X-ray diffraction and57Fe Mössbauer spectra of the system Fe2O3−Ga2O3

The influence of gallium substitution on the chemical and structural properties of haematite, α-Fe₂O₃, has been studied using X-ray diffraction and⁵⁷Fe Mössbauer spectroscopy. The presence of only α-(Ga _x Fe_1?x )₂O₃ phase is detected for the compositions withx between 0.01 and 0.90. A gradual decrease of the unit-cell parameters of α-(Ga _x Fe_1?x )₂O₃ with the increase of gallium substitution is measured.⁵⁷Fe Mössbauer spectra showed that the value of the magnetic hyperfine field of pure α-Fe₂O₃ decreases with increasing gallium for iron substitution. The hyperfine magnetic structure, which is observed for α-(Ga _x Fe_1?x )₂O₃ at room temperature, collapsed for the composition withx?0.50. The changes in the⁵⁷Fe Mössbauer spectra of the α-(Ga _x Fe_1?x )₂O₃ phase are discussed in the sense of the electronic relaxation and the superparamagnetic effects.     

Magnetic properties of the mixed spinel Co1+x Si x Fe2−2x O4

The structural and magnetic properties of the mixed spinel Co_1+x Si _x Fe_2?2x O₄ system for 0·1≤x≤0·6 have been studied by means of X-ray diffraction, magnetization, and Mössbauer spectroscopy measurements. X-ray intensity calculations indicate that Si⁴⁺ ions occupy only tetrahedral (A) sites replacing Fe³⁺ ions, and the added Co²⁺ ions substitute for (B) site Fe³⁺ ions. The Mössbauer spectra at 300 K have been fitted with two sextets in the ferrimagnetic state corresponding to Fe³⁺ at the A and B sites, forx≤0·3. The Mössbauer intensity data shows that Si possesses a preference for the A site of the spinel. The variation of the saturation magnetic moment per formula unit measured at 300 K with the Si content, is explained on the basis of Neel’s collinear spin ordering model forx≤0·3 which is supported by Mössbauer, and X-ray data. The Curie temperature decreases nearly linearly with increase of the Si content, forx=0·1–0·6.     

Interpretation of Mössbauer spectra of YBa2Cu3−x Fe x O7−d

We have studied the Mössbauer spectra of YBa₂Cu_3?x Fe _x O_7?d (11 compositions) and present here the results obtained from quantitative analyses of the paramagnetic and magnetic spectra.     

Mössbauer spectra and magnetic properties of Tm0.65Sr0.35Fe x Mn1 − x O3 (x = 0.3, 0.35, 0.4)

We report Mössbauer spectroscopy results for Tm_0.65Sr_0.35Fe _x Mn_{1 ? x} O₃ (x = 0.3?0.4) at 300 and 80 K. Like in the case of lighter lanthanide ferromanganites, we observe phase separation of the magnetic sub- system: a magnetic phase shows up in the spectra in the form of a Zeeman sextet and “paramagnetic” dou- blets.     

Superparamagnetic transition and local disorder in CuFe2O4 nanoparticles

We present X-ray diffraction (XRD), Mössbauer spectroscopy (MS) and d.c. magnetization measurements performed on ball-milled CuFe₂O₄ samples. The average particle size <d> was found to decrease to the nanometer range after t=15 min of milling. Room temperature Mössbauer data showed that the fraction of particles above the blocking temperature T_B increases with milling time, and almost complete superparamagnetic samples are obtained for <d> = 7(2) nm. Magnetization measurements below T_B suggest spin canting in milled samples. The values of saturation moment μ_S reveal that site populations are slightly affected by milling. Mössbauer resonant intensities are accounted for on the basis of local disorder of Fe³⁺ environments, and the development of sample inhomogeneities of Cu_xFe_3−xO₄ composition.     

Le systeme V1−xFexO2: Proprietes structurales et magnetiques

The system V_1?xFe_xO₂ (x ? 0.020) was investigated by X-ray diffraction, DTA, magnetic susceptibility measurements and Mössbauer spectroscopy. The phase diagram and the magnetic properties are similar to those of the V_1?xCr_xO₂ and V_1?xAl_xO₂ systems previously studied. In oxygen deficient materials appears a Magnéli type shear phase, the Fe³⁺ ions occupying the shear planes.     

Amorphous glasses with high iron content

The results of X-rays, electron microscope studies, magnetic and Mössbauer effect measurements on x Fe₂O₃ (1?x) [PbO.3B₂O₃] glasses with 50<x<65 mol% Fe₂O₃ are reported. All the samples are in amorphous state. The ratio between the number of ferrous and ferric ions decreases by increasing the Fe₂O₃ content. The magnetic behaviour of these glasses is analysed in connection with Mössbauer effect data.     

Magnetic and structural characterization of the solid solution CdFe2O4NiFe2O4

By means of Mössbauer spectroscopy magnetization measurements and powder diagrams, the structural and magnetic properties of the isomorphous series Cd_1?xNi_xFe₂O₄ has been studied. The cationic distribution depends on the relative concentration of Cd and Ni. The exchange integrals between the two sites in the spinel have been obtained.     

