Multiferroic Bismuth Ferrite Research Articles

Polar properties of hydrothermally synthesized BiFeO3 thin films

Multiferroic bismuth ferrite (BiFeO3) has at- tracted considerable attention due to applications related to the bulk photovoltaic effect in which the direction of polar- ization determines the direction of the photocurrent. Epitax- ial thin films are produced by means of techniques that usu- ally require high temperature processes. The hydrothermal method can be seen as an alternative route to obtain highly textured thin films in quantities compatible with batch pro- cessing; nevertheless, the structural, dielectric and electric properties are generally affected by the presence of hydro- gen and other reaction by-products. In this work, functional and highly textured BiFeO3 films were successfully pro- duced on metallic SrTiO3:Nb (0.5 wt.%) (100) substrates via hydrothermal synthesis. X-ray diffraction (XRD) and Atomic Force Microscopy (AFM) were used to analyze the structural properties of the films. Piezoresponse Force Microscopy (PFM) and Photoconductive Atomic Force Mi- croscopy (Pc-AFM) were used to determine their functional properties. We show the polarization switching and confirm

Effects of external magnetic field on the morphology and magnetic property of BiFeO3 particles prepared by hydrothermal synthesis

The perovskite multiferroic bismuth ferrite (BiFeO3) particles were synthesized by hydrothermal method under external magnetic fields with respective intensities of 0 T, 4 T, 8 T, and 12 T. High magnetic field greatly affected the growth behavior and magnetic property of BiFeO3. The morphology of BiFeO3 changed from regular cube to irregular sphere, and finally became a chain-like structure. Meanwhile, the magnetization of the as-prepared BiFeO3 particles measured under a 1.8 T magnetic field was enhanced from 0.1003 emu/g (0 T) to 0.2067 emu/g (12 T). The possible mechanisms of crystal growth and magnetic property were also discussed.

On the problem of coexistence of the weak ferromagnetism and the spin flexoelectricity in multiferroic bismuth ferrite

On the example of the prototypical multiferroic bismuth ferrite the microscopic origin of the spin flexoelectricity responsible for the spin cycloid ordering is analyzed. It is shown how the three basic structural distortions corresponding to the frozen-phonon modes of an ideal perovskite lattice result in the coexistence of the spin canting and the spin cycloidal ordering. On the basis of a simple formula for the antisymmetric Dzyaloshinskii-Moriya superexchange the inhomogenous magnetoelectric contribution to the theromodynamic potential is derived and the values of the microscopic parameters corresponding to the spin canting and the spin cycloid are determined.

Surface phase transitions in BiFeO3below room temperature

We combine a wide variety of experimental techniques to analyze two heretofore mysterious phase transitions in multiferroic bismuth ferrite at low temperature. Raman spectroscopy, resonant ultrasound spectroscopy, EPR, X-ray lattice constant measurements, conductivity and dielectric response, specific heat and pyroelectric data have been collected for two different types of samples: single crystals and, in order to maximize surface/volume ratio to enhance surface phase transition effects, BiFeO3 nanotubes were also studied. The transition at T=140.3K is shown to be a surface phase transition, with an associated sharp change in lattice parameter and charge density at the surface. Meanwhile, the 201K anomaly appears to signal the onset of glassy behaviour.

Local Mapping of Generation and Recombination Lifetime in BiFeO3 Single Crystals by Scanning Probe Photoinduced Transient Spectroscopy

Carrier lifetime in photoelectric processes is the average time an excited carrier is free before recombining or trapping. Lifetime is directly related to defects and it is a key parameter in analyzing photovoltaic effects in semiconductors. We show here a scanning probe method combined with photoinduced current spectroscopy that allows mapping with nanoscale resolution of the generation and recombination lifetimes. Using this method we have analyzed the mechanism of the abnormal photovoltaic effect in multiferroic bismuth ferrite, BiFeO(3). We found that generation and recombination lifetimes in BiFeO(3) are large due to complex generation and recombination processes that involve shallow energy levels in the band gap. The domain walls do not play a major role in the photovoltaic mechanism.

