BiFeO3 Phase Research Articles

Large piezoelectric response of BiFeO3/BaTiO3 polycrystalline films induced by the low-symmetry phase.

BaTiO3, BiFeO3 and BiFeO3/BaTiO3 polycrystalline films were prepared by the radio frequency magnetron sputtering on the Pt/Ti/SiO2/Si substrate. The phase structure, converse piezoelectric coefficient and domain structure of BaTiO3, BiFeO3 and BiFeO3/BaTiO3 thin films are characterized by XRD and PFM, respectively. The converse piezoelectric coefficient d33 of BiFeO3/BaTiO3 thin films is 119.5 pm V(-1), which is comparable to that of lead-based piezoelectric films. The large piezoelectric response of BiFeO3/BaTiO3 thin films is ascribed to the low-symmetry T-like phase BiFeO3, because the spontaneous polarization vector of T-like phase (with monoclinic symmetry) BiFeO3 can rotate easily under external field. In addition, the reduced leakage current and major domains with upward polarization are also attributed to the large piezoelectricity.

Structural and Magnetic Properties of BiFeO3 Thin Films by Sol-Gel

Bismuth iron oxide (BiFeO3) is amongst the class of multiferroic materials that has attracted world attraction because of its antiferromagnetic Neel temperature and high ferroelectric Curie temperature of 643K and 1103K respectively. However, synthesis of pure phase BiFeO3 is extremely difficult due to volatile nature of Bi2O3. This results in the presence of bismuth deficient and bismuth rich phases in pure bismuth iron oxide. In the past attention was focused on obtaining pure BiFeO3 phase with variation in annealing/calcination temperature in the range 400-700˚C, and very little consideration is given to the changes in Bi/Fe ratio during synthesis. We here report synthesis of pure phase bismuth iron oxide thin films using cost effective sol-gel method. The ratio of Bi/Fe is varied as 0.90, 1.0 and 1.10. The films are annealed at 300˚C in vacuum under the application of 500Oe magnetic field. XRD results show that with Bi/Fe ratio of 1.10 pure phase BiFeO3 can be obtained at low temperature of 300˚C. Bi/Fe ratios of 0.9 and 1.0 result in the presence of bismuth impurity phase. The films show ferromagnetic behavior as opposed to antiferromagnetic nature of bulk BiFeO3 due to suppression of spiral spin structure.

Sol-Gel Synthesis of BiFeO3 Nanoparticles

Among the various multiferroic materials bismuth iron oxide is a promising candidate due to its relatively high antiferromagnetic Neel temperature and high ferroelectric Curie temperature as compared to all other multiferroic materials. But synthesis of pure phase BiFeO3 is difficult due to the volatile nature of Bi2O3 that leads to bismuth deficient phases and if excess of bismuth is employed it give rise to bismuth rich phases. Moreover the synthesis of pure phase BiFeO3 requires high temperature annealing in the range of 400-700˚C. However, we here report sol-gel synthesis of bismuth iron oxide nanoparticles at low temperatures of 100-300˚C. These nanoparticles are characterized using XRD and VSM. The nanoparticles annealed at 100˚C results in amorphous behavior. Whereas peaks corresponding to pure phase bismuth iron oxide emerges at annealing temperatures of 200 and 300˚C. The nanoparticles show ferromagnetic behavior as opposed to antiferromagnetic behavior due to suppression of spiral spin structure.

The electronic, optical and ferroelectric properties of BiFeO3 during polarization reversal: A first principle study

