BiFeO3 Phase Research Articles

Heavy Mn-doping effect on spontaneous polarization in ferroelectric BiFeO3 thin films

Mn doping effect on the crystal structure and spontaneous polarization (Ps) in ferroelectric BiFeO3 (BFO) thin films are experimentally and theoretically investigated. Polarization hysteresis measurements for BiFe1−xMnxO3 (BFMO) films grown on (100) SrTiO3 substrates with SrRuO3 electrodes indicate that remanent polarization (Pr) remains unchanged in the range of 0 ≤ x ≤ 0.1. As a result of the density-functional theory (DFT) calculations for the rhombohedral, tetragonal, and monoclinic phases of BFO and BFMO (x = 0.5), we find that the monoclinic phase with compressive strain from the substrates yields the most reasonable Ps. The DFT calculations also indicate that heavy doping of up to 50% Mn does not lead to a considerable deterioration in Ps, which is consistent with the experiments. Our investigations demonstrate that the ferroelectric polarization in the BFMO system can be retained for a wide range of Mn content either in bulk or thin-film form.

Investigation of the properties of BiFeO3/intermediate-layer structures fabricated by magnetron sputtering

The properties of BiFeO3/intermediate layer/substrate structures fabricated by radio-frequency magnetron sputtering at low temperatures (450°C) and by deposition on substrates and intermediate layers between the substrate and the BiFeO3 film have been investigated. In the structures, glass substrates or commercial Pt/Ti/SiO2/Si(001) substrates have been used, and intermediate layers have been prepared from L10 FePt or Pt films. Intermediate layers of Pt and L10 FePt have the (111) and (001) textures, respectively, induced by rapid thermal annealing. It has been revealed that the deposition on the commercial substrates leads to the formation of BiFeO3 isotropic films that have a large surface roughness and consist of grains ∼200 nm in size with the BiFeO3 perovskite structure. In the case of the deposition of a BiFeO3 film on a Pt(111) intermediate layer, the BiFeO3 phase is suppressed. The deposition on an intermediate layer of the L10 FePt film with the (001) texture results in the formation of a single-phase BiFeO3 film with the (001) texture and the perovskite structure, which (as compared to the BiFeO3 films grown on the commercial substrate) has a less pronounced roughness, smaller grain sizes, and significantly better ferroelectric properties.

Effects on dielectric and magnetic properties of Cr-doped Bi0.9Ba0.1Fe1−xCrxO3 ceramic

In this paper, Cr-doped Bi0.9Ba0.1Fe1−xCrxO3 (x = 0.00, 0.05, 0.10, 0.15, 0.20) ceramic materials were prepared by traditional state solid synthesized method, and the effects of Cr3+ ion on magnetic and dielectric properties were investigated. All samples showed BiFeO3 phase formation were successful synthesized. The SEM images showed the shape of samples changed from regular to irregular shape. With increasing of Cr2+ ions, Saturation magnetization (Ms) increased from 5.24 to 8.6 emu/g, and then decreased to 7.31 emu/g, and coercivity (Hc) increased from 110.66 to 256.49 Oe. All the samples showed high dielectric constants at low frequency and the values of dielectric constants decreased slightly with frequency increasing. Delectric loss (tanδ) values kept a steady in a wide range frequency of 10–600 MHz. They ranged in tanδ from 0.01 to 0.07, which was a low dielectric loss in Bi0.9Ba0.1Fe1−xCrxO3 ceramics.

Structure evolution and magnetic property of cobalt-modified Bi0.9Gd0.1FeO3 nanocrystal at morphotropic phase boundary

Cobalt-modified Bi0.9Gd0.1FeO3 nanocrystal was prepared by facile co-precipitation method. The Rietveld refinement results show that minor Co-substitution concentration (x ∼ up to 0.05) in Bi0.9Gd0.1Fe1−xCoxO3 gradually promotes the structural transition from R3c to Pn21a and their magnetic performances are also enhanced. The magnetization of the as-prepared Bi0.9Gd0.1Fe1−xCoxO3 specimen (x = 0.05) at a 1.8 T testing magnetic field is 1.3450 emu/g, which is stronger than those of samples with x = 0 (0.3457 emu/g) and pure phase BiFeO3 (0.0867 emu/g). The possible mechanism of magnetic transition would mainly be contributed to the substitution-induced suppression of spiral spin structure and magnetically active substitution ions.

