BiFeO3 Phase Research Articles

Pure phase BiFeO3 thin films sputtered over Si: A new route towards high magnetization

We have investigated the structural and magnetic properties of BiFeO3 (BFO) thin films grown over (100)-oriented Si substrates by rf magnetron sputtering in a new route under O2 free low pressure Ar atmosphere. Single-phase BFO films were deposited in a heated substrate and post-annealed in situ. The new routed allows high deposition rate and produce polycrystalline BFO pure phase, confirmed by high resolution X-ray diffraction. Scanning electron and atomic force microscopy reveal very low surface roughness and mean particle size of 33 nm. The BFO phase and composition were confirmed by transmission electron microscopy and line scanning energy-dispersive X-ray spectroscopy in transmission electron microscopy mode. The surface chemistry of the thin film, analyzed by X-ray photoelectron spectroscopy, reveals the presence of Fe3+ and Fe2+ in a 2:1 ratio, a strong indication that the film contains oxygen vacancies. An hysteretic ferromagnetic behavior with room temperature high saturation magnetization ∼165 × 103 A/m was measured along the film perpendicular and parallel directions. Such high magnetization, deriving from this new route, is explained in the scope of oxygen vacancies, the break of the antiferromagnetic cycloidal order and the increase of spin canting by change in the surface/volume ratio. Understanding the magnetic behavior of a multiferroic thin films is a key for the development of heterogeneous layered structures and multilayered devices and the production of multiferroic materials over Si substrates opens new possibilities in the development of materials that can be directly integrated into the existent semiconductor and spintronic technologies.

Synthesis, optical and magnetic behavior of (BiFeO3)1−x(α-Fe2O3)x nanocomposites

(BiFeO3)1−x(α-Fe2O3)x nanocomposites were synthesized from dried gels of BiFeO3 and α-Fe2O3. Samples with x = (0.00 (BiFeO3), 0.25, 0.50 and 1.00 (α-Fe2O3)) were studied using X-rays diffractions (XRD), UV–vis spectroscopy, photoluminescence spectroscopy (PL), electron spin resonance (ESR) and vibrating sample magnetometer (VSM). Amounts of α-Fe2O3 phase were 23 and 35% for samples x = 0.25 and 0.50, respectively. Microstrain of BiFeO3 phase tended to decrease with increasing α-Fe2O3. Optical band gap reduced from 2.42 eV for BiFeO3 to 2.35 eV for sample x = 0.25 and then increased to 2.56 eV for sample x = 0.50. From PL, intensity of near band emission peak of BiFeO3 increased with increasing α-Fe2O3 content. From ESR and VSM, the g-value and magnetization saturation were enhanced with embedding of α-Fe2O3 into BiFeO3.

Enhanced Photocatalytic Activity in Bi1-xBa x FeO3 Prepared by a PEG400 Assisted Sol–Gel Method

Ferroelectric Bi1-xBa x FeO3 nanoparticles for x = 0, 0.01, 0.03, 0.05 and 0.10 were synthesized by a polyethylene glycol 400 (PEG400) assisted sol–gel method. X-ray diffraction reveals that Bi1-xBa x FeO3 nanoparticles exhibit a distorted rhombohedral structure with the R3c space group, and the diffraction peaks shift upon incorporation of Ba. Transmission electron microscope analysis shows that the particle size of Bi1-xBa x FeO3 nanoparticles is in the range of 30–60 nm, decreasing with an increase in Ba content. Bi1-xBa x FeO3 nanoparticles have band gaps in the range of 1.68–2.0 eV, which are capable of responding to visible light irradiation. The rate of the photocatalytic degradation of Bi1-xBa x FeO3 nanoparticles for x = 0.03 to methyl orange (MO) dye achieves about 81% under visible light irradiation for 3 h, which is higher than that of 66% for pure phase BiFeO3 (BFO). Moreover, the effects of Ba2+ modification on the band gap of BFO crystallites have been investigated and discussed.

