BiFeO3 Films Research Articles

Strain tuning of ferroelectric and optical properties of rhombohedral-like BiFeO3 thin films on SrRuO3-buffered substrates

Rhombohedral-like (R-like) BiFeO3 (BFO) thin films with a SrRuO3 (SRO) buffer layer have been deposited on a range of lattice mismatched substrates, including single crystalline (001)-oriented LaAlO3 (LAO), SrTiO3 (STO), MgO and TiN-buffered Si substrates. A wide range of strain states in BFO films have been achieved on these substrates. Ferroelectric measurements indicate that the film strain state affects the polarization of the BFO films. The bandgap of R-like BFO films follows the trend that highly compressive strain in BFO thin films leads to a larger bandgap. This study indicates that the physical properties including bandgap and ferroelectricity of BFO films, can be effectively tuned on large lattice mismatch substrates even with partial strain relaxation. Furthermore, the integration of ferroelectric BFO on SRO-buffered Si substrates suggests the possible application of strained BFO in Si-based electronic and optical devices.

Morphology-induced spin frustration in granular BiFeO3 thin films: Origin of the magnetic vertical shift

Pronounced room temperature vertical shifts in the magnetic hysteresis loops of granular, highly polycrystalline and ferromagnetic-like BiFeO3 thin films are observed upon field-cooling from a temperature above the Néel temperature of bulk BiFeO3. This is ascribed to the interplay between the preferential alignment, established by the field-cooling process, of the net magnetic moment, which arises from uncompensated antiferromagnetic spins, and the pinning of a fraction of these spins at the particle boundaries. Conversely, field-cooling of an epitaxially grown BiFeO3 film results in no vertical shift, confirming the effective role played by the particle boundaries (i.e., morphology) of the granular-like BiFeO3 films in the process of spin frustration.

Improving the photovoltaic effect by resistive switching

The mismatch between band structures of a light-absorptive layer and hole or electron transport layers will significantly deteriorate the performance of photovoltaic devices, which is generally alleviated by inserting a mitigating layer. In this report, we propose an alternative strategy to improve the efficiency by using resistive switching, which may decrease defect density in bulk and the lower barrier height at the interface due to the migration of defects to the interface under a certain electric field. By using a BiFeO3 film as a model light harvesting layer, a TiO2 mesoporous layer as an electron transport layer, and NiOx as a hole transport layer, bipolar resistive switching behavior has been observed. By setting the device in the low resistance state under certain applied voltages, performance has been significantly improved. Compared with the virgin device, the highest short-circuit current Jsc increases 2.3 times from 2.38 to 5.66 μA cm−2 and open-circuit voltage Voc increases 1.35 times from 0.39 to 0.525 V.

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 Co0.9Fe0.1 exists such that the magnetic anisotropy of the ferromagnet can be effectively controlled by engineering the orientation of the antiferromagnetic-spin axis.

Thermooptic properties of epitaxial BiFeO3 films with different orientations

We have fabricated epitaxial BiFeO3 (BFO) films with various orientations on a SrTiO3 substrate by chemical solution deposition, and evaluated their optical and thermooptic properties by spectroscopic ellipsometry. The refractive index of the epitaxial BFO films was estimated to range from 2.620 to 2.635 at a wavelength of 1550 nm at room temperature using six Gaussian oscillators, and a thermooptic coefficient ranging from −0.31 × 10−4 to −0.81 × 10−4 K−1 was obtained at a wavelength of 1550 nm. The refractive index increased with temperature. In addition, the rate of increase in refractive index changed at around the Neel temperature. It is suggested that this change is caused by the magnetovolume effect, because a similar change, the d-spacing change at around the Neel temperature, was also obseved. These results suggest that the BFO films have a high potential for application as an optical material with a high refractive index and that the effectual refractive index change can be controlled by the thermal expansion coefficient, even in the same material.