Effect of Ce Doping on the Magnetic Properties of NiFe2O4 Nanoparticles

In the present work, effect of different concentration of Ce ions on the magnetic properties of nickel ferrite nanoparticles is discussed. A series of NiCe _x Fe_2?x O₄ (x=0.0–0.10) samples were prepared by a chemical route. XRD patterns show that all the samples are in pure spinel phase except that with x=0.10. This indicates that rare earth ions have limited solubility in the spinel lattice. Magnetic properties are studied by electron paramagnetic resonance spectroscopy (EPR) and the results are explained in terms of a magnetic moment obtained by a vibrating sample magnetometer. Mössbauer spectroscopy was performed to have an idea about the distribution of ions between the tetrahedral and octahedral sites. Magnetic moment is found to decrease after doping with Ce ions. It was found that the samples show mixed spinel nature rather than the pure inverse character. Doping with Ce ions reduces the EPR linewidth of pure nickel ferrite, which indicates that eddy current losses are reduced.     

Structural and electrical properties of zinc-substituted cobalt ferrite

Polycrystalline samples of Co_1?x Zn _x Fe₂O₄ with stoichiometric proportion (x) varying from 0.0 to 1.0 were prepared through the thermal decomposition of their respective oxalates. The samples were calcined at 1000 °C for 3 h and characterized using X-ray diffraction (XRD) and Mössbauer spectroscopy techniques. The study of the cation distribution using Mössbauer spectroscopy showed that the ions at the tetrahedral site moved to the octahedral site by the addition of zinc and that the system varied from an inverse to a normal spinel structure. The values of the lattice parameter, X-ray density, oxygen parameter, inversion factor and radii of tetrahedral and octahedral sites were calculated using X-ray diffraction data. The temperature dependence of the conductivity showed a definite kink, except for the ZnFe₂O₄ sample, which can be attributed to the ferromagnetic–paramagnetic transitions. The calculated activation energy in the paramagnetic region was found to be smaller than that in the ferromagnetic region.     

151Eu Mössbauer studies on Zn-doped EuBa2Cu3O7−y

¹⁵¹Eu Mössbauer studies have been performed on the compounds EuBa₂(Cu_1?x Zn _x )₃O_7?y withx=0·0, 0·025, 0·05, 0·075 and 0·1. The parent compound, EuBa₂Cu₃O_7?y is superconducting with a transition temperature (T _c ) of 88 K.T _c is depressed as Zn is substituted for Cu in this system and the compounds withx>0·05 do not show superconductivity down to 12 K.¹⁵¹Eu Mössbauer studies at 295 K show a single Mössbauer line in all the compounds (whether superconducting or not) with isomer shift value typical of Eu^{3 +} ion. Further, the isomer shift values are nearly independent ofx and the temperatures down to 10 K. These observations imply that the Cu-O network responsible for superconductivity is very weakly coupled to the Eu sublattice.     

Thermal scanning studies of percolated Fe–Cu granular alloys

Fe–Cu alloys produced by mechanical alloying were studied with Mössbauer thermal scanning spectroscopy (MTS). This technique consists in recording Mössbauer effect absorption at a fixed energy while the temperature of the sample is changed. Hyperfine magnetic field behavior can be closely followed. Four mechanically alloyed samples Fe_xCu_100−x with iron concentration x=30, 35, 40, and 45 at.% were studied. Absorption graphs (intensity vs. temperature) are similar for all samples: intensity sharply falls with the increase of the temperature, following the collapse of the sextet lines. This collapse occurs at a very precise temperature, which coincides with a magnetic–nonmagnetic transition line in a former magnetic phase diagram. Conventional Mössbauer spectroscopy confirms the appointed phase change, displaying a doublet at temperatures above the critical.     

Investigations of Sr3Fe2−xMnxO7−δ the n=2 Ruddlesden–Popper phases with d3/d4 interactions

We have prepared a series of Mn substituted strontium ferrates, Sr₃Fe_2−xMn_xO_7−δ where x=2/3, 1 and 4/3, and investigated their properties with X-ray diffraction, X-ray absorption near edge spectroscopy (XANES), Mössbauer spectroscopy, variable-temperature resistivity and magnetic susceptibility. These compounds are metastable and have to be prepared at high temperatures, >1250°C, with solid state techniques followed by quenching in air to room temperature. As a consequence the compounds are oxygen deficient and contain some Fe³⁺. They are insulators and exhibit spin-glass like behavior at low temperatures due to frustrated magnetic interactions between the disordered array of transition metal ions on the B site. Mössbauer and XANES spectra show that the B cations are not fully oxidized and that the Fe⁴⁺ in these compounds are charge-disproportionated into Fe³⁺ and Fe⁵⁺ at low temperature.     