Preparation parameters optimization and structure investigation of multiferroic bismuth ferrite

Multiferroic bismuth ferrite, BiFeO3 (BFO), was prepared using conventional solid state reaction method. The obtained powders were thermally treated at different temperatures (600–850 °C) and times (15 min–15 h) with two heating regimes (slow and fast) to investigate the optimum synthesis conditions. Rietveld refinement was applied to the X-ray diffraction data and Raman spectroscopy has been used to verify the phase purity. Both the temperature and time of the heat treatment were critical parameters to obtain the high BFO phase content. Sample of 95 wt% of BFO phase has been achieved by using fast heating/cooling regime of 30 min at 800 °C and leaching with HNO3 to eliminate the impurity phases Bi2Fe4O9 and Bi25FeO40. Upper and lower calcinations temperatures and times result in higher content of secondary phases. The Raman active modes confirmed the presence of the single phase of BFO. Additional physical measurement (electrical and magnetic) prove the multiferroic feature of BFO. The estimated high activation energy (0.89 eV) indicates the antiferromagnetic order. The magnetic hysteresis loops gives indication for the existence of a weak canting moment.

Temperature controlled evolution of monoclinic to super-tetragonal phase of epitaxial BiFeO3 thin films on La0.67Sr0.33MnO3 buffered SrTiO3 substrate

Epitaxial BiFeO3 thin films of 130nm were deposited by pulsed laser deposition (PLD) technique on La0.67Sr0.33MnO3 buffered SrTiO3 (001) substrate at various temperatures under different ambient oxygen pressures. Reciprocal space mapping reveals that, with decreasing temperature and oxygen pressure, the broadly reported monoclinic phase (MA) of BiFeO3 thin film initially transforms to a tetragonal phase (T1) with c/a =1.05 (1) in a narrow girth of deposition condition and then to a super-tetragonal phase (T2) with giant c/a = 1.24 (1), as confirmed by reciprocal space mapping using high resolution x-ray diffraction. The surface morphology of the films reveals the island growth of the BiFeO3 films deposited at low temperatures. We propose that the transformation from monoclinic to the super-tetragonal phase is essentially due to the manifestation of excess local strain as a result of the island growth. This study offers a recipe to grow the super-tetragonal phase of BiFeO3, with giant c/a =1.24 (1) which exhibits exceptionally large ferroelectric polarization, on ferromagnetic layer La0.67Sr0.33MnO3. This phase of BiFeO3 can be utilized for the ferroelectric control of magnetism at the interface of BiFeO3 and La0.67Sr0.33MnO3.

MgFe2O4 phase effect on magnetic properties of BiFeO3 in ceramic composites of (1-x) BiFeO3–x MgFe2O4

A two-stage ceramic technology was used for producing a family of (1-x) BiFeO3 - x MgFe2O4 heterophase systems. The study of the magnetic hysteresis loop revealed the presence of a ferrimagnetic response. The experimentally obtained concentration and temperature dependences of the magnetic permeability made it possible to consider the influence of the BiFeO3 phase on the magnetic response of the studied samples.

Tensile stress effect on epitaxial BiFeO3 thin film grown on KTaO3

Comprehensive crystal structural study is performed for BiFeO3 (BFO) film grown on KTaO3 (KTO) substrate using transmission electron microscopy (TEM) and x-ray diffraction (XRD). Nano-beam electron diffraction (NBED) combined with structure factor calculation and high resolution TEM images clearly reveal that the crystal structure within BFO thin film is rhombohedral BFO, i.e., bulk BFO phase. Epitaxial relationship found by NBED indicates the BFO film grows in a manner that minimizes lattice mismatch with KTO. It further suggests BFO film is under slight biaxial tensile stress (~0.35%) along in-plane direction. XRD reveals BFO lattice is under compressive stress (~1.6%), along out-of-plane direction as a result of the biaxial tensile strain applied along in-plane direction. This leads to Poisson’s ratio of ~0.68. In addition, we demonstrate (1) why hexagonal notation rather than pseudocubic one is required for accurate BFO phase evaluation and (2) a new XRD method that shows how rhombohedral BFO can readily be identified among other phases by measuring a rhombohedral specific Bragg’s reflection.