Rhombohedral BiFeO3 which has a large spontaneous polarization is a candidate material for ferroelectric random access memory (FeRAM). And the polarization reversal is the key factor for the application of FeRAM. Here, the electronic, linear and nonlinear optical properties, structure and its stability of BiFeO3 during the continuous polarization reversal from reference cubic phase to ferroelectric R3c phase were systematically investigated by LSDA+U (local spin density approximation plus Hubbard U) and DFPT (density functional perturbation theory) method. We proposed that the nonlinear optical second order coefficients could be indicators to check the polarization reversal of BiFeO3 and our result showed that the coefficient reach its maximum between 20% distortion and 40% distortion during the half path of the reversal. The relationship between the nonlinear optical second order coefficient and the Bader charge was also compared and we found the coefficient is also dependent on the Bader charge. In addition, the linear optical absorption coefficient was calculated; the obvious change in absorption peaks was observed during the transition. By comparing the crystal orbital of the reference cubic phase and distorted R3c phase we found that the hybridized Bi 6s and O 2p is the main reason that causes the structure distortion. The local phonon density of states proves that the Bi and O interaction is the origin of the instability of cubic phase BiFeO3. The factor group analysis showed the change of Raman and IR modes during the transition.

Photocatalytic and magnetic behaviors of BiFeO3 thin films deposited on different substrates

Single phase polycrystalline BiFeO3 thin films were grown on three different substrates via chemical solution deposition. Our results indicate that the band gap of as-prepared BiFeO3 films can be tuned (2.02–2.67 eV) by the grain size effects caused by the substrates. These BiFeO3 films show good photocatalytic properties by the degradation of Congo red solution under visible-light irradiation (λ > 400 nm). Additionally, weak ferromagnetic behaviors can be observed at room temperature in all the films, which should be correlated to the destruction of the incommensurate cycloid spin structure of BiFeO3 phase and the coexistence of Fe3+ and Fe2+ as confirmed by X-ray photoelectron spectroscopy.

English

Bismuth ferrite nano particles were prepared by sol-gel route using solutions of iron nitrateand bismuth nitrate. The produced powder was dried at 150oC for 2 h and then calcined at various temperatures for 2 h as well.XRD was used tostudy the structure of BiFeO3Nanoparticles, it is found that the grain size were about 12.8, 39.9 and 47.6nmas calculated usingScherrer equation for samples calcined at 450˚C, 500 oC and 550 oC for 2 h respectively, this explain that grain size increase with increase in temperature and XRD too showed that besides the formation of single phase BiFeO3 an impurity phase was also observed.From diagnosis of samples, we foundthat BiFeO3have rhombohedral structure. Scanning electron microscope (SEM) surface morphology study indicatedabetter homogeneity with fine grain morphology besides the formation of single phase BiFeO3 an impurity phase. Less impurity phases was found in sample calcined at 450 ˚C. Atomic force microscope (AFM) declare that it hashomogenous particle distribution and coarseness of surface with change of particles to spherical form with dimension of 1.21nm, alsoGranularity Cumulation Distribution Report for same sample found that average diameter is equal to 45.31 nm. Finally the electrical properties resultsofBiFeO3such asthe dielectric constant , tangent loss and electrical resistivity and conductivitywere studied at different applied frequencies.

Conduction mechanisms and enhanced multiferroic properties of 2-2 type Bi0.89Sm0.11FeO3–NiFe2O4 composition thin films

Pure BiFeO3 (BFO), BiFeO3–NiFe2O4 (BFO–NFO) and Sm-doped Bi0.89Sm0.11FeO3–NiFe2O4 (BSFO–NFO) thin films were prepared on FTO/glass (SnO2:F) substrates via a sol–gel method. The effects of NFO layer and Sm-doping on the crystalline structure, leakage current, ferroelectric and ferromagnetic properties of all the thin films were investigated. Rietveld refinement of X-ray diffraction data indicated that BFO phase in BFO–NFO and BSFO–NFO films is trigonal with different space group of R3c:H and R3m:R. The leakage current of BFO film shows Ohmic emission at the entire electric field, whereas the dominant mechanism is changed to Fowler–Nordheim tunneling emission at the high electric field in the BFO–NFO and BSFO–NFO films. The improved ferroelectric and ferromagnetic properties have been observed in the BSFO–NFO film, e.g., large remanent polarization is 66.18μC/cm2 and large saturation magnetization is 31.86emu/cm3.

