developing magnonic devices.

Deterministic Manipulation of Multi‐State Polarization Switching in Multiferroic Thin Films (Adv. Funct. Mater. 8/2023)

Four-State Nonvolatile Memories In article number 2208244, Deyang Chen and co-workers report that four-state polarization states in multiferroic BiFeO3 thin films are deterministically achieved and reversibly controlled through precisely selecting 71° ferroelastic and 180° ferroelectric switching paths. This study provides the platform for the realization of multi-state memories and magnetoelectric coupling based devices.

Deterministic Manipulation of Multi‐State Polarization Switching in Multiferroic Thin Films

AbstractDeterministically controllable multi‐state polarizations in ferroelectric materials are promising for the application of next‐generation non‐volatile multi‐state memory devices. However, the achievement of multi‐state polarizations has been inhibited by the challenge of selective control of switching pathways. Herein, an approach to selectively control 71° ferroelastic and 180° ferroelectric switching paths by combining the out‐of‐plane electric field and in‐plane trailing field in multiferroic BiFeO3 thin films with periodically ordered 71° domain wall is reported. Four‐state polarization states can be deterministically achieved and reversibly controlled through precisely selecting different switching paths. These studies reveal the ability to obtain multiple polarization states for the realization of multi‐state memories and magnetoelectric coupling‐based devices.

Prediction of magnetoelectric properties of defect BiFeO3 thin films using Monte Carlo simulations

Magnetoelectric materials have received astonishing attention due to their great potential for nanotechnology and magnetoelectric applications. Herein, the ferroelectric, magnetic and magnetoelectric properties of multiferroic BiFeO3 (BFO) thin films have been carried out using Monte Carlo simulation investigations in the framework of the Heisenberg model. Exchange coupling interactions in magnetic and ferroelectric sublattices that correspond to the experimental critical temperature were estimated. Temperature dependence of the internal energy, specific heat, magnetization, electric polarization and their susceptibilities in BFO thin films have been systematically analyzed. Moreover, the effect of magnetoelectric coupling interaction Jme on M−H and P-H hysteresis loopsand magnetoelectric voltage coefficient were studied. A large magnetoelectric voltage coefficient of 104 mV/(cm.Oe) was predicted in BFO thin films with 25% defects. Furthermore, the defect mechanism can be used to control the polarization switching by the magnetic field in BFO system. The obtained results highlight the large magnetoelectric effect in BFO multiferroic thin films for multifunctional magnetoelectric devices.

Properties of stress-induced super tetragonal phase in epitaxial BiFeO3 thin film

Stress-induced structural phase transitions in multiferroic BiFeO3 thin films have attracted much attention for both fundamental research and promising applications in electronic devices. In this work, with the assistance of spherical aberration-corrected transmission electron microscopy, a tetragonal phase with the super large tetragonality (c/a ∼ 1.53) was found in the epitaxial BiFeO3 thin film on a LaAlO3/Si substrate. Analyses of atom positions showed that displacements of an Fe atom in this super tetragonal phase are much smaller than that in a regular tetragonal phase. First-principles calculations revealed that the structural phase transition occurs with the increasing lattice tetragonality c/a, accompanied by transitions in the atomic coordination and magnetic order.

Engineering of multiferroic BiFeO3 grain boundaries with head-to-head polarization configurations

Confined low dimensional charges with high density such as two-dimensional electron gas (2DEG) at interfaces and charged domain walls in ferroelectrics show great potential to serve as functional elements in future nanoelectronics. However, stabilization and control of low dimensional charges is challenging, as they are usually subject to enormous depolarization fields. Here, we demonstrate a method to fabricate tunable charged interfaces with ~77°, 86° and 94° head-to-head polarization configurations in multiferroic BiFeO3 thin films by grain boundary engineering. The adjacent grains are cohesively bonded and the boundary is about 1 nm in width and devoid of any amorphous region. Remarkably, the polarization remains almost unchanged near the grain boundaries, indicating the polarization charges are well compensated, i.e., there should be two-dimensional charge gas confined at grain boundaries. Adjusting the tilt angle of the grain boundaries enables tuning the angle of polarization configurations from 71° to 109°, which in turn allows the control of charge density at the grain boundaries. This general and feasible method opens new doors for the application of charged interfaces in next generation nanoelectronics.

