Structural and electrical properties of the sol-gel derived multiferroic BiFeO3 monolayer and NiTiO3-BiFeO3 bilayer thin films

In this paper, the multiferroic BeFiO₃ monolayer and NiTiO₃–BiFeO₃ bilayer thin films were fabricated by spin-coating method on the SrRuO₃/n⁺-Si substrate. The structural and ferroelectric properties of multiferroic BeFiO₃ monolayer and NiTiO₃–BiFeO₃ bilayer thin films were investigated. Both multiferroic films showed the typical XRD patterns of the perovskite structure without presence of the second phase. The electrical properties, such as leakage current and remnant polarization, of the NiTiO₃–BiFeO₃ bilayer film were superior to those of BeFiO₃ monolayer film, which those values were 1.94 × 10⁻⁴ A/cm² at electric field of 0.75 MV/cm and 14.05 μC/cm², respectively. This outcome is due to the NiTiO₃–BiFeO₃ bilayer film with a high Schottky barrier height as well as a top NiTiO₃ layer on the BiFeO₃ film inducing the strain-induced polarization rotation and forming the strong domain-wall pinning.     

Fast ferroelectric domain wall motion in BiAlO3

The ferroelectric domain wall motion was investigated in epitaxial PbTiO₃ and BiAlO₃ thin films on SrRuO₃/SrTiO₃ substrates. To determine the switching speeds of two ferroelectric capacitors consisting of PbTiO₃ and BiAlO₃ thin films, the switching currents of the two capacitors were measured as a function of time. The BiAlO₃ thin film showed faster switching behavior than the PbTiO₃ thin film. Data from a piezoelectric force microscope study indicated that the high domain wall motion of the BiAlO₃ thin film is due to its low activation energy.     

Nonvolatile-memory current characteristics of BiFeO3 nanodots switched by applying external bias and force

In the domain structure of ferroelectric materials, the directions of polarization are diverse and uniformly separated, and the current flows along the domain wall due to the domain wall's relatively low bandgap. To control the ferroelectric domain structure of ferroelectric materials, it is possible to apply not only an external electric field but also a flexoelectric field, which is externally applied by an external force. In this study, we controlled the domain structure of BiFeO₃ nanodots using an external electric field as well as a flexoelectric field, and these BiFeO₃ nanodots showed changes in resistance depending on the domain structure and domain wall. The formation of domain walls in BiFeO₃ nanodots caused relatively low-resistance states, whereas when domain walls were removed, BiFeO₃ nanodots showed high-resistance states. The experimental results show that nonvolatile memory devices can be applied by controlling the domain wall in BiFeO₃ nanodots.     

Photoresponse of high quality epitaxial BiFeO3 films grown through hydrothermal method and rapid microwave assisted method

High quality BiFeO₃ film with enhanced remanent polarization and magnetization attracts much interest for its great application potential in information storage and photoelectric devices. Here we grew single-crystal epitaxial BiFeO₃ film on SrTiO₃ substrate through hydrothermal method at low temperature, the film has a quality higher than any reported BiFeO₃ films grown through hydrothermal method. Furthermore, BiFeO₃ film was grown through a 25 min rapid microwave-assisted hydrothermal process. The two BiFeO₃ films exhibited obvious light response to periodically switching on and off of a LED light, and the film grown through microwave-assisted hydrothermal method produced a 4.7V open-circuit voltage exceeding the band gap of BiFeO₃ originating from the ferroelectric photovoltaic effect. These results demonstrate that hydrothermal and microwave-assisted hydrothermal method are simple, inexpensive and rapid to grow high quality epitaxial BiFeO₃ film with potential application in photoelectric detection and photovoltaic.     

The ferroelectric photovoltaic effect of BiCrO3/BiFeO3 bilayer composite films

This work involves an investigation of the structure, ferroelectricity, optical band-gap, photovoltaic spectral response and J-V performance of BiCrO₃/BiFeO₃ bilayer composite films prepared via the solution-gelation technique. It is shown that the enhanced ferroelectric properties are observed for BiCrO₃/BiFeO₃ bilayer composite films by interaction resulting from the coupling between BiFeO₃ and BiCrO₃ layers. The photovoltaic spectral responses of the normalized current for BiCrO₃/BiFeO₃ bilayer composite films presents a noteworthy red-shift towards visible region compared with the pure BiFeO₃ films and the pure BiCrO₃ films. Thus photovoltaic response is attributed to the narrow band-gap of BiCrO₃/BiFeO₃ bilayer composite films. The short circuit current density and open circuit voltage of the BiCrO₃/BiFeO₃ bilayer composite films under white-light illumination are much higher than the values of BiFeO₃ and BiCrO₃ films. The present work provides an available way of controlling photovoltaic response of ferroelectric films and accelerating its application in light sensors, light drivers and ferroelectric photovoltaic cells.     