Multiferroic BiFeO3 thin films: Structural and magnetic characterization

BiFeO3 (BFO) film has been deposited on indium tin oxide (ITO) substrate by a simple sol–gel spin-coating technique. The crystal phase composition, surface morphology, topography and magnetization measurements of the BFO thin film were investigated using grazing incidence X-ray diffraction (GIXRD), scanning electronic microscope (SEM), atomic force microscope and vibrating sample magnetometer, respectively. GIXRD analysis revealed that the film was fully crystallized and no impure phase was observed. Cross-section SEM results indicated that compact and homogeneous BFO thin film was deposited on ITO with a thickness of about 180nm. Moreover, most of A and E-symmetry normal modes of R3c BFO were assigned by Raman spectroscopy. We report here that the pure phase BFO film shows ferromagnetism at room temperature with remarkably high saturation magnetization of 63kAm−1. Our results are discussed mainly in correlation with the condition of processing technique and destruction of the spiral spin cycloid at interface layers and grain boundaries.

Photovoltaic effect and enhanced magnetization in 0.9(BiFeO3)–0.1(YCrO3) composite thin film fabricated using sequential pulsed laser deposition

We report on the photovoltaic effect and multiferroic properties of a 0.9(BiFeO3)–0.1(YCrO3) composite thin film deposited on a Pt/TiO2/SiO2/Si substrate by sequential ablation of BiFeO3 and YCrO3 ceramic targets using pulsed laser deposition. The desired composition of the composite was achieved by controlling the ablation time of respective targets. As confirmed by the x-ray diffraction pattern the resultant film was found to be polycrystalline in nature and composed of a mixture of both rhombohedral BiFeO3 and orthorhombic YCrO3 phases. Interesting multiferroic properties in terms of an enhanced saturation magnetization of ∼14 emu cm−3 and the remnant polarization of ∼4.5 µC cm−2 were observed where the enhancement in magnetization as compared to pristine BiFeO3 could be attributed to the super-exchange interaction between Fe and Cr-ions. The photovoltaic properties of the composite thin film were studied under white light illumination in both top–bottom and lateral electrode configurations. Short circuit current densities (JSC) = 1.48 µA cm−2 and 0.44 µA cm−2, and open circuit voltages (VOC) = 0.51 V and 0.32 V were observed in top–bottom and lateral electrode configurations, respectively.

Microwave-assisted sol–gel synthesis of BiFeO3 nanoparticles

Among the various multiferroic materials bismuth iron oxide is a promising candidate due to its relatively high antiferromagnetic Neel temperature and high ferroelectric Curie temperature as compared to all other multiferroic materials. However, synthesis of phase pure BiFeO3 is extremely difficult due to the volatile nature of Bi2O3 that leads to bismuth-deficient phases and if excess of bismuth is employed it gives rise to bismuth rich phases. Moreover, the synthesis of phase pure BiFeO3 requires high temperature annealing in the range of 400–700 °C. In order to overcome these difficulties, we here report microwave-assisted sol–gel synthesis of phase pure BiFeO3 nanoparticles. In the present study, power of microwaves is varied as 136, 264, 440, 616 and 800 W. XRD results show formation of phase pure BiFeO3 with rhombohedrally distorted perovskite structure at 264, 440 and 800 W. Crystallite size decreases to 21 nm with the increase in microwave power to 800 W. The presence of absorption bands at 470 and 580 cm−1 in FTIR spectra, corresponding to FeO6 and BiO6, indicate the formation of pure BiFeO3 phase. BiFeO3 nanoparticles show high dielectric constant (135 at 1 kHz) at 264 W. SEM images show the formation of spherical and cubic nanoparticles in the range of 100–150 nm with microwave powers of 136–440 W. Increasing the microwave power to 616 W gives spherical nanoparticles with sizes of 60 nm while further increasing the microwave power to 800 W results in nanoneedles with diameter of 30 nm. Ferromagnetic behavior, instead of antiferromagnetic nature of BiFeO3, is observed for the nanoparticles prepared at microwave power of 616 and 800 W. This demonstrates that microwave-assisted sol–gel technique gives phase pure BiFeO3 nanoparticles using low power and less time, along with excellent ferromagnetic and dielectric properties as compared to conventional heating method.