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.

Effects of dopant induced defects on structural, multiferroic and optical properties of Bi1−xPbxFeO3 (0 ≤ x ≤ 0.3) ceramics

Bi1−xPbxFeO3 (0 ≤ x ≤ 0.3) has been characterized in detail with an aim to identify role of defect such as dopant, various vacancies, grain boundaries etc, and their effect on structural, optical and multiferroic properties. Structural analysis revealed that Pb substitution transforms the rhombohedral phase of BiFeO3 to the pseudocubic phase for x ≥ 0.15, consistently all vibrational Raman modes associated with the rhombohedral phase are found disappeared. Optical response revealed weakening of the d–d transitions with Pb addition indicating change in the Fe atoms environment consistent with the transition from non-centrosymmetric to the centrosymmetric structure. Transport and dielectric responses are explained in terms of hopping due to the presence of defects like oxygen vacancies and grain boundary conduction. In the high temperature regime, grain boundary conduction led to decrease in resistivity with the presence of a hump that is associated with hopping conduction. Extrinsic contributions in the transport properties correlate well with dielectric response. Magnetic and ferroelectric responses are also presented where role of oxygen vacancies defects has been clearly identified.

A systematic study of structural, magnetic and electric properties of perovskite-spinel composites prepared by sol-gel technique

Multiferroics composite of perovskite BiFeO3 and spinel ferrites NiFe2O4/ZnFe2O4/CoFe2O4 with different composition were prepared by sol-gel method. Detailed investigations were made on the structural, magnetic and ferroelectric properties of these composites. The X-Ray Diffraction pattern confirms the formation of distorted perovskite and spinel phases of BiFeO3 and NiFe2O4, ZnFe2O4, CoFe2O4 respectively. Transmission Electron Microscopy (TEM) images reveals the particle size and the elemental idea of phase formation. The particle sizes calculated using TEM of NiFe2O4, ZnFe2O4, CoFe2O4 are 10–20 nm, 20–30 nm, 15–25 nm respectively and these are compatible with XRD results. The results of Scanning Electron Microscopy (SEM) images reveal that all the samples exhibit a very uniform distribution of perovskite and spinel phases in composites. The modified microstructure of composites is effective in reducing the leakage of electric charges which occurs due to a chain formation of the spinel-ferrite phase particle, so the electrical, magnetic and ferroelectric properties of composites may improve with spinel ferrites content.

Magneto-dielectric properties studies of the matrix composite [SrFe12O19(SFO)1-X – BiFeO3(BFO)X

The effects of different BiFeO3 (BFO) additions (0, 20, 40, 60 and 80 wt%) on the SrFe12O19 hexaferrite ceramic matrix (SFO) have been studied by Complex Impedance Spectroscopy (CIS), Mössbauer spectroscopy, magnetic hysteresis and X-Ray Diffraction (XRD) analysis. The solid state reaction method was used for the production of the SFO and BFO, the phase formation was confirmed by the XRD and Rietveld refinement, where the obtained fitting presented an effective refinement. The diffractogram for BFO-SFO composites presented all peaks in agreement with the BFO and SFO phases. The thermo-activated process was studied by the CIS and the Arrhenius equation in the composite series of BFO-SFO with SFO presenting higher values of activation energy between the analyzed samples and the frequency and temperature range studied was 1–10 MHz and 160 °C to 300 °C, respectively. The addition of BFO in the SFO ceramic matrix lead to an improvement of the thermal-stability of the composite, where a signal inversion of Temperature Coefficient of Capacitance (TCC), in the composites, was observed. The Nyquist plots showed the presence of a semicircle that was fitted by an equivalent circuit model using R - CPE association, showing the same profile in all samples. The Mössbauer spectrum of the composites demonstrate the typical five Fe3+ site for M-type hexagonal ferrites even after the addition of BFO in a ceramic matrix. The addition of BFO in the formation of composites, with SFO, showed interesting results in the radio frequency range and presented magnetic properties that can be explored in applications in the area of telecommunications, medicine and military industry.