Synthesis and mechanical properties characterization of multiferroic BiFeO3-CoFe2O4 composite nanofibers

Multiferroic nanofibers with excellent mechanical properties have great potential applications in multifunctional nanodevices. BiFeO₃-CoFe₂O₄ (BFO-CFO) composite nanofibers with different molar ratios were successfully synthesized by sol-gel-based electrospinning method. The mechanical properties of BFO-CFO composite nanofibers were examined by nanoindentation technique, and further investigated by amplitude modulation-frequency modulation (AM-FM) method based on atomic force microscopy (AFM). The results of AM-FM showed that the elastic moduli of BFO-CFO composite nanofibers increased with the increase of CFO ratio, which was consistent with the results of nanoindentation. These results indicated that AFM-based AM-FM is a powerful method for nondestructively investigating the mechanical properties of materials at nanoscale, and that the results of BFO-CFO composite nanofibers are also of practical importance for the future applications of multifunctional nanodevices.     

Optical properties of epitaxial BiFeO3 thin film grown on SrRuO3-buffered SrTiO3 substrate

The BiFeO₃ (BFO) thin film was deposited by pulsed-laser deposition on SrRuO₃ (SRO)-buffered (111) SrTiO₃ (STO) substrate. X-ray diffraction pattern reveals a well-grown epitaxial BFO thin film. Atomic force microscopy study indicates that the BFO film is rather dense with a smooth surface. The ellipsometric spectra of the STO substrate, the SRO buffer layer, and the BFO thin film were measured, respectively, in the photon energy range 1.55 to 5.40 eV. Following the dielectric functions of STO and SRO, the ones of BFO described by the Lorentz model are received by fitting the spectra data to a five-medium optical model consisting of a semi-infinite STO substrate/SRO layer/BFO film/surface roughness/air ambient structure. The thickness and the optical constants of the BFO film are obtained. Then a direct bandgap is calculated at 2.68 eV, which is believed to be influenced by near-bandgap transitions. Compared to BFO films on other substrates, the dependence of the bandgap for the BFO thin film on in-plane compressive strain from epitaxial structure is received. Moreover, the bandgap and the transition revealed by the Lorentz model also provide a ground for the assessment of the bandgap for BFO single crystals.     

Room-temperature magnetic control of ferroelectric polarization and enhanced ferromagnetic properties in 0.8BiFeO3-BaTiO3 ceramic

A multiferroic 0.8BiFeO₃ ??0.2BaTiO₃ (0.8BF-BaT) ceramic was prepared by solid-state reaction to study the magnetic control of its ferroelectric polarization and ferromagnetic properties. The influence of the magnetic field H on the ferroelectric polarization of 0.8BF-BaT ceramic was investigated. The maximum displacement current density Jm and the remanent polarization Pr of 0.8BF-BaT increased by 35% and 20% respectively after it being magnetized, indicating that the polarization was suppressed in the magnetic domain walls. Under a magnetic field of H?=?0.7?T, Jm and Pr decreased by 6.1% and 6.4% respectively, revealing coupling between polarization P and magnetic field H. The magnetic properties of BiFeO₃ were improved by introducing of BaTiO₃ and the magnetic Néel temperature T_N increased by 70?K because of structural effects.     

Effects of CuO Addition on Electrical Properties of 0.6BiFeO3–0.4(Bi0.5K0.5)TiO3 Lead‐Free Piezoelectric Ceramics

0.6BiFeO₃–0.4(Bi_0.5K_0.5)TiO₃ (0.6BF–0.4BKT) ceramic samples with 0.0–4.0 mol% CuO were prepared by the solid‐state reaction. The CuO addition aided the densification of the samples and slightly increased the lattice constant. The relaxor‐like defuse dielectric peak of 0.6BF–0.4BKT became sharper with increasing the CuO content. Polarization–electric field curve of the undoped 0.6BF–0.4BKT was a pinched loop in the as‐sintered state, while that was a square hysteresis with a large remanent polarization of 48 μC/cm² after the thermal quenching, demonstrating a strong domain wall pinning due to defect dipoles. We found that the CuO addition up to 2.0 mol% facilitates the polarization switching in the as‐sintered samples to increase the remanent polarization and the piezoelectric d₃₃ coefficient. The results of the structural and electrical investigations suggested that the copper ion acts as a donor in 0.6BF–0.4BKT by compensating the potassium vacancy created by the evaporation of K₂O during the calcination and sintering processes.     