Influences of Oxygen Pressure on Optical Properties and Interband Electronic Transitions in Multiferroic Bismuth Ferrite Nanocrystalline Films Grown by Pulsed Laser Deposition

Bismuth ferrite (BiFeO(3)) nanocrystalline films with the crystalline size of 27-40 nm have been grown on c-sapphire substrates under various oxygen pressures of 1 × 10(-4) to 1 Pa by pulsed laser deposition. The X-ray diffraction spectra show that the films are polycrystalline and present the pure rhombohedral phase. It was found that the Raman-active phonon mode E(TO1) shifts towards a higher energy side from 74 to 76 cm(-1) with increasing oxygen pressure, indicating a larger tensile stress in the films deposited at higher oxygen pressure. The X-ray photoelectron spectroscopy analysis suggests that the concentrations of both Fe(2+) ions and oxygen vacancies in the BiFeO(3) films increase with decreasing oxygen pressure. Moreover, the dielectric functions in the photon energy range of 0.47-6.5 eV have been extracted by fitting the transmittance spectra with the Tauc-Lorentz dispersion model. From the transmittance spectra, the fundamental absorption edge is observed to present a redshift trend with increasing the temperature from 8 to 300 K. Note that the optical band gap (E(g)) decreases with increasing the temperature due to the electron-phonon interactions associated with the interatomic distance in the BiFeO(3) films. However, the E(g) decreases from 2.88 to 2.78 eV with decreasing oxygen pressure at 8 K, which can be attributed to the increment of oxygen vacancies leading to the formation of some impurity states between the valence and conduction band. It can be concluded that the oxygen pressure during the film fabrication has the significant effects on microstructure, optical properties, and electronic band structure modification of the BiFeO(3) films.

One-dimensional multiferroic bismuth ferrite fibers obtained by electrospinningtechniques

We report the fabrication of novel multiferroic nanostructured bismuth ferrite (BiFeO3) fibers using the sol–gel based electrospinning technique. Phase pureBiFeO3 fibers were prepared by thermally annealing the electrospunBiFeO3/polyvinylpyrrolidone composite fibersin air for 1 h at 600 °C. The x-ray diffraction pattern of the fibers (BiFeO3) obtained showed that their crystalline structures were rhombohedralperovskite structures. Both scanning electron microscopy (SEM) andtransmission electron microscopy (TEM) images revealed that theBiFeO3 fibers were composed of fine grained microstructures. The grains were self-assembled andself-organized to yield dense and continuous fibrous structures. The magnetic hysteresis loops ofthese nanostructured fibers displayed the expected ferromagnetic behavior, whereby a coercivity of ∼ 250 Oe and a saturationmagnetization of ∼ 1.34 emu g − 1 were obtained. The ferroelectricity and ferroelectric domain structures of the fibers wereconfirmed using piezoresponse force microscopy (PFM). The piezoelectric hysteresisloops and polar domain switching behavior of the fibers were examined. Suchmultiferroic fibers are significant for electroactive applications and nano-scale devices.

Microstructural analysis of interfaces in a ferromagnetic-multiferroic epitaxial heterostructure

We report a study on multiferroic bismuth ferrite (BiFeO3, BFO)-ferromagnetic lanthanum strontium manganese oxide (La0.7Sr0.3MnO3, LSMO) epitaxial interfaces by scanning transmission electron microscopy-energy dispersive spectroscopy (STEM-EDS) and energy-filtered transmission electron microscopy (EFTEM). Epitaxial (001) oriented LSMO/BFO heterostructures were fabricated on a (001) strontium titanate (SrTiO3, STO) substrate using pulsed laser deposition (PLD). Different cooling conditions to room temperature (rapid or slow) were used to investigate the effect of fabrication conditions on the structural quality of the interfaces. The combined analysis of bright field transmission electron microscopy imaging, STEM-EDS and EFTEM data reveals that the LSMO-BFO heterostructure interface is free from any defects but the phases are chemically interdiffused over a length scale of ∼4 nm.

Synthesis of Bismuth Ferrite Nanoparticles via a Wet Chemical Route at Low Temperature

Nanoparticles (NPs) of multiferroic bismuth ferrite (BiFeO3) with narrow size distributions were synthesized via a wet chemical route using bismuth nitrate and iron nitrate as starting materials and excess tartaric acid and citric acid as chelating agent, respectively, followed by thermal treatment. It was found that BiFeO3NPs crystallized at∼350∘Cwhen using citric acid as chelating agent. Such crystallization temperature is much lower than that of conventional chemical process in which other types of chelating agent are used. BiFeO3NPs with different sizes distributions show obvious ferromagnetic properties, and the magnetization is increased with reducing the particle size.

