Finite-temperature properties of multiferroic BiFeO3

An effective Hamiltonian scheme is developed to study finite-temperature properties of multiferroic BiFeO3. This approach reproduces very well (i) the symmetry of the ground state, (ii) the Néel and Curie temperatures, and (iii) the intrinsic magnetoelectric coefficients (that are very weak). This scheme also predicts (a) an overlooked phase above Tc approximately 1100 K that is associated with antiferrodistortive motions, as consistent with our additional x-ray diffractions, (b) improperlike dielectric features above Tc, and (c) that the ferroelectric transition is of first order with no group-subgroup relation between the paraelectric and polar phases.     

Improved multiferroic properties of La-doped 0.6BiFeO3—0.4SrTiO3 solid solution ceramics

The 0.6（Bi1-xLax）FeO 3-0.4SrTiO 3（x = 0,0.1） multiferroic ceramics are prepared by a modified Pechini method to study the effect of substitution of SrTiO3 and La in BiFeO3.The X-ray diffraction patterns confirm the single phase characteristics of all the compositions each with a rhombohedral structure.The magnetic properties of the ceramics are significantly improved by a solid solution with SrTiO3 and substitution of La.The values of the dielectric constant ε r and loss tangent tan δ of all the samples decrease with increasing frequency and become constant at room temperature.The La-doped 0.6BiFeO3-0.4SrTiO3 ceramics exhibit improved dielectric and ferroelectric properties,with higher dielectric constant enhanced remnant polarization（Pr） and lower leakage current at room temperature.Compared with a anti-ferromagnetic BiFeO3 compound,the 0.6（Bi0.9La0.1）FeO3-0.4SrTiO3 sample shows the optimal ferromagnetism with remnant magnetization M r ～ 0.135 emμ/g and ferroelectricity with Pr ～ 5.94 μC/cm 2 at room temperature.     

Room-temperature multiferroic properties of Co-doped KNbO3 ceramics

KNb_0.95Co_0.05O₃ (KN–Co) ceramic was prepared via a solid-state reaction method, and the effect of cobalt dopant on the structural, electric, and magnetic properties was studied. The KN–Co ceramic with polycrystalline perovskite structure exhibited ferromagnetic and ferroelectric properties simultaneously at room temperature, and the coupling of them was confirmed by a large magnetocapacitance effect (about 13%) near the Curie temperature. The possible causes for the magnetism and magnetoelectric properties are discussed.     

Structural,optical, and multiferroic properties of single phased BiFeO3

A soft chemical coprecipitation method has been proposed for synthesis of nano-sized multiferroic BiFeO₃ (BFO) powders. The X-ray diffraction pattern confirms the perovskite structure of BFO and Rietveld refinement reveals the existence of rhombohedral R3c symmetry. Crystallite size and strain value are studied from Williamson–Hall (W–H) analysis. The transmission electron microscope (TEM) image shows that the particle size of BFO powders lies between 50–100 nm. 4A₁ and 7E Raman modes have been observed in the range 100–650 cm^?1 and a prominent band centered around 1150–1450 cm^?1 have also been observed corresponding to the two-phonon scattering. Differential Thermal Analysis (DTA) shows the existence of two prominent peaks at 330 ^°C and 837 ^°C corresponding to the magnetic and ferroelectric ordering, respectively. From the temperature dependent dielectric studies, an anomaly in the dielectric constant is observed at the vicinity of Neel temperature (T _N ) indicating a magnetic ordering. Also, BFO shows antiferromagnetic behavior measured from the magnetic studies.     

Pressure-induced phase transitions of multiferroic BiFeO3

Pressure-induced phase transitions of multiferroic BiFeO3 have been investigated using synchrotron radiation X-ray diffraction with diamond anvil cell technique at room temperature. Present experimental data clearly show that rhombohedral （R3c） phase of BiFeO3 first transforms to monoclinic （C2/m） phase at 7 GPa, then to orthorhombic （Pnma） phase at 11 GPa, which is consistent with recent theoretical ab initio calculation. However, we observe another peak at 2θ=7° in the pressure range of 5-7 GPa that has not been reported previously. Further analysis reveals that this reflection peak is attributed to the orthorhombic （Pbam） phase, indicating the coexistance of monoclinic phase with orthorhombic phase in low pressure range.     