In situ study of electric-field-induced ferroelectric and antiferromagnetic domain switching in polycrystalline BiFeO3

Antiferromagnetic domain switching induced by ferroelectric polarization switching has previously been observed in situ in both multiferroic BiFeO₃ single crystals and thin films. Despite a number of reports on macroscopic magnetoelectric measurements on polycrystalline BiFeO₃, direct in situ observation of electric-field-induced antiferromagnetic domain switching in this material has not been addressed due to the lack of high-quality samples capable of electrical poling. Here, the electric field control of antiferromagnetic domain texture is identified in polycrystalline BiFeO₃ using in situ neutron diffraction, showing the resultant magnetic domain reorientation induced by an electric field. An antiferromagnetic domain reorientation to a value of 2.2-2.5 multiples of a random distribution (MRD) is found to be induced by an electric field that provides a non-180° ferroelectric-ferroelastic domain texture of 2.2-2.5 MRD along the field direction. The current results show well-controlled coupling of multiferroic domain texturing in single-phase polycrystalline BiFeO₃.     

Growth and characterisation of multiferroic BiFeO3 films with fully saturated ferroelectric hysteresis loops and large remanent polarisations

Multiferroic BiFeO₃ films have been deposited on a number of substrates by RF magnetron sputtering. Two routes were adopted in order to obtain the films of high phase purity and accurate stoichiometry. The first was to sputter films at room temperature and then the BiFeO₃ phase was formed after sintering at various temperatures under controlled ambient atmosphere. The second was to grow BiFeO₃ in-situ at high temperature during sputtering. Although the sintered films grown on SrTiO₃ substrates were epitaxial and showed better texture than the in-situ films, they had much poorer ferroelectric properties, due to the residual traces of intermediate phases formed during heating. Under right growth parameters, the in-situ films grown on the LaNiO_3?x buffered SrTiO₃ showed fully saturated ferroelectric hysteresis loops with large remanent polarisation of 64 μC/cm². Piezoresponse force microscopy (PFM) was used to examine the ferroelectric domain structures. When scanned without DC bias, fine spontaneous domains were observed. Under ±10 V DC bias, PFM confirmed that the domains could be poled and switched.     

Polarization-enhanced photovoltaic response and mechanisms in Ni-doped (Bi0.93Gd0.07)FeO3 ceramics for self-powered photodetector

Multiferroic toxicity-free perovskite BiFeO₃-based materials reveal potentials in photoelectrical conversion, energy harvesting and photodetector. This work highlights polarization-enhanced photovoltaic (PV) effects and mechanisms in B-site Ni-doped (Bi_0.93Gd_0.07)FeO₃ ceramics with indium tin oxide (top electrode) and Au (bottom electrode) thin films. The enhanced PV effects are attributed to the reduced bandgap, polarization-modulated Schottky barrier, increased O 2p-Fe 3d orbital hybridization, and nucleation of domain walls to improve transportation of charge carriers. Improved PV parameters of responsivity (R) ～1.71 × 10^?2 A/W, detectivity (D*) ～9.56 × 10¹¹ Hz^1/2/W (Jones), open-circuit voltage (V_oc) ～0.65 V, and short-circuit current density (J_sc) ～80 μA/cm² were obtained from the ITO/ (Bi_0.93Gd_0.07)(Fe_0.97Ni_0.03)O₃/ Au heterostructure after 2 kV/cm poling under 405 nm irradiation at 10³ W/m² intensity. This work demonstrates that Ni-doped (Bi_0.93Gd_0.07)FeO₃ ceramics can be potential candidates for self-powered UV photodetector.     

Structural phase instability,mixed-phase,and energy band gap change in BiFeO3 under lattice strain effect from first-principles investigation

