Conditions favoring the polar weak ferromagnetic state in BiFeO<Subscript>3</Subscript>-type multiferroics

The crystal structure and magnetic properties of Bi_{1 − x} A _x FeO_{3 − x/2} (A = Ca, Sr, Pb, Ba), Bi_{1 − x} A _x (Fe_{1 − x} Ti _x )O₃, and Bi_{1 − x} A _x (Fe_{1 − x/2}Nb _x/2)O₃ solid solutions have been studied. It is shown that the homogeneous polar weak ferromagnetic state occurs in the vicinity of a morphotropic phase boundary in the systems where dopant ions lead to the reduction of the unit cell volume in the polar phase. In the case of A = Ca, the non-polar phase also exhibits weak ferromagnetism and the spontaneous magnetizations in the polar and nonpolar phases differ only slightly.     

Structural and optical properties of Ba(Ti1−x,Nix)O3 thin films prepared by sol–gel process

Ba(Ti_1−x,Ni_x)O₃ thin films were prepared on fused quartz substrates by a sol–gel process. X-ray diffraction and Raman scattering measurements showed that the films are of pseudo-cubic perovskite structure with random orientation and the change of lattice constant caused by Ni-doping with different concentrations is very small. Optical transmittance spectra indicated that Ni-doping has an obvious effect on the energy band structure. The energy gap of Ba(Ti_1−x,Ni_x)O₃ decreased linearly with the increase of Ni concentration. It indicates that the adjusting of band gap can be achieved by controlling the Ni-doping content accurately in Ba(Ti_1−x,Ni_x)O₃ thin films. This has potential application in devices based on ferroelectric thin films.     

Electrical properties of perovskite-type La(Cr1−xMnx)O3+δ

Perovskite-type La(Cr_1−xMn_x)O_3+δ (0.0x1.0) was synthesized using a sol–gel process. The crystal structure of La(Cr_1−xMn_x)O_3+δ changes from orthorhombic to rhombohedral at x=0.6. The Mn⁴⁺ ion content increases monotonically in the range 0.2x1.0. The magnetic measurement of La(Cr_1−xMn_x)O_3+δ indicates that a Mn³⁺ ion is a high-spin state with (d)³(dγ)¹. The variation of the average (Cr, Mn)-O distance is explained by ionic radii of the Cr³⁺, the Mn³⁺, the Mn⁴⁺ ions. Since the log σT–1/T curve is linear and the Seebeck coefficient (α) is independent of temperature, it is considered that La(Cr_1−xMn_x)O_3+δ is a p-type semiconductor and exhibits the hopping conductivity.     

High permeability and low power loss of Ti and Zn substitution lithium ferrite in high frequency range

The effect of Zn and Ti on the magnetic, power loss and structural properties of Li_0.5Zn_xTi_xMn_0.05Fe_2.45−2xO₄ ferrites (x=0.0 to 0.30 in step of 0.05)+0.5 wt% Bi₂O_3, prepared by standard ceramic technique, has been investigated. Complex permeability (μ*=μ′−jμ″) has been analyzed at room temperature in frequency range from 1 to 10³ MHz. It was found an enhancement in permeability with Ti and Zn concentration in Li_0.5Zn_xTi_xMn_0.05Fe_2.45−2xO₄ and exhibits the maximum value 106 for x=0.20 sample. Complex permeability of these ferrites exhibits stable frequency response up to 7 MHz beyond which the real part decreases sharply and imaginary part increases to have a peak at the relaxation frequency. Power loss measurements have been carried out in induction condition (B=10 mT) in frequency range of 50 kHz to 3 MHz. Power loss has been found to be quite low with the substitution of Ti and Zn in lithium ferrite.     

