Specific features of local structural,valence, and magnetic states of iron ions in the perovskite Bi0.5Ca0.5FeO3

The perovskite Bi_0.5Ca_0.5FeO₃ has been investigated using the Mössbauer effect at temperatures of 295 and 675 K. The measured temperature of the magnetic phase transition (Néel temperature) is T _N = 640 ± 10 K. Above the Néel temperature, there are two nonequivalent structural states of iron ions. In the perovskite Bi_0.5Ca_0.5FeO₃ at room temperature, there are seven most probable nonequivalent magnetic states of iron ions with significantly different values of the hyperfine interaction parameters. Four iron states correspond to Fe³⁺ ions in the octahedral oxygen environment, and three iron states correspond to Fe³⁺ ions in the tetrahedral oxygen environment.     

Features of local crystallographic, magnetic, and valence states of iron ions in Bi1 − x Sr x FeO3 perovskites at x = 0–0.67

Perovskites of the Bi_{1 ? x} Sr _x FeO₃ system (x = 0–0.67) at T = 295 K and T > T _N are studied using the Mössbauer effect. When the strontium content x = 0.1–0.15, the structural transition from the rhombohedral to the cubic phase takes place. It is found that in samples of the Bi_{1 ? x} Sr _x FeO₃ system (x = 0.07–0.67), there are only two structurally nonequivalent states of iron ions that correspond to Fe³⁺ ions in octahedral and tetrahedral oxygen environments.     

Magnetic and valence states of iron ions in the Perovskite Bi<Subscript>0.9</Subscript>Sr<Subscript>0.1</Subscript>FeO<Subscript>3</Subscript>

Single-phase rhombohedral perovskites (Bi_0.9Sr_0.1)FeO₃ were studied by Mössbauer spectroscopy at temperatures of 293, 87, and 680 K. The Neel temperature T _N = 652 ± 2 K of the magnetic transition was measured. Three states of trivalent iron ions in the octahedral states were discovered. Substitution of Sr²⁺ for 0.1 mol % Bi³⁺ breaks the spatially spin-modulated structure.     

Local states of iron ions in perovskite Bi0.815Y0.085La0.10FeO3

Bi_0.815Y_0.085La_0.10FeO₃ perovskite is studied by ⁵⁷Fe Mössbauer spectroscopy at 87, 295, and 670 K. The measured temperature of the magnetic phase transition (the Néel temperature) is T _N = 666 ± 5 K. It is found that substituting Y³⁺ ions for 0.085 at % Bi³⁺ in the Bi_0.9La_0.1FeO₃ perovskite destroys the spatially spin-modulated structure while the rhombohedral crystal structure is retained. Two structurally nonequivalent states of iron ions are found above the Neel temperature. Below the Neel temperature, there are four magnetically nonequivalent states of trivalent iron ions.     

Perovskites Bi0.8La0.2FeO3 and Bi0.8La0.2Fe0.95Cr0.05O3: Crystal structure and magnetic and charge states of iron ions

Perovskites of the Bi_0.8La_0.2Fe_{1 ? x} Cr _x O₃ system (x = 0, 0.05) were investigated by Mössbauer spectroscopy in the temperature range of 298–800 K. The samples were fabricated by solid-state synthesis and had a rhombic structure. Iron ions in Bi_0.8La_0.2FeO₃ and Bi_0.8La_0.2Fe_0.95Cr_0.05O₃ are situated in trivalent states. The magnetic transition temperatures (the Néel temperatures T _N ) T _N = 677.5 ± 2.5 K for Bi_0.8La_0.2FeO₃ and T _N = 647.6 ± 2.5 K for Bi_0.8La_0.2Fe_0.95Cr_0.05O₃ are measured. The substitution of trivalent iron ions from trivalent chromium ions in the amount x = 0.05 in Bi_0.8La_0.2Fe_0.95Cr_0.05O₃ perovskite decreases the hyperfine magnetic field at nuclei ⁵⁷Fe in Fe⁺³-O-Cr⁺³ chains by 30 kOe.     

Structural and electric-dielectric properties of some bismuth -phosphate glasses

In the present study, glasses with composition XBi₂O₃ (60?X)P₂O₅ 20Fe₃O₄ 20Li₂O (0≤X≤15) mol% have been prepared by the conventional melt quenching technique. The IR studies show the presence of FeO₄, FeO₆, BiO₆, PO₃, PO and PO₄ structural groups. The hyperfine structure of these glasses is investigated using Mössbauer spectroscopy. ME spectroscopy indicated the presence of two different oxidation states of iron (Fe²⁺ and Fe³⁺). The ferrous ions Fe²⁺ occupied tetrahedral coordination states, while the ferric ions Fe³⁺ occupied both tetrahedral and octahedral coordination states. The effect of partial replacement of P₂O₅ by Bi₂O₃ on the electric-dielectric properties is studied in more details. It is found that, unmonotonic variation in the σ_dc, (θ_D/2), σ_ac(ω), ε₁(ω), and ε₂(ω), as a function of Bi₂O₃ contents. Also it is found that, the power law exponent, s, is temperature dependent and the CBH model is the most applicable conduction mechanism in all glass samples. Pseudo Cole-Cole diagram of the investigated glassy samples exhibit similar behavior where all plots show a single semicircle indicating a single relaxation process.     