Enhancement of photovoltage by electronic structure evolution in multiferroic Mn-doped BiFeO3 thin films

The bulk photovoltaic effect (BPVE) is a mechanism of recent focus for novel solar cells that exceed the power conversion efficiency of p–n junction solar cells because of the quantum mechanical effect to generate photocurrent known as shift current. Ferroelectrics are receiving attention again because of their high voltage generation by the BPVE and converse piezoelectric effect to realize high performance optical actuators. We have investigated the BPVE in ferroelectric BiFeO3 (BFO) single crystal thin films, whereby the photovoltage was enhanced by Mn doping, and 852 V generation was demonstrated at 80 K. The enhancement mechanism was also investigated using soft and hard X-ray photoelectron spectroscopy (SXPES, HAXPES), and soft X-ray absorption spectroscopy with synchrotron radiation. This report reveals a way to new voltage source applications employing the BPVE for high impedance devices with ferroelectrics. Important aspects for designing ferroelectric materials by impurity doping are also discussed.

Antiferromagnetic textures in BiFeO3 controlled by strain and electric field

Antiferromagnetic thin films are currently generating considerable excitement for low dissipation magnonics and spintronics. However, while tuneable antiferromagnetic textures form the backbone of functional devices, they are virtually unknown at the submicron scale. Here we image a wide variety of antiferromagnetic spin textures in multiferroic BiFeO3 thin films that can be tuned by strain and manipulated by electric fields through room-temperature magnetoelectric coupling. Using piezoresponse force microscopy and scanning NV magnetometry in self-organized ferroelectric patterns of BiFeO3, we reveal how strain stabilizes different types of non-collinear antiferromagnetic states (bulk-like and exotic spin cycloids) as well as collinear antiferromagnetic textures. Beyond these local-scale observations, resonant elastic X-ray scattering confirms the existence of both types of spin cycloids. Finally, we show that electric-field control of the ferroelectric landscape induces transitions either between collinear and non-collinear states or between different cycloids, offering perspectives for the design of reconfigurable antiferromagnetic spin textures on demand.

Phonon-induced near-field resonances in multiferroic BiFeO3 thin films at infrared and THz wavelengths

Multiferroic BiFeO3 (BFO) shows several phonon modes at infrared (IR) to THz energies, which are expected to carry information on any sample property coupled to crystal lattice vibrations. While macroscopic IR studies of BFO are often limited by single-crystal size, scattering-type scanning near-field optical microscopy (s-SNOM) allows for IR thin film spectroscopy of nanoscopic probing volumes with negligible direct substrate contribution to the optical signal. In fact, polaritons such as phonon polaritons of BFO introduce a resonant tip–sample coupling in s-SNOM, leading to both stronger signals and enhanced sensitivity to local material properties. Here, we explore the near-field response of BFO thin films at three consecutive resonances (centered around 5 THz, 13 THz, and 16 THz), by combining s-SNOM with a free-electron laser. We study the dependence of these near-field resonances on both the wavelength and tip–sample distance. Enabled by the broad spectral range of the measurement, we probe phonon modes connected to the predominant motion of either the bismuth or oxygen ions. Therefore, we propose s-SNOM at multiple near-field resonances as a versatile and very sensitive tool for the simultaneous investigation of various sample properties.

Strain control of phase transition and magnetic property in multiferroic BiFeO3 thin films

BiFeO3 (BFO), a room-temperature antiferromagnetic-ferroelectric multiferroic, is widely researched due to its potential applications for electric-field control of the magnetism. In this work, the strain control of the phase transition and magnetic properties in the BFO/LaAlO3 heterostructures were investigated. The O K edge polarization-dependent X-ray absorption spectroscopy (XAS) spectra show that the Fe 3d level splits into five levels, which proves that the FeO5 pyramid is asymmetric in the highly strained tetragonal-like BFO. The spin canting induced by the asymmetric structure leads to the magnetic moment. Thus, an obvious magnetic signal in the 17-nm-thick BFO thin films was observed by the Quantum Design magnetic property measurement system. With the increase of the BFO film thickness, the clamping effect induced by the substrate becomes weak, further leading to the BFO phase transition. The O K edge polarization-dependent XAS spectra demonstrate that the orbital reconstruction exists at the mixed BFO phase boundaries. Since the orbital reconstructions can induce the strong magnetic coupling, the magnetic order of the different BFO phases will be coupled with each other. It causes a variation of the magnetic property at the phase boundaries or in the BFO phases.