Continuum theory and phase-field simulation of magnetoelectric effects in multiferroic bismuth ferrite

Multiferroic materials possess electric and magnetic orderings simultaneously, making it possible to manipulate the electric state of a multiferroic by magnetic field, or vice versa. Among all single-phase multiferroics, BiFeO 3 is particularly exciting, for its room temperature multiferroicity, excellent ferroelectric properties, and recently demonstrated electric control of antiferromagnetic domains, which opens door for its applications in spintronics. In this paper, we report a systematic theoretical and computational study on the structure and evolution of magnetoelectric domains in multiferroic BiFeO 3. A continuum description is developed for antiferromagnetic ordering first, which is then incorporated into an unconventional phase field method that couples ferroelastic, ferroelectric, and antiferromagnetic orderings through the characteristic function of variants. The internal elastic, electric, and magnetic fields are carefully analyzed, taking into account both bulk and thin film geometries and boundary conditions. The theory is implemented into numerical simulations, where we not only observe the coupled ferroelectric and antiferromagnetic domains, and demonstrate the electric control of antiferromagnetic ordering, but also reveal the switching of antiferromagnetic domains by mechanical stress that is yet to be reported in experiment. Our study offers deep insight into the microstructural evolution and macroscopic properties of BiFeO 3, and provides a powerful tool to study a wide range of multiferroic materials with magnetoelectric coupling.

Strain-induced isosymmetric phase transition inBiFeO3

We calculate the effect of epitaxial strain on the structure and properties of multiferroic bismuth ferrite, BiFeO3, using first-principles density functional theory. We find that, while small strains cause only quantitative changes in behavior from the bulk material, compressive strains of greater than 4% induce an isosymmetric phase transition accompanied by a dramatic change in structure. In striking contrast to the bulk rhombohedral perovskite, the highly strained structure has a c/a ratio of ~1.3 and five-coordinated Fe atoms. We predict a rotation of polarization from [111] (bulk) to nearly [001], accompanied by an increase in magnitude of ~50%, and a suppression of the magnetic ordering temperature. Intriguingly, our calculations indicate critical strain values at which the two phases might be expected to coexist.

First-principles study of ferroelectric domain walls in multiferroic bismuth ferrite

We present a first-principles density functional study of the structural, electronic and magnetic properties of the ferroelectric domain walls in multiferroic BiFeO3. We find that domain walls in which the rotations of the oxygen octahedra do not change their phase when the polarization reorients are the most favorable, and of these the 109 degree domain wall centered around the BiO plane has the lowest energy. The 109 degree and 180 degree walls have a significant change in the component of their polarization perpendicular to the wall; the corresponding step in the electrostatic potential is consistent with a recent report of electrical conductivity at the domain walls. Finally, we show that changes in the Fe-O-Fe bond angles at the domain walls cause changes in the canting of the Fe magnetic moments which can enhance the local magnetization at the domain walls.

First-principles study on the electronic and optical properties of BiFeO 3

The structural, electronic, and optical properties of multiferroic bismuth ferrite (BiFeO 3) are investigated using density functional theory within generalized gradient approximation (GGA). The calculated lattice parameters are in good agreement with the experimental data. The electronic structure shows that BiFeO 3 has an indirect (very close to direct) band gap of 1.06 eV. The complex dielectric function, absorption spectra, refractive index, extinction coefficient, energy-loss spectrum and reflectivity are calculated, and the results are compared with the available experimental data. Finally, the optical properties of BiFeO 3 are discussed based on the band structure calculations.

Hybrid functional study of prototypical multiferroic bismuth ferrite

We report a systematic comparison of various exchange-correlation functionals for the prediction of the structural, magnetic, electronic, and dynamical (phonons and Born effective charge tensors) properties of bismuth ferrite, a prototypical multiferroic compound. We have not only considered the usual approximations to density-functional theory such as the local-density approximation (LDA), generalized (GGA), and $\text{LDA}+U$, but also hybrid approaches such as B3LYP and B1. The recent B1-WC hybrid functional of Bilc et al. [Phys. Rev. B 77, 165107 (2008)], with the GGA functional of Wu and Cohen and an exact exchange mixing parameter of 0.16, provides very good overall agreement with experiments and can be considered as a valuable alternative to LDA, GGA, and $\text{DFT}+U$ for the study of bismuth ferrite. This does not only allow a reliable interpretation of the physical properties of this specific compound but also opens perspectives for further and more predictive first-principles investigations of multiferroic materials.