Synthesis and Characterization of Pure Single Phase BiFeO3 Nanoparticles by the Glyoxylate Precursor Method

Perovskite-type BiFeO 3 nanoparticles have been synthesized by thermal decomposition of glyoxylate precursor. The effects of different thermal decomposition temperatures (300–600 °C) on the phase evolution, morphology, and physical properties were investigated by thermal analysis, infrared spectroscopy, X-ray diffraction, electron microscopy, vibrating sample magnetometry, and UV–Vis spectroscopy methods. The high purity bismuth ferrite nanoparticles synthesized at 600 °C showed the weak ferromagnetism behavior, due to the size confinement effect. Furthermore, the BiFeO 3 nanoparticles also exhibited strong absorption in the visible region with the lowest band gap of about 2.08 eV.

Effect of Solvents on the Ferromagnetic Behavior of Undoped BiFeO<sub>3</sub> Prepared by Sol-Gel

We here report the preparation of undoped BiFeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (BFO) thin films with sol-gel method using two solvents, i.e., acetic acid (AA) and ethylene glycol (EG). Films are annealed in the temperature range 100-300 °C for 60 mins. Films deposited using AA show the presence of bismuth rich phase Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">46</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">72</sub> along with BFO in the intermediary conditions. Pure phase BFO is obtained when films are deposited using EG as a solvent even under as-deposited conditions, and the pure phase persists after annealing in the temperature range 100 °C-300 °C. This is the lowest temperature reported for the preparation of phase pure BFO. Bandgap of AA-based films lies in the range 2.58-2.64 eV and that of EG-based films in the range 2.77-2.81 eV. Reduced value of dielectric constant, in AA-based films, is because of the presence of bismuth rich impurity phase as compared with the films prepared using EG. Bands in FTIR spectra at 487 and 552 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> are characteristic bands of FeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> and BiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> groups, indicating the formation of BFO. Films deposited using AA show weak ferromagnetic behavior, while those deposited using EG show strong ferromagnetic behavior as opposed to antiferromagnetic nature of BFO, due to suppression of spiral spin structure.

Piezoelectric materials of (1 − x)Pb(Zr0.53Ti0.47)O3–xBi(Y0.7Fe0.3)O3 for energy-harvesting devices

The piezoelectric ceramics (1−x)Pb(Zr0.53Ti0.47)O3–xBi(Y0.7Fe0.3)O3 [(1−x)PZT–xBYF] (where, x=0.00–0.05) has been synthesized by a modified conventional solid state method. Initially the Pb(Zr0.53Ti0.47)O3 [PZT] and Bi(Y0.7Fe0.3)O3 [BYF] was pre-synthesized and mixed to prepare (1−x)PZT–xBYF ceramic composites. The effects of simultaneous addition of BYF on PZT system were measured as a function of sintering temperature, phase formation, microstructure and piezoelectric/dielectric properties. It was found that between 1150°C and 1190°C, these ceramics were well sintered showing a maximum density of approximately ⩾97.8% of the theoretical value. X-ray diffraction analysis revealed some minor second phases of BiYO3 and BiFeO3 at x>0.01. A uniform, fine grained and relatively pore-free microstructure is obtained at x=0.01 whereas, the average grain size abruptly decreased on further addition of BYF. All the (1−x)PZT–xBYF ceramics doped with various BYF content provided a high TC in the range of 382–387°C. It was found that the piezoelectric and the dielectric properties of (1−x)PZT–xBYF ceramics vary significantly with increasing BYF content. In addition, the piezoelectric voltage constant (g33), and transduction coefficient (d33×g33) of (1−x)PZT–xBYF ceramics have been calculated. For energy-harvesting materials, a high piezoelectric voltage constant expressed by g33=d33/(ε0×K33T) (where K33T is the dielectric constant and ε0 is dielectric permittivity of free space) is desirable. At the sintering temperature of 1170°C, the (1–x)PZT–xBYF ceramic with 0.01mol BYF content showed a considerably higher d33 and kp with lower K33T values which results in significantly higher g33 of 53.07×10−3Vm/N and (d33×g33) of 20,167×10−15m2/N. The large (d33×g33) indicates that the 0.99Pb(Zr0.53Ti0.47)O3–0.01Bi(Y0.7Fe0.3)O3 ceramic is a good candidate material for energy harvesting devices. The detail investigations and observations revealed that, the composition with x=0.01mol BYF could be the optimum magnitude of doping level in the PZT system.