Effect of Thermal Annealing on Photocatalytic Properties of BiFeO3 Thin Films

Improving photocatalytic activity of traditional photocatalysts is one of the actual topics. We fabricated the single phase multiferroic BiFeO3 thin films on SiO2/Si substrates using sol_gel method and analyzed the effect of thermal annealing on photocatalytic properties of the film. Experimental results show that all the samples had a single perovskite rhombohedral structure and are randomly oriented. The estimated band gap values of the films are 2.63, 2.88, and 2.92 eV, respectively. BiFeO3 thin films show good photocatalytic properties in the degradation of methylene blue solution under visible-light irradiation. These results indicate that the microstructure, optical, and photocatalytic properties of BiFeO3 thin films have a strong dependence on annealing temperature. We hope that this work could provide a new route for preparation of highly efficient photocatalysts.

Effect of pre-annealing and annealing temperature on microstructural and optical properties of multiferroic BiFeO3 thin films prepared by chemical solution deposition (CSD)

BiFeO3 thin films has been successfully grown on a quartz substrate using the chemical solution deposition (CSD) method. X-ray diffraction (XRD) used to determine the microstructure of BFO. The morphology and transmittance of BFO thin film were studied using SEM and UV-VIS. Deposition of the BiFeO3 thin film with pre-annealing in air and variation of annealing temperature showed that the increase in annealing temperature caused crystallinity to be higher and crystallite size to increase. High annealing temperature results in a larger grain size and lower thickness. Deposition of the BiFeO3 thin film with pre-annealing variation showed that BiFeO3 with pre-annealing in air had a higher crystallinity compared with in O2. The crystallite size of BiFeO3 with pre-annealing in O2 is greater than pre-annealing in air. BiFeO3 with pre-annealing in air has a smaller grain size and greater thickness compared to preannealing in O2. BiFeO3 band gap with pre-annealing in air and variation of annealing temperature were obtained 1.4-3.5 eV. While the BiFeO3 band gap with pre-annealing variation was obtained from 2.3-2.7 eV.

Activation energy mediated band structure in strained multiferroic BiFeO3 thin films

Although improved magnetic and piezoelectric performances were discovered in BiFeO3 rhombohedral-tetragonal mixed-phase, the associated electronic conduction mechanisms are less investigated. In the present work, we systematically investigated the leakage current and dielectric response as a function of temperature, frequency and bias field for pure rhombohedral and rhombohedral-tetragonal mixed-phase BiFeO3 films grown by pulse laser deposition. The multiferroic properties are demonstrated from their polarization and magnetization hysteresis. Both current-voltage and dielectric relaxation measurements indicate a lower activation energy observed for the mixed-phase BiFeO3 as compared to the rhombohedral-phase one. The present work for the first time indicates the distinguished transport properties as governed by different electronic conduction mechanisms for BiFeO3 at various phases, and improves the understandings associated to the strain engineering related electrical properties.

Tuning Effective Spin Hall Angles via Oxygen Vacancies in Multiferroic BiFeO3‐Based Heterostructures

AbstractRecently, magnetization switching driven by spin–orbit torque (SOT) has been intensely studied. However, it is still a challenge to effectively control the spin Hall angle (SHA) and critical current density for SOT switching. With the help of multiferroic BiFeO3 (BFO) thin films, a method to adjust SHA and the switching current is proposed. The BFO‐based heterostructures with opposite spontaneous polarization fields show huge changes in both perpendicular magnetic anisotropy and SOT‐induced magnetization switching. The variation of the effective SHAs for the heterostructures with opposite polarizations is estimated to be 272%, which can be attributed to the distribution of oxygen vacancies inside the BFO films. The possible applications of these structures in memory and reconfigurable logic devices are also demonstrated.