The effect of crystal distortion and domain structure on piezoelectric properties of BiFeO3 thin films

BiFeO3 films are promising piezoelectric materials for vibration energy harvesting. In this study, the effects of the crystallographic symmetry and domain structure, which significantly depends on film thickness owing to the strain from the substrate, on the direct piezoelectric properties of (100) BiFeO3 epitaxial films were investigated. The BiFeO3 epitaxial films with thicknesses from 100 to 1000 nm were grown by rf magnetron sputtering. The crystal and domain structures of the films were characterized by reciprocal space mapping of X-ray diffraction and piezoelectric force microscopy. It was found that the highest e31,f coefficient of −3.9 C/m2 was obtained on the 600-nm-thick film, which has an intermediate structure between tetragonal and rhombohedral. This suggests that the phase transition by lattice relaxation is effective for improving the piezoelectric properties of BiFeO3 films.

Novel type of spin cycloid in epitaxial bismuth ferrite films

We report on the novel cycloids that can be realized in epitaxial BiFeO3 films. We found two featured (1 1 0) and (11¯0) plane cycloids in the (1 1 0) oriented BiFeO3 film and considered the action of compressive and tensile deformations on the areas of the cycloids stability. Our findings show that additional magnetic anisotropy induced by the strains allocates cycloids with the definite directions of spin rotation and electric polarization and change the properties of space modulated structures. Until now, it was believed that polarization is linked with antiferromagnetic order throughout the plane of spin rotation, namely it was considered that polarization always lies in the cycloid spin rotation plane. We show that due to the action of epitaxial strains spin rotational plane of the cycloid deviates from the plane containing intrinsic spontaneous polarization Ps.

Direct observation of room-temperature out-of-plane ferroelectricity and tunneling electroresistance at the two-dimensional limit

Out-of-plane ferroelectricity with a high transition temperature in nanometer-scale films is required to miniaturize electronic devices. Direct visualization of stable ferroelectric polarization and its switching behavior in atomically thick films is critical for achieving this goal. Here, ferroelectric order at room temperature in the two-dimensional limit is demonstrated in tetragonal BiFeO3 ultrathin films. Using aberration-corrected scanning transmission electron microscopy, we directly observed robust out-of-plane spontaneous polarization in one-unit-cell-thick BiFeO3 films. High-resolution piezoresponse force microscopy measurements show that the polarization is stable and switchable, whereas a tunneling electroresistance effect of up to 370% is achieved in BiFeO3 films. Based on first-principles calculations and Kelvin probe force microscopy measurements, we explain the mechanism of polarization stabilization by the ionic displacements in oxide electrode and the surface charges. Our results indicate that critical thickness for ferroelectricity in the BiFeO3 film is virtually absent, making it a promising candidate for high-density nonvolatile memories.

Conductive tail-to-tail domain walls in epitaxial BiFeO3 films

The complex conductive behavior of ferroelectric domain walls is attracting more and more attention for their potential application as an independent nanoelectronic component. For the (001) epitaxial BiFeO3 films, we find that the domain wall conductivity varies among 71° domain walls, with tail-to-tail (T-T) domain walls more conductive than head-to-head (H-H) and head-to-tail (H-T) ones. Furthermore, it is observed that most of the conductive areas are composed of two parallel lines around the T-T domain walls. These experimental results can be well simulated by our theoretical model based on the polarization configuration and a tunneling mechanism. Our work will help to understand the mechanism of domain wall conductance in ferroelectric materials and further promote the usage of domain walls in advanced nano-devices.

Ferroelectric polarization reversal tuned by magnetic field in a ferroelectric BiFeO3/Nb-doped SrTiO3 heterojunction

Interfacial resistive switching of a ferroelectric semiconductor heterojunction is highly advantageous for the newly developed ferroelectric memristors. Moreover, the interfacial state in the ferroelectric semiconductor heterojunction can be gradually modified by polarization reversal, which may give rise to continuously tunable resistive switching behavior. In this work, the interfacial state of a ferroelectric BiFeO3/Nb-doped SrTiO3 junction was modulated by ferroelectric polarization reversal. The dynamics of surface screening charges on the BiFeO3 layer was also investigated by surface potential measurements, and the decay of the surface potential could be speeded up by the magnetic field. Moreover, ferroelectric polarization reversal of the BiFeO3 layer was tuned by the magnetic field. This finding could provide a method to enhance the ferroelectric and electrical properties of ferroelectric BiFeO3 films by tuning the magnetic field.