Lattice strain effects drive a variety of novel functional responses in epitaxial BiFeO₃ thin films and have attracted significant interest and attention from researchers in experimental and theoretical studies. However, the difficulty in designing experimental techniques in addition to facing problems in the first principles approach, such as output accuracy and high computational costs, constitute the discovery of new functional responses in epitaxial BiFeO₃ thin films not entirely understood. Therefore, in this study, we perform a first principles calculation based on the less expensive LDA+U method to investigate the structural phase instability and electronic properties change in BiFeO₃ under the lattice strain effect. The structural phase transformation of BiFeO₃ under volumetric and compressive/tensile lattice strain was examined established on the calculated lower energy phases. Importantly, we demonstrated that the change of crystal structure phases of BiFeO₃ was extremely sensitive to the volumetric and compressive/tensile lattice strain, comparable with various experiment data, as reported in the literature. Moreover, we revealed for the first time from the first principles prediction the coexistence of mixed R-T phases in the region of moderate compressive ζ_in-plane of ?2.9% (e.g. LaAlO₃ substrates with ɑ = 3.79 Å). From the prediction of electronic properties obtained by the LDA+U and PBE0 methods, we found that the energy band gap increased when the compressive in-plane lattice strain is increased while, in contrast, the energy band gap decreased when BiFeO₃ was under the tensile in-plane lattice strain effect. We also demonstrate that our computational technique based on the first principles study was sufficiently accurate enough, helping to speed up the process of designing new materials having an excellent multifunctional response (piezoelectric, magnetic, photovoltaic, spintronic).     

Modified electrical properties of chemical solution deposited epitaxial BiFeO3 thin films by Mn substitution

The effect of Mn substitution on microstructure and electrical properties of epitaxial BiFeO₃ (BFO) thin films grown by an all-solution approach was investigated. Raman analysis reveals that the Mn atoms substitution at Fe sites can result in Jahn-Teller distortion and thus lead to the weakness of long-range ferroelectric order. In addition, the break-down characteristics of BFO thin films are improved with the increase of Mn atoms content, although the leakage current is gradually increased. Meanwhile, the grain size, the dielectric constant and loss are also increased with the increase of Mn content. The P-E hysteresis loops and PUND results demonstrate that the intrinsic ferroelectric polarization is effectively improved with Mn atoms substitution as the grain size increased and Mn atoms play a role of nucleation sites. However, the ferroelectric properties are deteriorated with the excess substituted Mn content due to the higher leakage current.     

Facile synthesis of a novel flower-like BiFeO3 microspheres/graphene with superior electromagnetic wave absorption performances

In recent years, porous or layered magnetic materials have received increasing attention due to their low density and lightweight. In this work, porous BiFeO₃ microspheres and three-dimensional porous BiFeO₃ microsphere-reduced graphene oxide (RGO) composite (3D porous BiFeO₃/RGO) were prepared by one-step etching processing using pure BiFeO₃ particles as precursors. The precursor undergoes dissolution-recrystallization/reduction process, resulting in large amount of BiFeO₃ fragments and graphene hybrid product, which forms 3D porous BiFeO₃/RGO composite. Electromagnetic (EM) absorption performance measurements exhibit that at low thickness of 1.8?mm, porous BiFeO₃/RGO composite can achieve reflection loss (R_L) value up to ?46.7?dB and absorption bandwidth (defined by R_L <?10?dB) exceeding 4.7?GHz (from 12.0 to 16.7?GHz), testifying outstanding microwave absorbing performance. Compared with pure porous BiFeO₃, improved EM wave absorption ability of as-prepared porous BiFeO₃/RGO composite is attributed to interfacial polarization, multiple reflections, scattering, and appropriate impedance matching.     

Electrochromic properties of Au-WO3 nanocomposite thin-film electrode

We observed proton transfer phenomenon of WO₃ in Au-WO₃ nanocomposite thin-film electrode prepared by sputtering deposition method. The Au-WO₃ nanocomposite electrode formed using both the Au and WO₃ targets consisted of a nano-sized Au crystalline phases and a tungsten oxidative phase, indicating the formation of crystalline Au nanophases, as confirmed by X-ray diffraction analysis and X-ray photoelectron spectroscopy. In particular, due to Au metallic nanophases, the modified electrochromic and electrochemical properties of WO₃ were observed. The Au-WO₃ electrode showed a reverse optical modulation with respect to applied potential compared to that of WO₃ electrode.     

Thickness‐dependent domain wall reorientation in 70/30 lead magnesium niobate‐ lead titanate thin films