Growth and physical properties of BiFeO3 thin films directly on Si substrate

Multiferroic BiFeO3 thin films have been widely studied for their intriguing fundamental physics and exotic functional properties. Integration of the BiFeO3 thin films with semiconductor Si is crucial for the practical development of novel electronic and photosensitive devices. Here, we report the fabrication and physical properties of BiFeO3 thin films grown directly on bare Si substrate by laser-molecular beam epitaxy. It was found that a predeposition process performed at room temperature and high vacuum conditions is effective and necessary for obtaining the pure-phase BiFeO3 thin films. X-ray diffraction measurements indicate that the obtained BiFeO3 thin films are in a single-phase polycrystalline state and show improved crystallinity with increasing thickness, and a transmission electronic microscopy image illustrates an interface layer around 3 nm in thickness existing between the BiFeO3 thin films and Si substrate. Electrical measurements show that the BiFeO3 thin films have good ferroelectricity and low leakage current. Moreover, optical properties investigated by spectroscopic ellipsometry demonstrate that the direct band gap of the thin films increases with decreasing thickness.

Expansion of the spin cycloid in multiferroic BiFeO3 thin films

Understanding and manipulating complex spin texture in multiferroics can offer new perspectives for electric field-controlled spin manipulation. In BiFeO3, a well-known room temperature multiferroic, the competition between various exchange interactions manifests itself as non-collinear spin order, i.e., an incommensurate spin cycloid with period 64 nm. We report on the stability and systematic expansion of the length of the spin cycloid in (110)-oriented epitaxial Co-doped BiFeO3 thin films. Neutron diffraction shows (i) this cycloid, despite its partly out-of-plane canted propagation vector, can be stabilized in thinnest films; (ii) the cycloid length expands significantly with decreasing film thickness; (iii) theory confirms a unique [11bar 2] cycloid propagation direction; and (iv) in the temperature dependence the cycloid length expands significantly close to TN. These observations are supported by Monte Carlo simulations based on a first-principles effective Hamiltonian method. Our results therefore offer new opportunities for nanoscale magnonic devices based on complex spin textures.

Towards Oxide Electronics: a Roadmap

Giant photovoltaic effect due to bulk photovoltaic effect observed in multiferroic BiFeO3 thin films has triggered a renewed interest on photoferroelectric materials for photovoltaic applications. Tremendous advance has been done to improve power conversion efficiency (up to up to 8.1%) in photoferroelectrics via absorption increase using narrow bandgap ferroelectrics. Other strategies, as it is the more efficient use of ferroelectric internal electric field, are ongoing. Moreover, as a by-product, several progress have been also achieved on photostriction that is the photo-induced deformation phenomenon. Here, we review ongoing and promising routes to improve ferroelectrics photoresponse.

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

Multiferroic Y-doped BiFeO3 (BY10FO) thin films were deposited on FTO coated glass substrate using sol-gel spin coating technique. Y-doping causes structural distortion without changing the structure of the parent BiFeO3 (rhombohedral: R3c). The M-H hysteresis curve reveals that the BY10FO film exhibits saturation magnetization at a low-coercive field by suppressing the spiral spin modulated structure. The bipolar resistive switching behavior has been investigated on a Ag/BY10FO/FTO hetero-structure through conventional I-V curve measurements and the device can produce an ON/OFF ratio of around 12 over 30 complete testing cycles. The space charge limited current and Schottky barrier emission conduction mechanism play a crucial role in switching the states between HRS and LRS. The impedance spectroscopy analysis at HRS and LRS confirms the significant degradation of resistance from MΩ to kΩ. The switching mechanism in the hetero-structure is due to migration and recombination of oxygen vacancies present in the film. The non-degradation of the Ag/BY10FO/FTO device after several testing cycles confirms that the switching of resistance between ON and OFF states is reproducible, reversible and controllable to be used for possible future non-volatile resistive random access memory application.