Control of Domain Structures in Multiferroic Thin Films through Defect Engineering.

Domain walls (DWs) have become an essential component in nanodevices based on ferroic thin films. The domain configuration and DW stability, however, are strongly dependent on the boundary conditions of thin films, which make it difficult to create complex ordered patterns of DWs. Here, it is shown that novel domain structures, that are otherwise unfavorable under the natural boundary conditions, can be realized by utilizing engineered nanosized structural defects as building blocks for reconfiguring DW patterns. It is directly observed that an array of charged defects, which are located within a monolayer thickness, can be intentionally introduced by slightly changing substrate temperature during the growth of multiferroic BiFeO3 thin films. These defects are strongly coupled to the domain structures in the pretemperature-change portion of the BiFeO3 film and can effectively change the configuration of newly grown domains due to the interaction between the polarization and the defects. Thus, two types of domain patterns are integrated into a single film without breaking the DW periodicity. The potential use of these defects for building complex patterns of conductive DWs is also demonstrated.

Characterization of domain distributions by second harmonic generation in ferroelectrics

Domain orientations and their volume ratios in ferroelectrics are recognized as a compelling topic recently for domain switching dynamics and domain stability in devices application. Here, an optimized second harmonic generation method has been explored for ferroelectric domain characterization. Combing a unique theoretical model with azimuth-polarization-dependent second harmonic generation response, the complex domain components and their distributions can be rigidly determined in ferroelectric thin films. Using the proposed model, the domain structures of rhombohedral BiFeO3 films with 71° and 109° domain wall, and, tetragonal BiFeO3, Pb(Zr0.2Ti0.8)O3, and BaTiO3 ferroelectric thin films are analyzed and the corresponding polarization variants are determined. This work could provide a powerful and all-optical method to track and evaluate the evolution of ferroelectric domains in the ferroelectric-based devices.

Long range ordering of 71° domain walls in epitaxial BiFeO3 thin films

Substrate-related aspects are often utilized to tune domain architectures in ferroelectric thin films. In this work, however, we report on the role of background pressure during film growth in stabilizing certain domain states. The growth of BiFeO3 films in high-background pressure conditions results in c-oriented films in conjunction with a long-range ordering of 71° domain walls. The importance of high-pressure is highlighted by replacing half of the oxygen background gas with argon. The proposed mechanism takes into account the enhanced surface diffusivity and screening of depolarization fields during high-pressure growth.

Dielectric properties and resistance mechanism of BiFeO3 films with (1 1 0) orientation

The Pt/BiFeO3/La0.5Sr0.5CoO3/STO (Pt/BFO/LSCO/STO) heterojunction capacitors were fabricated on (1 1 0) SrTiO3 (STO) substrates by off-axis RF magnetron sputtering. The structure and physical properties of BiFeO3 thin films were investigated. It was found that the BFO film showed single phase (1 1 0) epitaxial perovskite structure with high crystal quality. The Pt/BFO/LSCO capacitor exhibited a saturated butterfly loop and a stable bipolar resistance switching behavior after post-annealing. The current-voltage characteristic of Pt/BFO/LSCO heterostructure is well explained by the space-charge-limited conduction mechanism.

Dry lubrication of friction on ferroelectric BiFeO3 film

Epitaxial BiFeO3 film shows excellent ferroelectric properties, which is not only studied extensively in magnetoelectric modulation but also shows good tribological performance. In our experiment, we find that the surface of BiFeO3 film can be more lubricant with the application of bias voltages or stress through the AFM probe with a reduction (up to ∼40%) of the friction between the sample and the probe tip. Based on the Prandtl-Tomlinson model, the mechanism of the dry lubrication on BiFeO3 film originates from the reduction of van der Waals force between sample surface and probe tip. The findings here could be advantageously used in promoting the performance and durability of Micro/Nano Electromechanical devices based on ferroelectric thin films.