Strain effects on multiferroic BiFeO3 films

The field of multiferroics has experienced a rapid progress resulting in the discovery of many new physical phenomena. BiFeO₃ (BFO) compound, which is one of the few room-temperature single-phase multiferroics, has contributed subsequently to this progress. As a result, significant review articles have been devoted specifically to this famous system. This chapter is dedicated to the strain effects on the structure stability and property changes of BFO thin films. It is a short and non-exhaustive topical overview that may be seen as an invitation for interested readers to go beyond. There is a very active and prolific research in this field and we apologize to the authors whose relevant work is not cited here. After a short introduction, we will thus review the effect of strain on BFO films by describing the consequences on the structure and the phase transitions as well as on polar, magnetic and magnetoelectric properties.     

High-temperature heat capacity of the BiFeO3 multiferroic

Data on the heat capacity of the BiFeO₃ multiferroic have been obtained within a broad temperature interval. Correlation has been established between the composition of the Bi₂O₃-Fe₂O₃ pseudobinary system and the specific heat capacity of the oxide compounds.     

Aftereffects of an electric shock in BiFeO3 multiferroic

Delayed (up to several months) transformations in the atomic structure induced by the external action (electric shock) are observed for the first time in a class of nonmetals in BiFeO₃ multiferroic.     

Investigation of complex impedance and modulus properties of Nd doped 0.5BiFeO3-0.5PbTiO3 multiferroic Composites

0.5BiNd _x Fe_1?x O ₃ ? 0.5PbTiO₃ (BN _x F_1?x ? PT)(x = 0.05, 0.10, 0.15, 0.20) composites were successfully synthesized by a solid state reaction technique. At room temperature, X-ray diffraction shows tetragonal structure for all concentrations of Nd doped 0.5BiFeO₃ ? 0.5PbTiO₃ composites. The nature of Nyquist plot confirms the presence of bulk effects only for BN _x F_1?x ? PT (x = 0.05, 0.10, 0.15, 0.20) composites. The bulk resistance is found to decreases with the increasing temperature as well as Nd concentration and exhibits a typical negative temperature coefficient of resistance (NTCR) behavior. Both the complex impedance and modulus studies have suggested the presence of non-Debye type of relaxation in the composites. Conductivity spectra reveal the presence of hopping mechanism in the electrical transport process of the composites. The activation energy calculated from impedance plot of the composite decreases with increasing Nd _x concentration and found to be 0.89, 0.76, 0.71 and 0.70 eV for x=0.05, 0.10, 0.15 and 0.20 respectively.     

Co掺杂Bi5Ti3FeO15多铁陶瓷的磁电性能

采用了传统的固相烧结工艺制备出Bi₅Fe_1-xCo _{x Ti3O15(BFCT-x,x=0.0—0.6)多铁陶瓷样品,研究了Co掺杂对Bi5FeTi3O15(BFTO)微观结构、铁电和磁性能的影响.X射线衍射谱显示样品均已形成四层铋系层状钙钛矿相,且随着掺杂量的增加发生了结构变化.拉曼光谱进一步证实掺入的Co占据了< 关键词： 固相烧结 多铁陶瓷 剩余磁化 剩余极化}     

Enhanced microwave absorption properties in BiFeO3 ceramics prepared by high-pressure synthesis

Multiferroic BiFeO₃ nanoparticles and ceramics have been successfully prepared by sol–gel method and following high-pressure synthesis. The denser samples, good crystallization and crystal structure deformation have been obtained via high-pressure synthesis proved by XRD, SEM and Raman spectra. The enhanced magnetization of high-pressure samples attributes to crystal structure deformation; moreover, the enhanced dielectric loss of high-pressure samples results from good crystallization. The better microwave absorption properties can be obtained by high-pressure synthesis, and the minimum reflection loss elevates from ?13 dB at 12.4 GHz to ?17 dB at 11.2 GHz. It means that the high-pressure synthesis can effectively improve microwave absorption properties of multiferroic materials.     