Remarkable influence on the dielectric and magnetic properties of lithium ferrite by Ti and Zn substitution

The effect of Zn and Ti substitution on the magnetic and electrical properties of Li_0.5Zn_xTi_xMn_0.05Fe_2.45−2xO₄ ferrites (x=0.0 to 0.30 in steps of 0.05) +0.5wt% Bi₂O₃ prepared by a standard ceramic technique has been investigated. Electrical conductivity and dielectric measurements at different temperatures from 300 K to 700 K in the frequency range from 100 Hz to 2 MHz have been analysed. The variation of the real part of dielectric constant (ε^′) and loss tangent (tanδ) with frequency and temperature has been studied; it follows the Maxwell–Wagner model based on the interfacial polarization in consonance with the Koops phenomenological theory. It is found that the permittivity of zinc and titanium substituted lithium ferrite improves and shows a maximum value ( 1.5×10⁵) at 100 Hz for the x=0.25 sample. The dielectric transition temperature (T_d) depends on the concentration of Ti and Zn in Li_0.5Zn_xTi_xMn_0.05Fe_2.45−2xO₄. The saturation magnetization and Curie temperature both decrease with increase in the concentration of Ti and Zn in the ferrite.     

Structural, thermal and conductive properties of Bi4−xMxV2O11 (M = La, Gd; 0 x 0.4) compounds

Bi_4−xM_xV₂O₁₁ (M = La, Gd) was prepared by solid state reactions. The amount of La and Gd in the (Bi_4−xM_xV₂O₁₁) was varied in the range of (0 x 0.4). The addition of La and Gd to Bi₄V₂O₁₁ electrolyte was found to stabilize the β crystalline phase for x 0.3. In addition, the phase transition corresponding β- to γ-phases are evident in the ionic conductivity plots as well as in XRD, DSC profiles of x 0.3 samples. The highest ionic conductivity was observed in Bi_3.9La_0.1V₂O₁₁ and Bi_3.8Gd_0.2V₂O₁₁ samples in the range of 10⁻³–10⁻⁴ S/cm for 700–500 °C. These results were supported by impedance spectroscopy, X-ray diffraction (XRD) and differential scanning calorimetry (DSC).     

M?ssbauer and X-ray studies of the structural phase transformations and suppression of polymorphism in La1 ? xSrxMn0.98Fe0.02O3 + δ (x = 0.05?0.30)

The specific features of the structural transformations in La_{1 − x} Sr _x Mn_0.98Fe_0.02O_{3 + δ} (x = 0.05−0.30) as functions of the Sr content have been investigated using M?ssbauer spectroscopy and X-ray diffraction. It has been shown that, when the Sr content is lower than 10%, three phases are formed: PnmaII, PnmaI, and R-3c. In the Sr concentration range from 10 to 20%, the PnmaI and R-3c phases are formed. At a strontium content of 20%, the rhombohedral phase is formed. When the strontium content increases to 30%, the presumably monoclinic (distorted cubic) phase is formed in addition to the rhombohedral phase. The specific features of the formation and suppression of the phases in the LaMn_0.98Fe_0.02O_{3 + δ} compound are compared with those observed in the strontium-doped La_{1 − x} Sr _x Mn_0.98Fe_0.02O_{3 + δ} compound. The general regularities of the structural transformations and substantial differences between them have been revealed.     

Micro‐Raman study of exfoliated Bi2Sr2CaCu2O8+δ single crystals with different thicknesses

Exfoliated Bi₂Sr₂CaCu₂O_8+δ (Bi‐2212) single crystals were prepared by micromechanical cleavage of bulk Bi‐2212 single crystals on SiO₂/Si substrates. Room temperature micro‐Raman spectra were collected using a 532‐nm laser source. The evolutions of the spectra of A_1g (Bi), A_1g (Sr), and A_1g (O_Bi) Raman modes with different thicknesses of the samples were studied. The refractive index of Bi‐2212 single crystal was obtained by studying the intensity evolutions based on the interference effect. The observed wavenumber shifts of the A_1g (Bi), A_1g (Sr), and A_1g (O_Bi) modes were analyzed. Copyright © 2011 John Wiley & Sons, Ltd.     