Local states of iron ions in multiferroics Bi1−x La x FeO3

Mössbauer studies of perovskites Bi_1?x La _x FeO₃ (x = 0, 0.10, 0.20, 0.61, 0.90, 1.00) were conducted at 295 and 87 K. The spatial spin-modulated structure (SSMS) observed in perovskites BiFeO₃ and Bi_0.9La_0.1FeO₃ leads to a specific distribution of hyperfine fields P(B) with two peaks. Substitution of La for Bi (x = 0.2) destructs the SSMS. The concentration dependences of the hyperfine field (B), isomer shift (?) and quadrupole shift (δ) were measured. The iron ions are in the trivalent state. The local magnetic moments μ(Fe) of the Fe³⁺ ions are determined.     

Sol–gel derived nanocrystalline multiferroic BiFeO3 and R (R=Er and Tm) doped therein: Magnetic phase transitions and enhancement of magnetic properties

Nanocrystalline BiFeO₃ and rare earth ion doped BiFeO₃ (Bi_0.9R_0.1FeO₃, R=Er and Tm) were prepared by sol–gel method. Rietveld analysis of the X-ray diffractograms of the samples revealed that small amount of impurity phase of Bi₂Fe₄O₉ was formed together with the desired phase. In the thermal variation of magnetic mass susceptibility (χ_m) of the samples, one sharp transition below T_M (T_M∼100 K, 50 K and 30 K for BiFeO₃, Bi_0.9Er_0.1FeO₃ and Bi_0.9Tm_0.1FeO₃, respectively) was observed, which clearly hint the change of the domination of the ferromagnetic exchange interaction over the usual antiferromagnetic exchange interaction. Also, static magnetization (M) and susceptibility of each doped sample have been drastically enhanced compared to that of BiFeO₃. The values of χ_m and M measured at different temperatures confirmed that the magnetic behavior of the doped systems has been dominated by the paramagnetic/ferromagnetic clusters below ∼T_M. Another phase transition were observed in the χ_m vs. T curve of the samples at relatively higher temperature T_B (∼260 K for BiFeO₃, ∼220 K for Bi_0.9Er_0.1FeO₃ and ∼180 K for Bi_0.9Tm_0.1FeO₃), which may be attributed to the charge ordering transition. Ferroelectric hysteresis loops of the samples observed at 100 Hz confirmed the presence of ferroelectric ordering of the samples. Measured values of dielectric constants at 1 kHz of each sample in presence and absence of magnetic field confirmed a substantial magnetoelectric coupling of all the samples.     

Effect of Sm,Co codoping on the dielectric and magnetoelectric properties of BiFeO3 polycrystalline ceramics

Multiferroic Bi_0.95Sm_0.05Fe_1−xCo_xO₃ (x=0−0.1) ceramics were prepared by the rapid liquid phase sintering method. For all the samples studied, the dielectric constant and dielectric loss decrease with increasing frequency in the range from 1 kHz to 1 MHz. It shows that the dielectric constant of Bi_0.95Sm_0.05FeO₃ at 10 kHz is about forty times larger than that of pure BiFeO₃. This dramatic change in the dielectric properties of Bi_0.95Sm_0.05Fe_1−xCo_xO₃ (x=0−0.1) samples can be understood in terms of the space charge limited conduction associated with crystal defects, which was indicated by the increase of magnetoelectric effect with doping Co³⁺ under applied magnetic field from 1 to 8 kOe. It was believed that the ferroelectric polarization enhancement comes from the exchange interaction between the Sm³⁺ and Fe³⁺ or Co³⁺ ions for Bi_0.95Sm_0.05Fe_0.95Co_0.05O₃ at room temperature.     

Simulation of Al/Fe disorder in Ca2FexAl2−xO5

A method is described which is capable of simulating disorder across a range of composition. It is used to study the disorder of iron and aluminium on the octahedral and tetrahedral cation sites in the Brownmillerite phase Ca₂Fe_xAl_2−xO₅. The arrangements with the lowest lattice energies are those that maximize the number of Fe³⁺ on octahedral sites throughout the composition range. An exchange of one Fe³⁺ with one Al³⁺, which results in a decrease in the number of iron ions on octahedral sites, increases the lattice energy by an amount that is dependent on x but is independent of the number of exchanges. The exchange of trivalent cations also results in an expansion in the b lattice direction, accompanied by a decrease in the a and c directions across the full range of composition.     