Continued reduction in length scales associated with many ferroelectric film‐based technologies is contingent on retaining the functional properties as the film thickness is reduced. Epitaxial and polycrystalline lead magnesium niobate‐lead titanate (70PMN‐30PT) thin films were studied over the thickness range of 100‐350 nm for the relative contributions to property thickness dependence from interfacial and grain‐boundary low permittivity layers. Epitaxial PMN‐PT films were grown on SrRuO₃/(001)SrTiO₃, while polycrystalline films with {001}‐Lotgering factors >0.96 were grown on Pt/TiO₂/SiO₂/Si substrates via chemical solution deposition. Both film types exhibited similar relative permittivities of ~300 at high fields at all measured thicknesses with highly crystalline electrode/dielectric interfaces. These results, with the DC‐biased and temperature‐dependent dielectric characterization, suggest irreversible domain wall mobility is the major contributor to the overall dielectric response and its thickness dependence. In epitaxial films, the irreversible Rayleigh coefficients reduced 85% upon decreasing thickness from 350 to 100 nm. The temperature at which a peak in the relative permittivity is observed was the only measured small signal quantity which was more thickness‐dependent in polycrystalline than epitaxial films. This is attributed to the relaxor nature present in the films, potentially stabilized by defect concentrations, and/or chemical inhomogeneity. Finally, the effective interfacial layers are found to contribute to the measured thickness dependence in the longitudinal piezoelectric coefficient.     

Oxygen-vacancy-controlled magnetic properties with magnetic pole inversion in BiFeO3-based multiferroics

Oxygen vacancies which are generally present in ferrite oxide may significantly affect their magnetic properties. A comprehensive understanding of the relationship between oxygen vacancies and magnetism is of great importance. In this work, we report an oxygen vacancy concentration dependence of magnetism in a single-phase multiferroic BiFeO₃ (BFO)-based system. The BiFeO₃-DyFeO₃ (BDFO) solid solution is synthesized with controlled oxygen vacancies and is characterized by X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and AC impedance spectroscopy. The magnetic properties, especially magnetic pole inversion, are found to be highly dependent on oxygen vacancy concentration. The oxygen vacancies generate a Weiss molecular field on the Dy³⁺ ions in the magnetic field range of 570-1000 Oe depending on the oxygen vacancy concentration, and result in a residual net magnetization by breaking the balance between the two nearly antiparallel spin lattices of Fe³⁺ ions. This work demonstrates an effective way to control oxygen vacancies and thereby the magnetic properties and sheds light on the relationship between oxygen vacancies and magnetism in BFO-based multiferroics.     

Effect of SrRuO3 layer thickness on electrical properties of Pb(Zr0.52Ti0.48)O3/SrRuO3 superlattices

Artificial ferroelectric superlattices have received considerable attention because of the potential to realize macro-performance adjustments and bring exotic phenomena. In recent years, it has been reported that metallic SrRuO₃ with single-unit-cell thickness can be used as the constituent material of ferroelectric superlattices, but the effect of SrRuO₃ thickness on ferroelectric properties is not clear. In this study, Pb(Zr_0.52Ti_0.48)O₃/SrRuO₃ (PZT/SRO) ferroelectric/metallic superlattices with the thickness of SRO layers ranging from 1 to 3 unit cells have been grown on Nb:SrTiO₃ substrates by pulsed laser deposition (PLD), and their structural and electrical properties are investigated. The experimental results show that the electrical properties of the PZT/SRO superlattices are greatly affected by the thickness of SRO layers, and the PZT₁₀/SRO₂ superlattice possesses the best electrical properties. The X-ray photoelectron spectroscopy (XPS) results show that oxygen vacancies are accumulated at the PZT/SRO interfaces, which can reduce the depolarization fields and the leakage current. The results demonstrate that the thickness of metallic conductive oxide in the ferroelectric/metallic superlattices is a key factor affecting their ferroelectric properties.     

Growth and electrical properties of high-Curie point rhombohedral Mn-Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 thin films

High-quality ternary relaxor ferroelectric (100)-oriented Mn-doped 0.36Pb(In_1/2Nb_1/2)O₃-0.36Pb(Mg_1/3Nb_2/3)O₃-0.28PbTiO₃ (Mn-PIMNT) thin films were grown on SrRuO₃-buffered SrTiO₃ single-crystal substrate in a wide deposition temperature range of 550-620°C using the pulsed laser deposition method. The phase structure, ferroelectric, dielectric, piezoelectric properties, and nanoscale domain evolution were studied. Under the deposition temperature of 620°C, the ferroelectric hysteresis loops and current-voltage curves showed that the film owned significantly enhanced remnant ferroelectric polarization of 34.5 μC/cm² and low leakage current density of 2.7 × 10⁻¹⁰ A/cm². Moreover fingerprint-type nanosized domain patterns with polydomain structures and well-defined macroscopic piezoelectric properties with a high normalized strain constant of 40 pm/V was obtained. Under in situ DC electric field, the domain evolution was investigated and 180° domain reversal was observed through piezoelectric force microscope. These global electrical properties make the current Mn-PIMNT thin films very promising in piezoelectric MEMS applications.