Photocatalytic Degradation of Phenol by Impure BiFeO<sub>3</sub> under Visible Light Irradiation

Impure BiFeO3 as a visible light photocatalyst for phenol degradation was synthezied via solid state method. The calcined temperatures of catalysts were studied. The characteristics of the catalysts were determined by XRD, SEM/EDS and BET. The catalysts were mainly composed of rhombohedral distorted perovskite-type BiFeO3 phase with impurity phases (B2Fe4O9, Bi25FeO40 and Bi2O3). Photocatalytic activity of phenol (5 mg/l) was studied in the impure BiFeO3 illuminated with 200 w fluorescence lamps. The catalyst amount and initial pH value of phenol solution on the photocatalytic effiency have been also investigated. In the present experiments, the catalyst calcined at 800°C shows the highest activity. The optimum loading of impure BiFeO3 and pH value were obtained to be 0.5 g/l and 6, respectively.

Antiferromagnetic–weak ferromagnetic transition in lightly doped BiFeO3: role of structural defects

Chemical substitution is commonly considered as favoring the suppression of the cycloidal magnetic structure specific to the polar phase of multiferroic BiFeO3. To reveal the factors underlying the substitution-driven instability of the antiferromagnetic order, synthesis and investigation of the crystal structure, microstructure, local ferroelectric, and magnetic properties of the ceramic samples of Bi0.95A0.05Fe0.95B0.05O3 (A = Pr, Ca; B = Mn, Ti) were carried out. The investigation did not reveal any link between the size of the substituting ions and the magnetic behavior of the compounds. However, it found a clear correlation between the magnetic ground state and the morphology/defect structure of the ceramics, as traced by the scanning probe microscopy. The results of the work suggest that the magnetic cycloid should be less stable in those doped materials, in which the charge-compensating mechanism involves the formation of crystal lattice defects.

Structural evolution and physical properties of multiferroic Bi0.9−xLa0.1PbxFeO3−x/2 ceramics

The polycrystalline samples of multiferroic Bi0.9−xLa0.1PbxFeO3−x/2 (x = 0–0.35) were prepared by the solid state reaction method and characterized by x-ray diffraction, field emission scanning electron microscopy, dielectric, magnetic, magnetodielectric (MD) and magnetoelectric (ME) measurements. A structural evolution from rhombohedral to pseudocubic structure was found to happen near x = 0.20. The changes and anomalies observed in magnetization were correlated with structural evolution and the development in microstructure. The ferroelectromagnetic measurements demonstrated Pb2+ doping to be a very effective method to realize the coexistence of weak ferromagnetism and ferroelectric in the ferroelectric R3c phase of BiFeO3. The MD and ME effects of Bi0.9−xLa0.1PbxFeO3−x/2 ceramics were first reported. A maximum ME voltage coefficient has been observed at x = 0.30. This work is helpful for understanding the ferroelectromagnetic behaviors and ME effect with complicated spin structures.

Synthesis, structures and properties of single phase BiFeO3 and Bi2Fe4O9 powders by hydrothermal method

Single phase BiFeO3 and Bi2Fe4O9 powders have been synthesized via hydrothermal method by carefully controlling the reaction conditions. It is found that the BiFeO3 shows dense microstructure with irregular, larger grains, the average grain size is ~5.0 μm, whereas the Bi2Fe4O9 displays small, porous microstructure with the sheet shaped grains, the average thickness is ~55 nm and length is ~500 nm. X-ray photoemission spectra confirm that in both compositions, the Bi and Fe cations have the charge valence of +3. The magnetization–magnetic field (M–H) and magnetization–temperature (M–T) measurements reveal that the BiFeO3 has weak ferromagnetism even at room temperature due to its canted antiferromagnetic interaction, whereas the Bi2Fe4O9 shows antiferromagnetic nature in the temperature range of 10–300 K. Our results provide interesting supplements for controlling the morphology and understanding the structure–property relationship of BiFeO3 and Bi2Fe4O9.