Temperature-dependent Fe Mössbauer spectroscopy and local structure of the mullite-type Bi2(FexGa1−x)4O9 (0.1≤x≤1) solid solution

The Bi₂(Fe_xGa_1−x)₄O₉ oxide solid solution possessing a mullite-type structure has been investigated by ⁵⁷Fe Mössbauer spectroscopy in dependence of composition (0.1≤x≤1) and temperature (293≤T/K≤1073). The spectra have been fitted with two doublets for tetrahedrally and octahedrally coordinated high-spin Fe³⁺ ions, respectively. The experimental areas of the subspectra were used to determine the distribution of iron on the two inequivalent structural sites. The fraction of iron cations occupying the octahedral site is found to increase with decreasing Fe content and the cation distribution is almost independent of temperature. The unusual temperature dependence of the quadrupolar splitting, QS, observed for the octahedral site with dQS/dT>0 is discussed in connexion with structural data for Bi₂Fe₄O₉. The temperature dependence of Mössbauer isomer shifts and signal intensities is examined in the context of local vibrational properties of iron on the two inequivalent sites of the mullite-type lattice structure.     

Multiferroic characteristics of the strained epitaxial Bi0.9Ho0.1FeO3 thin film

We fabricated high quality epitaxial Bi_0.9Ho_0.1FeO₃ thin films which exhibited the tetragonally stained structure with a c/a ratio of about 1.04. The Bi_0.9Ho_0.1FeO₃ thin film showed a good ferroelectric property with the high remanent polarization (P_r) of about 80 μC/cm². The ferromagnetic hysteresis loop with a clear remanent magnetization was shown. The coercive field and the remanent magnetization of the Bi_0.9Ho_0.1FeO₃ film are 6200 Oe and 1.7 emu/g, respectively. The abrupt conduction due to space charge limited (SCL) was revealed in leakage current density versus electric field.     

Investigation of La1−x Ca x FeO3−y (0≦x≦0.50) by means of X—ray diffraction and Mössbauer spectroscopy

Compounds La_1?x Ca _x FeO_3?y (0≤x≤0.50) were prepared and characterized by X-ray diffraction and Mössbauer measurements. The diffraction patterns were defined to be orthorhombic. The lattice constants of orthorhombic perovskite La_1?x Ca _x FeO_3?y decrease linearly with increasingx. The Mössbauer spectra at room temperature indicate that the Néel temperature drops and the isomer shift decreases with increasingx. The spectrum of La_0.50 Ca_0.50 FeO_3?y at 80 K shows two hyperfine splitting patterns which may be related to the Fe³⁺ and Fe⁵⁺ ions.     

Mössbauer study of Cd0.1Ni0.9Sn y Fe2−2y O4 ferrites

Mössbauer spectra of Cd_0.1Ni_0.9Sn_yFe_2?2yO₄ (y=0.0 to 0.5) ferrite system have been studied. The spectra suggest the existence of two hyperfine fields, one due to Fe³⁺ tetrahedral (A) site ions and the other due to Fe³⁺ octahedral (B) site ions. The variation of isomer shift, quadrupole interaction and internal magnetic fields of⁵⁷Fe³⁺ ions in both A and B sites have been determined as function of tin concentration. The systematic decrease in Curie temperature observed in the above system with tin concentration is explained on the basis of exchange interaction.     

Local defect structures and EPR studies on (FeO6)9− and (FeO4)5− clusters in YGG: Fe3+ system

The octahedral (FeO₆)^9? and tetrahedral (FeO₄)^5? clusters in yttrium gallium garnet (YGG): Fe³⁺ system are investigated based on the 252 × 252 complete energy matrices for d⁵ configuration ions in trigonal and tetragonal ligand fields, moreover, the EPR and optical spectra are made unified calculation. The results indicate that the defect structures around Fe³⁺ centres display expansion effects at different temperatures 4.2 and 295 K, and which are close to those in YIG garnet, respectively. Simultaneously, the defect structure parameters for Fe³⁺ centres in YGG are determined, and the relationship between the defect structure and the temperature has been discussed.     

Charge disproportionation induced exchange bias in La0.5Ca0.5FeO3-δ

We observed an exchange bias effect in La0.5Ca0.5FeO3 perovskite compound.The exchange bias is associated with the charge disproportionation transition from Fe4+ions to Fe3+and Fe5+ions below 175 K.The competition between the ferromagnetic interaction of Fe3+and Fe5+ions and the antiferromagnetic one of Fe3+and Fe3+ions results in a unidirectional anisotropy in the cluster-glass system.An antiferromagnetically interfacial exchange coupling constant Ji1.95 meV at the cluster-glass region was yielded by fitting the cooling field-dependence of the exchange bias field.     