BiFeO 3 ceramics synthesized by mechanical activation assisted versus conventional solid-state-reaction process: A comparative study

Multiferroic bismuth ferrite, BiFeO 3, was prepared using conventional solid-state-reaction and mechanical activation assisted solid-state-reaction method. Room temperature X-ray diffraction patterns for these samples at various stages of processing were collected to analyze the phase evolution. The patterns showed that as compared to the conventionally processed samples, perovskite structured BiFeO 3 phase formation temperature decreases by ∼100 °C in the samples produced by mechanical activation assisted process. Differential thermal analysis (DTA) measurements of both samples show a ferroelectric transition at ∼825 °C, characteristic of ferroelectric BiFeO 3. Ferroelectric measurements and the leakage measurements reveal that despite the presence of predominantly BiFeO 3, mechanical activation assisted samples show higher leakage over the conventionally processed samples, attributed to the decreased grain size and higher defect concentration of the mechanical activation assisted samples. Mechanical activation assisted samples also show enhanced magnetization and a clear magnetic transition at ∼375 °C.

An in situ study of the formation of multiferroic bismuth ferrite using high resolution synchrotron X-ray powder diffraction

The synthesis of multiferroic BiFeO 3 has been undertaken in situ using both vacuum and argon atmospheres. We have collected and refined synchrotron X-ray powder diffraction data from pressed pellets of the starting powders of Bi 2O 3 and Fe 2O 3 at intervals in the reaction to form the final multiferroic BiFeO 3 product. Data were collected at 3-min intervals following as realistically as possible the sintering regimes used in industry. The difference in the transformation temperature of monoclinic Bi 2O 3 to cubic Bi 2O 3 was found to be 650 and 700 °C when samples were sintered in a vacuum and argon environments, respectively. It was found that this reaction was 75% complete before the multiferroic product began to form. In both cases it was found that the quantity of Fe 2O 3 was unaffected by increasing temperature until after the transition of monoclinic to cubic Bi 2O 3 had reached its maximum value. After this transition the quantities of cubic Bi 2O 3 and Fe 2O 3 were found to decrease at very similar rates yielding the final BiFeO 3 structure. An SEM study of the bulk microstructure of the BiFeO 3 product showed a poor densification due to incomplete reactions between the remaining Bi 2O 3 and Fe 2O 3.

Effect of samarium doping on the properties of solid-state synthesized multiferroic bismuth ferrite

Problems in the synthesis of phase-pure BiFeO3, a multiferroic material, are well known and presence of other phases often leads to inferior properties. Work reported here was carried out to obtain pure phase BiFeO3 via a solid-state-reaction method in both pure and Sm-doped form. X-ray diffraction (XRD) of the calcined undoped samples showed that maximum amount of pure BiFeO3 phase was obtained at a calcination temperature of 825 °C while a deviation in calcination temperature led to the increased amount of secondary phases such as Bi25FeO40. On the other hand, XRD patterns of Sm doped calcined samples of composition Bi0.9Sm0.1FeO3 revealed that secondary phases do not appear on calcination at and above 800 °C for 1 h in air. Vibrating sample magnetometry (VSM) measurements showed that Bi0.9Sm0.1FeO3 samples possess higher room temperature remnant magnetization than undoped samples. Temperature dependent measurements suggested antiferromagnetic behavior of Sm doped samples with Neèl temperature of ∼370 °C. Ferroelectric measurements showed that Sm doping improves the polarization but enhances the leakage current.

Phase Transitions in Multiferroic BiFeO3 Crystals, Thin‐Layers, and Ceramics. Enduring Potential for a Single Phase, Room‐Temperature Magnetoelectric ′Holy Grail′

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Phase transitions in multiferroic BiFeO3 crystals, thin-layers, and ceramics: enduring potential for a single phase, room-temperature magnetoelectric ‘holy grail’

Magnetic phase transitions in multiferroic bismuth ferrite (BiFeO3) induced by magnetic field, epitaxial strain, and composition modification are considered. These transitions from a spatially modulated spin spiral state to a homogenous antiferromagnetic one are accompanied by tghe release of latent magnetization and a linear magnetoelectric effect that makes BiFeO3-based materials efficient room-temperature single phase multiferroics.

Influence of strain and oxygen vacancies on the magnetoelectric properties of multiferroic bismuth ferrite

The dependencies on strain and oxygen vacancies of the ferroelectric polarization and the weak ferromagnetic magnetization in the multiferroic material bismuth ferrite, BiFeO_3, are investigated using first principles density functional theory calculations. The electric polarization is found to be rather independent of strain, in striking contrast to most conventional perovskite ferroelectrics. It is also not significantly affected by oxygen vacancies, or by the combined presence of strain and oxygen vacancies. The magnetization is also unaffected by strain, however the incorporation of oxygen vacancies can alter the magnetization slightly, and also leads to the formation of Fe^{2+}. These results are discussed in light of recent experiments on epitaxial films of BiFeO_3 which reported a strong thickness dependence of both magnetization and polarization.