57Fe Mössbauer studies on MgAlxCrxFe2-2xO4 spinel system

Based on ⁵⁷Fe Mössbauer spectroscopy studies conducted at 300 K, the magnetic behaviour of the spinel ferrite system MgAl_xCr_xFe_2-2xO₄ (x=0.0–0.5) has been investigated. The Mössbauer spectra for x=0.0–0.2 exhibit two Zeeman sextets, one due to the Fe³⁺ octahedral ions and the other due to the Fe³⁺ tetrahedral ions, while the compositions x=0.3, 0.4 and 0.5 showed central paramagnetic doublet superimposed on the magnetic sextets. The variation of hyperfine parameters and origin of the central doublet on the magnetic sextets have been discussed.     

Effect of heat treatment procedures on site preferences of Fe in YBa2Cu3O7−D

Mössbauer spectra of YBa₂Cu_2·85Fe_0:15O_7?d prepared using different heat treatment procedures have been obtained. The transition temperature varies from 23 to 53 K, depending on the procedure adopted for preparation. Associated with this are changes in the lattice structure. Mössbauer spectra reveal population of the four components and their relation to the decrease in the transition temperatures.     

Investigation of mixed valency ferrites La1−xCaxFeO3−y (0<x<0.50) with the perovskite structure

Investigation of mixed ferrites La_1?xCa_xFe^III_1?τFe^IV_τO_3?y (0 ≤ x ≤ 0.05, 0 ≤ y ≤ 0.07) has been carried out by magnetic, electrical and Mössbauer resonance measurements. Mössbauer spectra at low temperature can be interpreted assuming a disproportionation of tetravalent iron according to a scheme previously proposed for CaFeO₃: 2 Fe^IV → Fe^III + Fe^V. Electrical measurements show an increasing electronic localization when the temperature is decreased. This result corroborates the proposed model which moreover corresponds to a stabilization of the isotropic configurations t³_2ge²_g of Fe^III and t³_2ge⁰_g of Fe^V in regular octahedral sites. The fit of the Mössbauer spectra at 4.2 K is also discussed in relation to the distribution of Fe^III and Fe^V in the lattice.     

Magnetic properties of magnesium-cobalt ferrites synthesized by co-precipitation method

The alternating current (a.c.) low field susceptibility vs temperature, magnetization and⁵⁷Fe Mössbauer effect measurements are reported for the spinel solid solution series Mg _x Co_1?x Fe₂O₄ synthesized by a wet-chemical method before and after high temperature annealing. The observed features for the wet samples, such as the coexistence of paramagnetic doublet and magnetic sextets in Mössbauer spectra and lower saturation magnetization values confirm small particle ferrite behaviour. Especially, Mössbauer spectra of wet samples reveal the presence of superparamagnetic particles which exist simultaneously with ferrimagnetic regions in the materials well supported by a.c. susceptibility data. The high temperature annealing changes the wet-prepared ferrites into the ordered magnetic structure of ceramic ferrites.     

Effects of Gd2O3 on structure and magnetic properties of Ni-Mn ferrite

The emulsion method was used to prepare nanocrystalline Ni_0.7Mn_0.3Gd _x Fe_2-x O₄ ferrites. The growth of particles, the structure and the magnetic properties were investigated by X-ray diffraction (XRD), Mössbauer spectroscopy and vibrating sample magnetometer (VSM). Furthermore, the influence of Gd₂O₃ on magnetic properties of Ni-Mn ferrite powders has been investigated in detail. When the crystallite sizes are about 30–40 nm, all the samples have the similar Ms values. The variational rules of saturation magnetization (Ms) and coercivity (Hc) along with doped-Gd contents at different sintering temperatures show that the maximum Gd ions content doped into ferrite lattices is x = 0.06. When Gd-doped content x is larger than 0.06, the doped Gd ions can’t enter into the ferrite lattice totally but reside at grain boundary, as the ionic radii of the Gd³⁺ ions are larger than that of Fe³⁺ ions. The ferrimagnetism have not disappeared completely, even if the crystallite size is 7.8 nm.     

X-ray diffraction,Mössbauer spectrometry and thermal studies of the mechanically alloyed (Fe1−xMnx)2P powders

(Fe_1?xMn_x)₂P phosphide powders in the composition range 0.15?≤?x ≤ 0.75 have been mechanically alloyed and their structural, magnetic and thermal changes with composition have been investigated by means of X-ray diffraction, ⁵⁷Fe Mössbauer spectrometry, magnetization measurements and differential scanning calorimetry. The milling process induces changes in the crystal phase diagram of the (Fe_1?xMn_x)₂P system. The XRD results reveal the coexistence of a bcc Fe(Mn)-type, hexagonal (Fe₂P and Mn₂P-type), orthorhombic (MnP-type) and tetragonal Fe₃P-type structures for all compositions. The room temperature Mössbauer spectra confirm the formation of the Fe(Mn)-type, non-stoichiometric Fe₂P-type, FeP-type and Fe₃P-type structures. Saturation magnetization exhibits a comparable behavior to that of the average hyperfine magnetic field. The DSC scans show the existence of several endothermic and exothermic peaks in the temperature range (100–700)?°C related to different phase transitions. The endothermic peak at about 582–589?°C can be related to the ferromagnetic/paramagnetic transition temperature (Curie temperature, T_C) of the Fe(Mn)-type structure.