The structure and magnetic properties of pure single phase BiFeO3 nanoparticles by microwave-assisted sol-gel method

The pure single phase BiFeO3 nanoparticles have been prepared by microwave-assisted sol-gel process and its structure and magnetic properties have been studied. X-ray diffractometer, infrared and transmission electron microscope results show that the synthesized BiFeO3 nanoparticles at the calcining temperature of 450 °C exhibits a rhombohedrally distorted perovskite structure without secondary phase and the size is about 40 nm. Meanwhile, the pure single phase BiFeO3 particles exhibit typical ferromagnetic properties at room temperature and the obvious large exchange bias phenomenon at 60 K. A exchange bias field (HE) of 302 Oe at 60 K for pure single phase BiFeO3 nanoparticles at calcining temperature 450 °C after field cooling at 20 KOe has been observed. The MT curve at H = 100 Oe shows that the FC and ZFC magnetization curves start to differ in the temperature range from 60 K to 300 K revealing spin glass behavior of BiFeO3 nanoparticles at calcining temperature 450 °C, 500 °C and 550 °C. The ferromagnetic behavior is attributed to size of effects, which partially destroy the long-wavelength cycloid spin structure expected in bulk BiFeO3. The exchange bias effect in single crystalline BiFeO3 nanoparticles arises from co-existence of different magnetic phases of an antiferromagnetic core and a ferromagnetic surface. The ferromagnetic, exchange bias effect and spin glass behavior of BiFeO3 nanoparticles are assigned to the uncompensated or canted spins at the ferromagnetic surface and an antiferromagnetic core of BiFeO3 nanoparticles.

Chelation and calcination promoted preparation of perovskite-structured BiFeO3 nanoparticles: a novel magnetic catalyst for the synthesis of dihydro-2-oxypyrroles

Perovskite-structured Bismuth ferrite (BiFeO3) nanoparticles as a novel heterogeneous catalyst were designed by an auto combustion route using a different chelating agent and calcination temperature. The effect of different chelating agents like disaccharide (sucrose), α-hydroxy acid (citric acid, tartaric acid), amide (urea) and calcination (150–750 °C) temperature on structure and the catalytic performance of BiFeO3 have been analyzed. The catalytic performance of BiFeO3 has been increased by modifying its synthesis with the addition of suitable organic compound and calcination. BiFeO3 synthesized without the use of chelating agent gave very poor yield, i.e., 36.89%. The augmented effect of the chelating agent on the catalytic performance of BiFeO3 was obtained in the order of blank 750 °C. The calcination temperature results in augmentation in yield of approximately 30% with model reaction on increasing temperature from 150 to 650 °C. Different calcination temperatures (150–750 °C) have been employed to obtain single phase BiFeO3 nanoparticles. All synthesized BiFeO3 nanoparticles were fully characterized by FT-IR; XRD; VSM; BET; TGA; XPS and Raman spectroscopy. For the very first time ever we have used them as a recyclable magnetic nanocatalyst in the formation of highly substituted dihydro-2-oxypyrrole by using one-pot, three-component reaction of DMAD, aniline and formaldehyde in methanol at room temperature with 63–88% yield. All the synthesized oxypyrroles have been characterized by various spectroscopic techniques.