Destructive Interactions between Pore Forming Agents and Matrix Phase during the Fabrication Process of Porous BiFeO3 Ceramics

Porous bismuth ferrite ceramics were synthesized by sacrificial pore former method. A mixture of BiFeO3 and 20 wt% of various pore formers including high density polyethylene, polyethylene glycol, polyvinyl alcohol, urea and graphite was intensively milled for 10 h in a planetary ball mill, uniaxially cold pressed and then subjected to the multi-stage heat treatment. The results revealed that urea and polyvinyl alcohol are appropriate candidates for maintaining the strength of the final porous structure. Density and porosity measurements showed that by employing 20 wt% of high density polyethylene and graphite, a porous sample with a maximum porosity of nearly 40% could be obtained. Mercury porosimetry results showed that using urea as a pore former gives porous bismuth ferrite with a mean pore diameter of 7 μm, uniform pore distribution as well as interconnected pores. Moreover, reactions between BiFeO3 matrix phase and thermal decomposition products of pore formers can lead to degradation of the BiFeO3 phase in the final porous samples. Analysis of X-ray diffraction patterns illustrated that in the samples processed with graphite, high density polyethylene and polyvinyl alcohol as pore former, BiFeO3 matrix phase partially or completely decomposed to intermediate phases of Bi2Fe4O9 and Bi25FeO40. Using of urea did not damage the matrix phase and porous BiFeO3 within the original perovskite structure could be prepared. Furthermore, thermodynamic investigation was carried out for prediction of possible interactions between matrix phase and pore former at elevated temperatures.

Low-energy phases, electronic and optical properties of Bi1−xLaxFeO3 solid solution: Ab-initio LDA+U studies

Modification of BiFeO3 by substituting Bi (III) with La (III) was studied using ab-initio method based on self-interaction corrected local density approximation plus Hubbard-like U approach, as implemented in plane-wave pseudopotential Cambridge Serial Total Energy Package code. Possible stable phases of BiFeO3 and Bi1−xLaxFeO3 were determined from the calculated total energy. Phase transformation of rhombohedral to orthorhombic in Bi1−xLaxFeO3 solid solution occurred at 0.57 La concentration ratio for rhombohedral R3c and orthorhombic Pn21a phases, consistent with the available experimental and other calculation studies. The electronic band structure, density of states, and optical properties of Bi1−xLaxFeO3 were also investigated. Results showed that the electronic and optical properties of Bi1−xLaxFeO3 were mainly influenced by La concentration, but slightly affected by structural phase transformation. These results would provide a theoretical reference for fundamental science and technological application of Bi1−xLaxFeO3 solid solution.

Observation of coexistence of ferroelectric and antiferroelectric phases in Sc substituted BiFeO3

Polycrystalline BiFe1−xScxO3 (x=0, 0.05, 0.10 and 0.15) compounds were prepared using solid state reaction route. The X-ray diffraction patterns showed that Sc substitution stabilized the crystal structure in rhombohedral structure with R3c space group along with nominal percentage of monoclinic structure with Cm space group. Raman and FTIR measurements revealed that the substituent replaced Fe-ions and caused structural distortions. Co-existence of ferroelectric and antiferroelectric phases were observed in Sc substituted BiFeO3 and antiferroelectric phase is found to be evolved at the expense of ferroelectric phase with the Sc content. The electric field driven effects indicated that antiferroelectric phase was suppressed and ferroelectric phase was enriched. Frequency dependence of dielectric constant (ɛr) and dielectric loss tangent (tanδ) at different temperatures were investigated. Further, grain resistances and grain boundary resistances were increased with the Sc content. Activation energies estimated from dielectric, impedance and modulus data indicated that these relaxations originated presumably due to the oxygen vacancy movements.

The effect of the ethylene glycol to metal nitrate molar ratio on the phase evolution, morphology and magnetic properties of single phase BiFeO3 nanoparticles

In this work, single phase BiFeO3 nanoparticles have been synthesized by thermal decomposition of a glyoxylate complex achieved by the redox reaction between ethylene glycol and nitrate anions. The effects of different molar ratios of ethylene glycol to metal nitrate anions on the phase evolution, morphology and magnetic properties were investigated by infrared spectroscopy, thermal analysis, X-ray diffraction, electron microscopy and vibrating sample magnetometry methods. The single phase bismuth ferrite nanoparticles synthesized with the ethylene glycol to nitrate anions molar ratio of 5 showed the weak ferromagnetism behavior with saturation magnetization of 1.3emu/g, due to the size confinement effect. Furthermore, the BiFeO3 nanoparticles were used for the degradation of methylene blue (MB) as a typical dye pollutant under direct sunlight irradiation.