Optimization of excess Bi doping to enhance ferroic orders of spin casted BiFeO3 thin film

Multiferroic Bismuth Ferrite (BiFeO3) thin films with varying excess bismuth (Bi) concentration were grown by chemical solution deposition technique. Room temperature multiferroic properties (ferromagnetism, ferroelectricity, and piezoelectricity) of the deposited BiFeO3 thin films have been studied. High resolution X-ray diffraction and Raman spectroscopy studies reveal that the dominant phases formed in the prepared samples change continuously from a mixture of BiFeO3 and Fe2O3 to pure BiFeO3 phase and, subsequently, to a mixture of BiFeO3 and Bi2O3 with increase in the concentration of excess Bi from 0% to 15%. BiFeO3 thin films having low content (0% and 2%) of excess Bi showed the traces of ferromagnetic phase (γ-Fe2O3). Deterioration in ferroic properties of BiFeO3 thin films is also observed when prepared with higher content (15%) of excess Bi. Single-phased BiFeO3 thin film prepared with 5% excess Bi concentration exhibited the soft ferromagnetic hysteresis loops and ferroelectric characteristics with remnant polarization 4.2 μC/cm2 and saturation magnetization 11.66 emu/g. The switching of fine spontaneous domains with applied dc bias has been observed using piezoresponse force microscopy in BiFeO3 thin films having 5% excess Bi. The results are important to identify optimum excess Bi concentration needed for the formation of single phase BiFeO3 thin films exhibiting the improved multiferroic properties.

Multiferroic BiFeO3 glass-ceramics: Phase formation and physical property

A production of polycrystalline phase with glassy precursor, i.e., glass-ceramics processing, has a great potential to fabricate the sophisticated materials. In this study, we have prepared the Bi2O3–Fe2O3–BaO–B2O3 system precursor, in which BiFeO3 phase is crystallizable, and examined the phase formation and physical properties in resulting glass-ceramics. We found both ferroelectricity and ferromagnetism in the glass-ceramics consisting of BiFeO3 phase, i.e., multiferroic glass-ceramics. This study has demonstrated an alternative method for synthesis of polycrystalline BiFeO3 material.

Structure, conduction mechanisms and multiferroic properties of (Sm, Cr) co-doped Bi0.89Sm0.11Fe0.97Cr0.03O3–NiFe2O4 composition thin films

(Sm, Cr) co-doped Bi0.89Sm0.11Fe0.97 Cr0.03O3 (BSFC)–NiFe2O4 (NFO) composition thin films were successfully prepared on FTO/glass (SnO2:F) substrates via a sol–gel method. The structure, surface morphology, leakage current, ferroelectricity and ferromagnetism of BSFC–NFO composition thin film have been investigated. X-ray diffraction analysis indicates that the thin film is polycrystalline and consisted of a rhombohedral perovskite (R3m space group) BiFeO3 phase and a cubic (Fd-3m space group) inverse spinel NiFe2O4 phase. The BSFC–NFO composition thin film is promising in practical application because of its well saturated ferromagnetic (Ms = 19.45 emu/cm3) and ferroelectric (Pr = 39 µC/cm2) hysteresis loops with low order of leakage current density (J = 6.73 × 10−6 A/cm2, at an applied electric field of 100 kV/cm). Which suggest the ferroelectric and ferromagnetic properties can be improved by this composition thin film structure. Moreover, the various conduction mechanisms of BSFC–NFO composition thin film have also been studied.