Structural, Magnetic and Photocatalytic Properties of BiFeO3 Nanoparticles

Single phase BiFeO3 nanoparticles as a visible light photocatalyst were successfully synthesized by thermal decomposition of the glyoxylate precursor. The glyoxylate precursors were formed by the redox reaction between ethylene glycol and nitrate ions. The phase evolution, structure and optical properties of BiFeO3 nanoparticles were characterized by X-ray diffraction, electron microscopy and UV–Vis spectroscopy methods. The BiFeO3 nanoparticles showed the quasi spherical shape. The BiFeO3 nanoparticles synthesized at 500 °C showed the weak ferromagnetism behavior, due to the size confinement effect, in spite of the antiferromagnetic behavior of the BiFeO3 nanoparticles synthesized at 600 °C. The BiFeO3 nanoparticles exhibited strong absorption in the visible region with the optical band gap calculated from Tauc’s plot. The optical band gap decreased from 2.10 to 2.08 eV by the increasing of thermal decomposition temperature. Moreover, the BiFeO3 nanoparticles were used for the degradation of methylene blue as a typical dye pollutant under direct sunlight irradiation.

Structural, Magnetic and Photocatalytic Properties of BiFeO3 Nanoparticles

Single phase BiFeO3 (BFO) nanoparticles as a visible light photocatalyst were successfully synthesized by thermal decomposition of the glyoxylate precursor. The glyoxylate precursors were formed by the redox reaction between ethylene glycol and nitrate ions. The phase evolution, structure and optical properties of BFO nanoparticles were characterized by X-ray diffraction, electron microscopy and UV–Vis spectroscopy methods. The BFO nanoparticles showed the quasi spherical shape. The BFO nanoparticles synthesized at 500 °C showed the weak ferromagnetism behavior, due to the size confinement effect, in spite of the antiferromagnetic behavior of the BFO nanoparticles synthesized at 600 °C. The BFO nanoparticles exhibited strong absorption in the visible region with the optical band gap calculated from Tauc’s plot. The optical band gap decreased from 2.10 to 2.08 eV by the increasing of thermal decomposition temperature. Moreover, the BFO nanoparticles were used for the degradation of methylene blue (MB) as a typical dye pollutant under direct sunlight irradiation.

Structural, ferroelectric and magnetic properties of BiFeO3-ZnFe2O4 nano-composites

ABSTRACTMultiferroic BiFeO3-ZnFeO3 nano-composites in different composition were prepared by sol-gel method. Detailed investigations were made on the structural, magnetic and ferroelectric properties of these nanocomposites. The X-Ray Diffraction (XRD) pattern confirms the formation of distorted perovskite and spinel phases of BiFeO3 and ZnFe2O4 respectively. Transmission Electron microscopy reveals the particle size and the elemental idea of pure ferrites. The particle sizes calculated using TEM of ZnFe2O4 is 20–30 nm and match with XRD result. An enhancement of polarization in nano-composites is observed from Polarization Vs Electric Field loops. Magnetic polarization versus Magnetic field curves indicates the improved magnetic properties.

Parasitic phases at the origin of magnetic moment in BiFeO3 thin films grown by low deposition rate RF sputtering

A series of epitaxial BiFeO3 thin films has been grown under high partial pressure in a pure O2 atmosphere, which leads to a low deposition rate. The samples grown under these conditions have presented an evolution of the quality of the epitaxy as the deposition temperature increases, however, spurious β–Bi2O3 and supertetragonal BiFeO3 phases are present in the films grown at higher temperatures. The presence of γ–Fe2O3 is reported in one growing condition, and has been attributed to the origin of hysteretic ferromagnetic behavior. A second kind of magnetism, with higher magnetic moment and anhysteretic behaviour, is attributed to the presence of mixed phases of BiFeO3.

Low-Temperature Solid-State Synthesis of High-Purity BiFeO3 Ceramic for Ferroic Thin-Film Deposition.