Structural and magnetic phases of Bi<Subscript>1−x</Subscript>A<Subscript>x</Subscript>FeO<Subscript>3−</Subscript><Subscript>δ</Subscript> (A = Sr,Pb) perovskites

The Bi_1−xA_xFeO_{3− δ} (A = Sr, Pb) systems have been studied using the X-ray, neutron powder diffraction and magnetization measurements in a magnetic field up to 14 T. It was found that around x ∼ 0.06 the crystal symmetry changes from a rhombohedral (space group R3c) to pseudo-tetragonal. In the composition range 0.07 ≤ x ≤ 0.14 the phases with different symmetry of the unit cell coexist independent of synthesis conditions. The neutron powder diffraction shows that the iron ions have average oxidation state close to 3+. The magnetic structure for Bi_0.5Sr_0.5FeO_{3− δ} is found to be G-type antiferromagnetic with magnetic moment of about 3.8 μ_B/Fe³⁺. The weak ferromagnetic state due to magnetoelectric interactions was revealed in the lightly doped rhombohedrally distorted compositions. No evidence for a spontaneous magnetization was observed for the pseudo-tetragonal phases. These compositions show irreversible nonlinear magnetization vs. field behavior apparently due to small local deviations from the collinearity of the magnetic moments.     

Defect chemistry and oxygen transport of (La0.6Sr0.4 − xMx)0.99Co0.2Fe0.8O3 − δ, M = Ca (x = 0.05, 0.1), Ba (x = 0.1, 0.2), Sr: Part I: Defect chemistry

This paper is the first part of a two part series, where the effects of varying the A-site dopant on the defect chemistry, the diffusion coefficient and the surface catalytic properties of the materials (La_0.6Sr_0.4 − xM_x)_0.99Co_0.2Fe_0.8O_3 − δ, M = Sr, Ca (x = 0.05, 0.1), Ba (x = 0.1, 0.2) (LSMFC) have been investigated. In part I, the findings on the defect chemistry are reported, while the transport properties are reported in part II. Substitution of Sr²⁺ ions with Ca²⁺ ions (smaller ionic radius) and Ba²⁺ ions (larger ionic radius) strains the crystal structure differently for each composition while keeping the average valence of the cations constant. The Ba²⁺ containing materials show the largest oxygen loss at elevated temperatures, while the purely Sr²⁺ doped material showed the smallest oxygen loss. This was reflected in the partial oxidation entropy of the materials. The measured oxygen loss was modelled with point defect chemistry models. Measurements at very low p_O2 showed several phase transitions.     

Crystallization of amorphous Fe100−xPx (13x24) alloys

A study of the structure change with temperature in amorphous Fe₁₀₀−xP_x (13 x 24) alloys was carried out by measuring magnetization and thermal expansion and also by structural analysis using X-ray diffraction and differential thermal analysis (DTA). The structure of the amorphous alloys relaxes (the decrease of excess free volume) at temperatures 100–150 K below the crystallization temperatures. The alloys with x 15 transform into (α-Fe + amorphous) at about 600 K. The alloys with x15 transform into (α-Fe+amorphous+Fe₃P) at about 600 K. With further heating, the alloys transform into (α-Fe+Fe₃P) both of which are stable phases from the equilibrium phase diagram.     

Oxygen nonstoichiometry of Sr(Co,Fe)O3−δ-based perovskites: I. Coulometric titration of SrCo0.85Fe0.10Cr0.05O3−δ by the two-electrode technique

The p(O₂)–T–δ diagram of perovskite-type SrCo_0.85Fe_0.10Cr_0.05O_3−δ was determined by the coulometric titration technique in the temperature range 770–1250 K at oxygen partial pressures from 8 10⁻¹⁰ to 0.5 atm. Stability of the cubic perovskite phase of SrCo_0.85Fe_0.10Cr_0.05O_3−δ, existing down to the oxygen pressures of 10⁻³–10⁻⁵ atm, was found to be slightly higher than that of SrCo_0.80Fe_0.20O_3−δ, probably due to stabilization of oxygen octahedra neighboring Cr⁴⁺ cations. When the oxygen nonstoichiometry of the Cr-containing perovskite decreases from 0.47 to 0.38, the partial molar enthalpy and entropy for overall oxygen incorporation reaction vary in the ranges −165 to −60 kJ mol⁻¹ and 90 to 150 J mol⁻¹ K⁻¹, respectively. Within the stability limits of the single perovskite phase, the p(O₂)–T–δ diagram can be adequately described by equilibrium processes of oxygen incorporation, cobalt disproportionation and interaction of cobalt and iron cations, with the thermodynamic functions independent of defect concentrations. Increasing grain size in SrCo_0.85Fe_0.10Cr_0.05O_3−δ ceramics from submicron size to 100–200 μm has no effect on the oxygen thermodynamics. The two-electrode coulometric titration technique, based on the alternate use of electrodes for oxygen pumping and e.m.f. measurements, is described and verified by studying oxygen nonstoichiometry of La_0.3Sr_0.7CoO_3−δ and PrO_x.     