Mechanisms of exchange bias with multiferroic BiFeO3 epitaxial thin films

We have combined neutron scattering and piezoresponse force microscopy to show that the exchange field in CoFeB/BiFeO_{3} heterostructures scales with the inverse of the ferroelectric and antiferromagnetic domain size of the BiFeO3 films, as expected from Malozemoff's model of exchange bias extended to multiferroics. Accordingly, polarized neutron reflectometry reveals the presence of uncompensated spins in the BiFeO3 film at the interface with CoFeB. In view of these results, we discuss possible strategies to switch the magnetization of a ferromagnet by an electric field using BiFeO3.     

Rietveld analysis and multiferroic properties of Fe doped Ba0.95Bi0.05TiO3 ceramics

We report on the structural, magnetic and ferroelectric properties of Fe doped Ba_0.95Bi_0.05TiO₃ (Ba_0.95Bi_0.05Ti_1−yFe_yO₃ (0 ≤ y ≤ 0.1)) ceramics prepared by solid state reaction. Rietveld analysis shows that compounds with y ≤ 0.07 have a single phase-tetragonal structure (space group P4mm), and the tetragonality (c/a) decreases as x increases. Ba_0.95Bi_0.05Ti_0.93Fe_0.07O₃ ceramic shows ferromagnetic order with a Curie temperature of about 700 K and ferroelectric order at room temperature. These results indicate that the Ba_0.95Bi_0.05Ti_0.93Fe_0.07O₃ can be useful as a multiferroic material.     

Raman tensor elements for multiferroic BiFeO3 with rhombohedral R3c symmetry

A quantitative assessment of the Raman spectrum emitted from a coarse‐grained polycrystal of multiferroic BiFeO₃ has been carried out by means of a polarized Raman microprobe. The dependence of the intensity of Raman phonon modes has been first theoretically modeled as a function of crystal rotation. Then, the Raman tensor elements have been experimentally determined from the analysis of the A_g and E_g vibrational modes. Copyright © 2009 John Wiley & Sons, Ltd.     

Surface enhanced strong visible photoluminescence from one-dimensional multiferroic BiFeO3 nanostructures

The surface defect dominated visible emission from one-dimensional multiferroic BiFeO₃ (BFO) nanowires (NWs) is characterized by photoluminescence (PL) spectroscopy. The PL spectra of BFO NWs exhibit a weak near band emission (NBE) along with a strong defect-level emission (DLE). It is suggested that excess surface defects exist in BFO NWs which are responsible for strong visible green emission. Passivation of BFO NW surface with H₂ significantly improves the NBE emission while suppressing surface recombination. Such a surface enhanced emission promises many potential applications of BFO NWs not only in photonic devices such as LEDs but also in fluorescence-based chemical sensing.     

Effects of Co-substitutes on multiferroic properties of Bi5FeTi3O15 ceramics

The samples Bi₅FeTi₃O₁₅ (BFTO) and Bi₅Fe_0.5Co_0.5Ti₃O₁₅ (BFCT) were prepared by incorporating BiFeO₃ (BFO) and BiFe_0.5Co_0.5O₃ (BFCO) into the host Bi₄Ti₃O₁₂ (BTO) using the solid state reaction technique. The Raman shift data indicate that the as-prepared materials are of the four-layer Aurivillius phase with an orthorhombic symmetry. At room temperature (RT) and under a driving electric field of 230 kV/cm, the half substitution of Fe by Co ions is found to result in the increase in the remanent polarization $\left(2P_r\right)$ by about 35% and simultaneously the decrease in $2E_c$ by about 41%, respectively. The magnetic moment at least tripled its value by substituting half Fe ions by Co ions. The $2M_r$ of BFCT is about three thousand times the value of BFTO.     

Effect of BiFeO3 ceramics morphology on electrodynamic properties in the terahertz frequency range

The nature of the contribution (additional to the phonon one) to the permittivity of bismuth ferrite ceramics has been studied using submillimeter and Fourier transform infrared spectroscopy. It has been shown that its numerical value depends on morphological features of samples, and the nature is determined by the defectness and crystallite interfaces. The effect of structure-phase features of bismuth ferrite ceramics on antiferromagnetic resonance line frequencies is indicated.