Synthesis and characterization of coprecipitation-prepared La-doped BiFeO3 nanopowders and their bulk dielectric properties

Nanocrystalline Bi1−xLaxFeO3 (x = 0, 0.05, 0.1, 0.2, and 0.3) powders with particle sizes of about 50–90 nm were prepared by a simple co-precipitation method. The prepared powders were characterized by X-ray diffraction (XRD), Fourier trans form infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray absorption spectroscopy (XAS), and vibrating sample magnetometer (VSM) techniques. The morphologies of all the Bi1−xLaxFeO3 powders were spherical in shape. The main phase of BiFeO3 with rhombohedral structure was observed in all Bi1−xLaxFeO3 powders. A pure BiFeO3 phase was observed in the powder with x = 0.3 only whereas impurity phases of Bi2O3, Bi24Fe2O29, and Bi25FeO40 were observed in other powders. Grain size of Bi1−xLaxFeO3 ceramics decreased with increasing La doping concentration. The undoped BiFeO3 sample exhibited a high dielectric constant of 1.1 × 104 at room temperature in a low frequency range. The dielectric constant of BiFeO3 ceramics increased after substitution by La3+. A Debye-type dielectric relaxation at low temperatures was observed, which was attributed to the carrier hopping between Fe2+ and Fe3+ ions.

A new chiral phase of BiFeO3 evidenced from resonant x-ray Bragg diffraction

Results that support a new chiral phase in the only multiferroic material known above room temperature have been obtained by a resonant x-ray Bragg diffraction experiment, with the incoming x-ray beam tuned near the Fe K-edge (7.1135 keV), performed in its ferroelectric phase. The R3c forbidden reflection (0,0,9)H was studied as a function of the rotation of the crystal about the Bragg wavector in both phases, paramagnetic (700 K) and antiferromagnetic (300 K). The data gathered is consistent with a chiral structure formed by a circular cycloid propagating along (1,1,0)H. Templeton and Templeton (T&T) scattering at 700 K is attributed in part to charge-like quadrupoles absent in a standard model of a cycloid in which a material vector generates all electronic states of the resonant ion. Extensive sets of azimuthal-angle data are used to infer values of three atomic multipoles in a satisfactory minimal model of the iron electronic structure, with a quadrupole (E1-E1 event) and a hexadecapole (E2-E2 event) contributing T&T scattering, plus a magnetic dipole (E1-E1).

Structure, leakage mechanism and multiferroic properties of (Mn, Cr) co-doped BiFe0.93Mn0.04Cr0.03O3/NiFe2O4 bilayer film by sol–gel

(Mn, Cr) co-doped BiFe0.93Mn0.04Cr0.03O3/NiFe2O4 (BFMC/NFO) bilayer film has been prepared on FTO/Glass substrate via a sol–gel method. Enhanced ferroelectric and ferromagnetic properties were obtained in the (Mn, Cr) co-doped BFMC/NFO bilayer film, e.g., large remnant polarization value (Pr=55μC/cm2) and large saturation magnetization (Ms=27.04emu/cm3). Rietveld refinement of X-ray diffraction data of the bilayer film showed that the BFMC layer was conformed to a BiFeO3 phase with tetragonal lattice type with P4 space group, and NFO layer was a NiFe2O4 phase with cubic lattice type with Fd-3m: 1 space group. The tetragonal phase is in favor of enhanced ferroelectric properties in the BFMC/NFO bilayer film. Moreover, leakage mechanism of the BFMC/NFO bilayer film was also investigated.

Preparation and electric properties of BiFeO3 film by electrophoretic deposition

BiFeO3 (BFO) films were synthesized by electrophoretic deposition process (EPD) using suspensions containing BFO powders. The BFO powders were prepared by sol–gel method and the effects of calcining temperature and additive on phase composition, particle size and size distribution of BFO powders have been systematically investigated. It is found that the BFO powders prepared with citric acid as chelating agent and calcined at 600°C is single phase and have smaller particle size and narrow size distribution. And the well-dispersed and stable suspensions containing BFO particles were obtained by adjusting the pH value to 6.5. Single phase BFO films were deposited by EPD. The SEM micrographs show there are some pores in the BFO films and the grain size of films is in the range of 250–400nm. The dielectric constant of BFO films decreases with the increase of frequency and there is a peak of dielectric loss when frequency is about 10kHz. Moreover, the electric field and frequency dependences of ferroelectric properties in BFO films have been investigated.