BiFeO3 thin films via aqueous solution deposition: a study of phase formation and stabilization

This paper reports a thorough microstructural investigation of bismuth ferrite (BFO) thin films subjected to various processing conditions and discusses their influence on the stability of the BiFeO3 perovskite phase. The formation of secondary phases in BFO thin films is studied as a function of annealing temperature and time, film thickness, Bi excess, and Ti substitution. While films annealed at 600 °C consist of the desired BiFeO3 phase, higher temperatures induce the decomposition leading to a significant amount of secondary phases, particularly the iron-rich Bi2Fe4O9 phase. A longer annealing time at 700 °C further enhances the decomposition of BiFeO3. Qualitative microstructural analysis of the films is performed by electron backscattered diffraction which provides phase analysis of individual grains. The morphology of the single-crystalline Bi2Fe4O9 grains that are embedded in the BiFeO3 matrix drastically changes as a function of the film thickness. Nucleation of these Bi2Fe4O9 grains probably occurs at the film/substrate interface, after which grain growth continues toward the surface of the film through the depletion of the BFO phase. Addition of Bi excess or the substitution of Fe with Ti in the precursor solutions significantly reduces the formation of an iron-rich secondary phase. Influence of the secondary phases as well as Ti substitution on magnetic properties of BFO films was investigated.

Synthesis, microstructural characterization, and dielectric properties of BiFeO3 microcrystals derived from molten salt method

In this work, we report on the synthesis of BiFeO3 microcrystals by a facile molten-salt technique with the mixed medium of NaCl–KCl as the molten salt. The microstructures of BiFeO3 microcrystals were characterized by X-ray diffraction, Raman spectra, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The results show that almost pure phase BiFeO3 powders with the average particle size of 1.0μm were synthesized at temperatures of 700–800°C. However, the prolonged holding time and the change of molten salt ratios with respect to the starting oxides have no remarkable effects on the size and morphology of the BiFeO3 powders. Raman spectra of the BiFeO3 multiferroic ceramics prepared by the BiFeO3 microcrystals derived from the molten-salt method, reveal five Raman peaks at around 125, 145, 212, 268, and 504cm−1. With increasing sintering temperature, the Raman peak at 145cm−1 disappeared, a slight blue frequency shift was observed in the Raman peak at 212cm−1, but no frequency shift was observed in the Raman peaks at around 125, 268, and 504cm−1. The dielectric constants of the BiFeO3 multiferroic ceramics prepared from the BiFeO3 microcrystals synthesized by the molten-salt method, are in the range of 60–250 (measured @103Hz and room temperature), and the corresponding dielectric losses are in the range of 0.05–0.2, which are dependent upon the molten-salt processing parameters.

Controllable phase evolution of bismuth ferrite oxides by an organic additive modified hydrothermal method

Bismuth ferrite crystallites were synthesized by an organic additive modified hydrothermal method using Fe(NO3)3·9H2O and Bi(NO3)3·5H2O as starting materials with NaOH as the mineralizer. The phase evolution of bismuth ferrites in the process of hydrothermal reactions was controlled by adjusting the process parameters, such as NaOH concentration, reaction temperature and duration time. X-ray diffraction (XRD) analysis indicated that the pure phase of Bi25FeO40, BiFeO3 and Bi2Fe4O9 were synthesized at the temperature of 200°C for 24h with NaOH concentration of 2, 5 and 10M, respectively, with the addition of polyvinyl alcohol (PVA). It was found that the addition of PVA postponed the evolution progress between different phases of bismuth ferrites and widened the individual Bi–Fe oxides existence range of processing parameters. Moreover, the mechanism of forming different bismuth ferrites in the hydrothermal process was investigated and discussed.

First principles simulation of temperature dependent electronic transition of FM-AFM phase BFO.