Direct piezoelectric properties of BiFeO3 epitaxial films grown by combinatorial sputtering

We have proposed that BiFeO3 films are suitable for piezoelectric vibration energy harvesting applications, because of the low dielectric permittivity and excellent ferroelectricity. In this paper, the improvement of the piezoelectric properties of the domain-engineered BiFeO3 films was discussed. The (100) epitaxial films were prepared at growth temperatures between 500 and 650°C on Nb-doped (100) SrRuO3 single crystals. The combinatorial sputtering method was employed in order to prevent Bi defects during high-temperature deposition. The domain structure and e31,f piezoelectric coefficients of the films were characterized by a piezoelectric force microscopy and direct piezoelectric response, respectively. The highest e31,f coefficient of -4.3 C/m2 and figures of merit of 14 GPa were obtained in the film deposited at 600°C, which has the highest domain wall density. This suggests that the introduction of the domain walls is effective to enhance the piezoelectric properties of BiFeO3 films.

In situ observation of atomic movement in a ferroelectric film under an external electric field and stress

Atomic movement under application of external stimuli (i.e., electric field or mechanical stress) in oxide materials has not been observed due to a lack of experimental methods but has been well known to determine the electric polarization. Here, we investigated atomic movement arising from the ferroelectric response of BiFeO3 thin films under the effect of an electric field and stress in real time using a combination of switching spectroscopy, time-resolved X-ray microdiffraction, and in situ stress engineering. Under an electric field applied to a BiFeO3 film, the hysteresis loop of the reflected X-ray intensity was found to result from the opposing directions of displaced atoms between the up and down polarization states. An additional shift of atoms arising from the linearly increased dielectric component of the polarization in BiFeO3 was confirmed through gradual reduction of the diffracted X-ray intensity. The electric-field-induced displacement of oxygen atoms was found to be larger than that of Fe atom for both ferroelectric switching and increase of the polarization. The effect of external stress on the BiFeO3 thin film, which was controlled by applying an electric field to the highly piezoelectric substrate, showed smaller atomic shifts than for the case of applying an electric field to the film, despite the similar tetragonality.

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.

Electronic excitation induced modifications in the ferroelectric polarization of BiFeO3 thin films

We report the effect of electronic excitations induced modifications in the ferroelectric polarization in BiFeO3 (BFO) multiferroic films grown on 0.2% Nb doped SrTiO3 (SNTO) substrates by pulsed laser deposition. The BFO/SNTO films were irradiated with 200 MeV Ag+15 ions with ion fluences of 5 × 1010 to 5 × 1012 ions/cm2 and characterized by using X–ray diffraction (XRD), atomic force microscopy (AFM), ferroelectric polarization and near-edge X-ray absorption fine structure (NEXAFS) measurements. The XRD and AFM results show the creation of structural defects and oxygen vacancies by ion irradiation. Such defects induced enhancement in structural strain and reduction in mobility of charge carriers leading to the improvement in ferroelectric behaviour in BFO films. In addition, the local electronic structures investigated using NEXAFS studies reveal the significant change in spectral features suggesting the role of Bi-O and Fe-O hybridizations that modifies the ferroelectric behaviour of BFO films. These results are used to understand the mechanism.

Memory improvement in lead-free BiFeO3 ferroelectric with high-k Al2O3 buffer layer for non-volatile memory applications

We report the deposition of ferroelectric lead-free BiFeO3 perovskite film on Si (100) substrate by RF magnetron sputtering using Al2O3 as buffer layer. X-ray diffraction and multiple-angle ellipsometric analysis show the pure ferroelectric phase and maximum refractive index of the BiFeO3 film at the annealing temperature of 500 °C. For different annealing temperature of Al2O3 film, amorphous film structure and refractive index in the range 1.7–1.77 has been observed. Metal–ferroelectric–metal (MFeM), metal–ferroelectric–silicon (MFeS), metal–insulator–silicon (MIS) and metal–ferroelectric–insulator–silicon (MFIS) structure have been fabricated with BiFeO3 ferroelectric and Al2O3 insulator layer to investigate the electrical characteristics. Improvement in the memory window from 2.3 V in MFeS structure to 4.8 V in MFeIS structure has been observed with 10 nm buffer layer at the annealing temperature of 500 °C. The MFe(100 nm)I(10 nm)S structure annealed at 500 °C shows the breakdown voltage of 38 V and no degradation in the polarization charge upto the read-write cycles of 108.