The synthesis of high-purity BiFeO3 (BFO) ceramic by solid-state reaction is known to be very difficult due to inevitable formation of the secondary phases, mostly mullite-type Bi2Fe4O9 and sillenite-type Bi25FeO39. In particular, it is very difficult to completely remove the Bi-deficient Bi2Fe4O9 phase from sintered ceramic BFO targets. This problem consequently leads to the difficulty of fabricating high-quality BFO thin films using these sintered targets. Herein, we introduce a simple but effective low-temperature processing scheme for removing impurity phases in which optimized processing conditions are obtained by chemically correlating the first calcination step with the subsequent leaching and sintering steps. More specifically, we suitably avoid the formation of the high-temperature-stable Bi2Fe4O9 phase by performing the calcination at significantly low temperatures (between 650 and 675 °C) with Bi-excess starting powders. We have then fabricated epitaxially grown BFO thin films using these phase-pure ceramic targets and consequently achieved high-quality ferroelectricity and switchable photovoltaic responses. On the basis of the present experimental observations, we suggest that a low impurity concentration in the sintered BFO ceramic target, even with a low relative density, is advantageous for high-quality thin-film fabrication.

Band gap engineering of Gd and Co doped BiFeO3 and their application in hydrogen production through photoelectrochemical route

The present report deals with the synthesis of Gd and Co doped BiFeO3 (BFO) i.e. Bi1-xGdxFe1-yCoyO3 (BGFCO, x = 0.0, 0.1; y = 0, 0.05, 0.10, 0.20, 0.25) nanoparticles by sol–gel method. The co-doping leads to band gap engineering of BiFeO3 with the band gap varying from 2.23 eV to 1.77 eV. The band gap engineering coupled with UV–Vis spectroscopy has been used to find the optimum material. The significant lowering in the band gap of the doped BFO is attributed to the deformation produced in Fe–O octahedron geometry as well as rearrangement in its molecular orbitals. The band gap engineering leads to materials with improved solar spectral response which in turn results in better harvesting of solar energy. X-ray diffraction (XRD) patterns indicate the formation of pure phase of BiFeO3 and its doped variants. The surface morphologies and particle sizes of different compositions have been investigated through scanning electron microscope (SEM). The as synthesized BFO as well as its doped variants have been used as photoanodes for hydrogen production through photoelectrochemical (PEC) splitting of water. The optimum material Bi0.9Gd0.1Fe0.75Co0.25O3 (BGFCO-25) with band gap of 1.77 eV has been used as photoanode having PEC configuration of 1 mol/L NaOH as the electrolyte solution and the Pt as cathode using 1.5 AM UV–Vis illumination. This has produced the photocurrent density of 2.03 mA/cm2 and hydrogen production rate of 74.57 μmol cm−2 h−1. The maximum photo-conversion efficiency has been found to be 2.29% for BGFCO-25 which is higher than that of BFO in which it is 0.76%. This noteworthy enhancement in the photoelectrochemical properties is ascribed to narrowing of the band gap which improves the solar spectral response and allows the absorption of higher density of photons. The stability test of the photoanode has been done through chronoamperometry technique.

Microstructure and temperature stability of highly strained tetragonal-like BiFeO3 thin films

A highly strained tetragonal (T)-like BiFeO3 (BFO) phase interspersed between two rhombohedral (R)-like phase regions in BFO films grown on LaAlO3 substrates was confirmed by transmission electron microscopy (TEM). The typical width of the T-like phase was about 5–35nm and the areal fraction of the T-like phase was estimated to be 35%. A lattice misalignment of 3.8° between adjacent R- and T-like phases was observed in both high-resolution TEM images and selected-area electron diffraction patterns. Superlattice reflection spots were observed along the [110] direction in the R-like phase, which were attributed to the antiphase tilting of oxygen octahedra about the pseudocubic [111] axis of the R-like phase. A structural phase transition from the T-like phase to the R-like one at around 435°C was observed by TEM during in situ heating and cooling experiments.