Understanding how temperature affects the electronic transitions of BFO is important for design of BiFeO3 (BFO)-based temperature-sensitive device. Hitherto, however, there have been only very limited reports of the quantitative simulation. Here, we used density functional theory (DFT) and two-dimensional correlation analysis (2D-CA) techniques to calculate the systematic variations in electronic transitions of BFO crystal, over a range of temperature (50~1500 K). The results suggest that the heat accumulation accelerates the O-2p(4) orbital splitting, inducing the Fe(3+)-3d(5) → Fe(2+)-3d(5)d(0) charge disproportionation. The origin is observed as the temperature-dependent electron transfer process changes from threefold degeneracy to twofold degeneracy. Additionally, the crystallographic orientation (111) can be used to control the 2p-hole-induced electronic transition as O → unoccupied Fe(3+)-3d(5), in comparison to the O → Bi-6p(3) + Fe(3+)-3d(5)d(0) on the orientations (001) and (101). This study offers new perspective on the improvement of BFO-based temperature-sensitive device.

Electrical, magnetic and magnetoelectric properties of 0.6BaTiO3–0.4BiFeO3/CoFe2O4 particulate composites

0.6BaTiO3–0.4BiFeO3/CoFe2O4 composites were synthesized by the conventional solid-state sintering method and the sintering temperature was 1100 °C. The electrical, magnetic and magnetoelectric properties were investigated. The results show that BaTiO3–BiFeO3 and CoFe2O4 phases can coexist in the composites and all the synthesized composites exhibit excellent electrical, magnetic and magnetoelectric properties. The magnetoelectric behaviors are strongly dependent on the concentration of CoFe2O4 and the dc magnetic field. The maximum magnetoelectric coefficient of the composites reaches up to about 8.47 mV/cm Oe.

High frequency permeability and permittivity spectra of BiFeO3/(CoTi)-BaM ferrite composites

Low magnetic loss ferrite composites consisting of Ba(CoTi)1.2Fe9.6O19 and BiFeO3 (BFO) ferrite were investigated for permeability, permittivity, and high frequency losses at 10 MHz–1 GHz. The phase fraction of BiFeO3 was quantitatively analyzed by X-ray diffraction measurements. An effective medium approach was employed to predict the effective permeability and permittivity for the ferrite composites, which was found to be in good agreement with experimental data. The experiment demonstrated low magnetic losses (<0.128), modified by BFO phase fraction, while retaining high permeability (∼10.86) at 300 MHz. More importantly, the BFO phase resulted in a reduction of magnetic loss by 32%, as BFO phase increased from 2.7 vol. % to 12.6 vol. %. The effect of BFO phase on magnetic and dielectric properties revealed great potential for use in the miniaturization of high efficiency antennas.

Magnetic and dielectric properties of BiFeO3 nanoparticles

Single crystalline nano-sized multiferroic BiFeO3 (BFO) powders were synthesized through simple chemical co-precipitation method using polyethylene glycol (PEG) as capping agent. We obtained pure phase BiFeO3 powder by controlling pHand calcination temperature. From X-ray diffraction studies the nanoparticles were unambiguously identified to have a rhombohedrally distorted perovskite structure belonging to the space group of R3c. No secondary phases were detected. It indicates single phase structure. EDX spectra indicated the appearance of three elements Bi, Fe, O in 1:1:3. From the UV-Vis diffuse reflectance spectrum, the absorption cut-off wavelength of the BFO sample is around 558nm corresponding to the energy band gap of 2.2 eV. The size (60-70 nm) and morphology of the nanoparticles have been analyzed using transmission electron microscopy (TEM).   Linear M−H behaviour and slight hysteresis at lower magnetic field is observed for BiFeO3 nanoparticles from Vibrating sample magnetometer studies. It indicates weak ferromagnetic behaviour at room temperature. From dielectric studies, the conductivity value is calculated from the relation s = L/RbA Sm-1 and it is around 7.2 x 10-9 S/m.

Composition- and pressure-induced ferroelectric to antiferroelectric phase transitions in Sm-doped BiFeO3 system

A three-dimensional phenomenological model is proposed to describe both ferroelectricity and antiferroelectricity based on the Ginzburg-Landau-Devonshire theory. Its application to the multiferroic Sm-doped BiFeO3 system describes the temperature-, pressure-, and composition-induced ferroelectric to antiferroelectric phase transitions. The constructed temperature-composition and temperature-pressure phase diagrams show that compressive hydrostatic pressure and Sm doping have similar effects on the ferroelectric and antiferroelectric phase transitions. It is also indicated from the temperature-pressure phase diagram that the experimentally observed phase of BiFeO3 under the hydrostatic pressure from 3 GPa to 10 GPa is a PbZrO3-like antiferroelectric phase.