Thermogalvanic power and fast ion conduction in δ-Bi2O3 and δ-(Bi2O3)1−x(R2O3)x with R = Y, Tb---Lu

The thermogalvanic power (Seebeck coefficient) of O^2- conducting δ-Bi₂O₃ and δ-(Bi₂O₃)_1−x(Y₂O₃)_x has been measured directly as a function of temperature and partial oxygen pressure in N₂---O₂ mixtures. The of δ-(Bi₂O₃)_0.75(R₂O₃)_0.25 with R = Tb---Lu was indirectly determined using an isothermal concentration cell technique. Except for pure δ-Bi₂O₃, the heat of transport is much smaller than the activation energy for O^2- conduction for all materials. The vibrational freedom of O²⁻ ions in all δ-stabilized materials is reflected in their IR spectra at room temperature. Two prototypes of a thermogalvanic P_O2 meter were tested.     

Structural and magnetic characterization of Bi1–xLaxFeO3 and BiFe1–yMnyO3 nanoparticles synthesized via a sol–gel method

Here, La-doped Bi_1–xLa_xFeO₃ and Mn-doped BiFe_1–yMn_yO₃ (x, y?=?0.1, 0.2, 0.3) nanoparticles were synthesized by a sol–gel process. For Bi_1–xLa_xFeO₃, structural analysis suggested that its structure changed from rhombohedral to tetragonal phase without secondary phase when x increased from 0 to 0.3. In addition, with the increasing doping concentration of La, transmission electron microscopy exhibited the doped nanoparticle size reduced, while the magnetic properties were correspondingly enhanced. However, for synthesis of BiFe_1–yMn_yO₃, phase analysis showed that there easily existed secondary phase for nanoparticles with 10% and 30% Mn-doped, while 20% Mn-doped nanoparticles indicated high crystallinity without any impure phases. Similarly, we observed the enhanced magnetic properties with the increase of Mn concentration.     

The Fe3O4 origin of the “Biphase” reconstruction on α-Fe2O3(0 0 0 1)

The so-called Biphase termination on α-Fe₂O₃ has been widely accepted to be a structure with a 40 Å unit supercell composed of coexisting islands of Fe_1−xO and α-Fe₂O₃. Based on thermodynamic arguments and experimental evidence, including transmission electron diffraction, imaging, magnetic and spectroscopic information, it is found that the previously proposed structure model is inaccurate. The actual Biphase structure is instead a layered structure related to the reduction of α-Fe₂O₃ to Fe₃O₄. A model for the Biphase termination is proposed which does not contain islands of Fe_1−xO but instead consists of bulk α-Fe₂O₃ and a Fe₃O₄-derived overlayer. The proposed model is consistent with all current and previously reported experimental findings.     

The relationship between magnetic behaviour and local structure around Fe ions in Fe-doped TiO2 rutile

The magnetic and structural characterization of Ti_1−xFe_xO₂ (x=0.025, 0.05, 0.07, 0.125, and 0.15) samples prepared by mechano-synthesis using TiO₂ and Fe₂O₃ as starting materials are reported. XANES measurements performed at the Fe K-edge show that Fe ions are in 3+ oxidation state in the 7 at% Fe-doped sample and in a mixture of 2+ and 3+ oxidation states in the other samples. EXAFS results show the incorporation of Fe ions substituting Ti ones in the rutile TiO₂ structure. They also reveal a strong correlation between the number of oxygen nearest neighbours and the Fe²⁺ fraction, i.e the number of oxygen near neighbours decreases when the Fe²⁺ fraction increases. All samples present ferromagnetic-like behaviour at room temperature. We found a clear dependence between saturation magnetization and coercivity with the fraction of Fe²⁺ and/or the number of Fe near neighbour oxygen vacancies.     