Site occupancy and magnetic study of Al and Cr co-substituted Y3Fe5O12

Single-phased polycrystalline Y₃Fe_5−2xAl_xCr_xO₁₂ garnet samples (x=0, 0.2, 0.4 and 0.6) have been prepared by the conventional ceramic technique. Rietveld refinement of X-ray diffraction patterns of the samples shows them to crystallize in the Ia3d space group and the corresponding lattice constant to decrease with increasing Al³⁺ and Cr³⁺ contents (x). Mössbauer results indicate that Cr³⁺ substitutes for Fe³⁺ at the octahedral sites whilst Al³⁺ essentially replaces Fe³⁺ at the tetrahedral sites. This result indicates that co-doping of Y₃Fe₅O₁₂ does not affect the preferential site occupancy for separate individual substitution of either Cr³⁺ or Al³⁺. The magnetization measurements reveal that the Curie temperature (T_c) monotonically decreases with increasing x while the magnetic moment per unit formula decreases up to x=0.4 and then slightly increases for x=0.6. This reflects a progressive weakening of the ferrimagnetic exchange interaction between the Fe³⁺ ions at octahedral and tetrahedral sites due to co-substitution. The magnetic moment was calculated using the cations distribution inferred from the Mössbauer data and the collinear ferrimagnetic model, and was found to agree reasonably with the experimentally measured value. The phenomenological amplitude crossover, characterized by the temperature T*, has also been observed in the doped YIG and briefly discussed.     

Anisotropy of hyperfine field in diamagnetically substituted YIG

The results of the nuclear magnetic resonance of ⁵⁷Fe in Al-, Ga- and In-substituted Y₃Fe₅O₁₂ garnets are reported. When diamagnetic Ga³⁺ or Al³⁺ ions are substituted for tetrahedral Fe³⁺ ions, four satellite lines in the NMR spectrum of octahedral Fe³⁺ ions appear. The substitution of In³⁺ ions for octahedral Fe³⁺ ions leads to three satellite lines in the spectrum of tetrahedral Fe³⁺ ions. The hyperfine fields of these satellites are explained using the independent bond model for the hyperfine interaction. Suprisingly large change of the anisotropic part of the hyperfine field relative to the isotropic one was found.     

Structural,electrical and magnetic properties of Zr–Mg cobalt ferrite

Spinel cobalt ferrite, CoFe_2−xM_xO₄ has been synthesized by substitution of the combination of metallic elements M=Zr–Mg by the microemulsion method using polyethylene glycol as a surfactant. Powder X-ray diffraction analysis reveals that the substitution results in shrinkage of the unit cell of cobalt ferrite due to higher binding energy of the synthesized samples. The energy-dispersive X-ray fluorescence analysis confirms the stoichiometric ratios of the elements present. The thermogravimetric analysis shows that the minimum temperature required for the synthesis of these substituted compounds is 700 °C. A two-point probe method was employed for the measurement of the electrical resistivity in a temperature range of 293±5 to 673±5 K. It appears that there is a decrease in the number of Fe²⁺/Fe³⁺ pairs at the octahedral sites due to the substitution and corresponding migration of some of the Fe³⁺ ions to tetrahedral sites, consequently increasing the resistivity and the activation energy of hopping of electron at the octahedral sites. The susceptibility data also suggest migration of Fe³⁺ to tetrahedral site in the initial stage, which results in an increase in A–B interactions leading to large increase in the blocking temperature (T_B) as observed in samples having dopant content x=0.1.     

Photoacoustic spectra of Fe3+ and Cr3+ in lithium chromo-ferrites,Li0.5Fe2.5−xCrxO4 (0≤x≤2.5)

Photoacoustic spectra of polycrystalline Li_0.5Fe_2.5−xCr_xO₄ (x=0, 0.5, 1.25, 2.0 and 2.5) and LiFeO₂ have been investigated at 300K in the wavelength range 200–2500 nm. The spectra of x=0 composition arises from octahedral and tetrahedral Fe³⁺ ions in the spinel lattice. The Crsubstituted Li-ferrite phases exhibit broad maxima in the range 1400–1600nm and these are attributed to tetrahedral Cr³⁺ present in these phases besides the usual octahedral Cr³⁺ ions. The results are discussed in the light of recent work on tetrahedral Cr³⁺. The results of ferrimagnetic resonance spectra of Li ferrites and their Cr-substituents are also reported and correlated with photoacoustic spectra.