First-principles prediction of coexistence of magnetism and ferroelectricity in rhombohedral Bi2FeTiO6

First principles calculations based on the density functional theory within the local spin density approximation plus U(LSDA + U) scheme, show rhombohedral Bi₂FeTiO₆ is a potential multiferroic in which the magnetism and ferroelectricity coexist. A ferromagnetic configuration with magnetic moment of 4μB per formula unit has been reported with respect to the minimum total energy. Spontaneous polarization of 27.3 μC/cm², caused mainly by the ferroelectric distortions of Ti, was evaluated using the berry phase approach in the modern theory of polarization. The Bi-6s stereochemical activity of long-pair and the ‘d⁰-ness’ criterion in off-centring of Ti were coexisting in the predicted new system. In view of the oxidation state of Bi³⁺, Fe²⁺, Ti⁴⁺, and O²⁻ from the orbital-resolved density of states of the Bi-6p, Fe-3d, Ti-3d, and O-2p states, the valence state of Bi₂FeTiO₆ in the rhombohedral phase was found to be Bi³⁺₂Fe²⁺Ti⁴⁺O₆.     

Magnetic properties of Ni-substituted BiFeO3

BiFe_1−xNi_xO₃ ceramic powders with x up to 0.10 have been prepared by the sol-gel technique. The band gap of BiFeO₃ is 2.23 eV, and decreases to 2.09 eV for BiFe_0.95Ni_0.05O₃ and BiFe_0.90Ni_0.10O₃. The Mössbauer spectra show sextet at room temperature, indicating the magnetic ordering and the presence of only Fe³⁺ ions. Superparamagnetism with blocking temperature of 31 K for BiFe_0.95Ni_0.05O₃ and 100 K for BiFe_0.90Ni_0.10O₃ was observed. Enhanced magnetization at room temperature have been observed (1.0 emu/g for BiFe_0.95Ni_0.05O₃ and 2.9 emu/g for BiFe_0.90Ni_0.10O₃ under magnetic field of 10,000 Oe), which is one order larger than that of BiFeO₃ (0.1 emu/g under magnetic field of 10,000 Oe). The enhanced magnetization was attributed to the suppression of the cycloidal spin structure by Ni³⁺ substitution and the ferrimagnetic interaction between Fe³⁺ and Ni³⁺ ions.     

Study of the modulated structure of Bi2Sr2CaFe2Ox

The ceramics and single crystals of the Bi₂Sr₂CaFe₂O_x compound were synthesized. The X-ray diffraction data showed orthorhombic symmetry. The lattice parameters are equal toa=5.464 Å,b=5.453 Å,c=31.313 Å. The crystal structure is described by the primitive Bravais lattice with the possible space groups Pbmm. Pbm2 and Pb2₁m. The obtained X-ray patterns show the presence the incommensurate structural modulation with the following parameters: the modulation vector lies in the (100)-plane, the value of the wave-vector componentq _b=0.22(3). The comparison of the obtained results with data for Fe-doped Bi₂Sr₂CaCu₂O_x and Bi₂Sr₃Fe₂O_x are presented. No magnetic peculiarities of the compound were found. Mssbauer measurements of the ceramic samples indicate the presence mainly of Fe³⁺ oxidation state and Fe⁴⁺ (about 20%). The decrease of the quadrupole-splitting values for 2212Fe in comparison with Fe-doped 2212Cu was revealed that may be connected with higher symmetry of the local environment of Fe atoms in 2212Fe.     

Influence of doping concentration on room-temperature ferromagnetism for Fe-doped BaTiO3 ceramics

Ba(Ti_1−xFe_x)O₃ ceramics (x=7, 30 and 70 at%) were prepared by solid-state reaction. All samples are single-phase with 6H-BaTiO₃-type hexagonal perovskite structure. Mössbauer spectra show all Fe atoms to be present as Fe³⁺ in BaTiO₃ lattice, occupying M1 octahedral and pentahedral sites. Room-temperature ferromagnetism is exhibited and saturation magnetization gradually decreases with increasing Fe content. The observed ferromagnetism is considered to be an intrinsic property of Ba(Ti_1−xFe_x)O₃, originating from super-exchange interactions between Fe³⁺ in different occupational sites associated with oxygen vacancies. The variation in magnetization with Fe content is related to the ratio of pentahedral to octahedral sites and oxygen vacancies.