The magnetic properties of the MgZn ferrite system by Mössbauer spectroscopy

Samples of the magnetism-zinc ferrite series Zn_xMg_1?xFe₂O₄ (x = 0.0 to 1.0) have been studied by the Mössbauer effect technique at 77 K. Mössbauer spectra for x = 0.0 to 0.6 suggest the existence of two hyperfine fields, one due to the Fe³⁺ tetrahedral ions (A-sites) and the other due to the Fe³⁺ octachedral ions (B-sites), while for x=0.7 it shows relaxation behaviour and for x?0.8 it exhibits a paramagnetic quadrupole doublet. The variation of nuclear magnetic fields at the A and B sites is explained on the basis of the AB and BB supertransferred hyperfine interactions. Analysis of the average Mössbauer line width as a function of zinc concentration suggests that the relaxation spectrum observed at x=0.7 (77 K) is possibly due to domain wall oscillations.     

Magnetic properties of CuZn ferrite investigated by Mössbauer spectroscopy

The magnetic properties of the Zn_xCu_1?xFe₂O₄ ferrite system (x = 0 to 1) have been investigated by means of Mössbauer Spectroscopy. Mössbauer spectra of the CuZn ferrite system, taken at room temperature for x = 0.0 to 0.4 suggest the existence of two hyperfine fields, one due to the Fe³⁺ tetrahedral ions (A-sites) and the other due to the Fe³⁺ octahedral ions (B-sites), while for x = 0.5 it shows relaxation behaviour and for x ? 0.6 it exhibits a paramagnetic quadrupole doublet. The systematic dependence of the isomer shifts, quadrupole interactions and nuclear magnetic fields of ⁵⁷Fe³⁺ ions in both tetrahedral and octahedral sites has been determined as a function of zinc content. The variation of nuclear magnetic fields at the A and B sites are explained on the basis of A-B and B-B supertransferred hyperfine interactions. Analysis of the relaxation spectrum observed at x = 0.5 (300 K) suggests that the relaxation mechanism is due to domain wall oscillations.     

Low-temperature Mössbauer study of the MgCd ferrite system

Samples of the magnesium-cadmium ferrite series Cd_xMg_1?xFe₂O₄ (X = 0 to 0.8) have been studied by the Mössbauer-effect technique at 4.2 K. Mössbauer spectra for X = 0.0 to 0.8 suggest the existence of two hyperfine fields, one due to the Fe³⁺ tetrahedral ions (A-sites) and the other due to Fe³⁺ octahedral ions (B-sites). The systematic dependence of the isomer shifts, quadropole interactions and nuclear magnetic fields of ⁵⁷Fe³⁺ ions in both A- and B-sites has been determined as a function of cadmium content. The variations in the isomer shifts with Cd content are consistent with the variations in the Fe³⁺ -O^-₂ internuclear separations. It has been found here that supertransferred hyperfine interactions do not contribute to the systematics of the A- and B-site hyperfine fields.     

Mössbauer study of the spinel system GexCu1−xFe2O4

The magnetic properties of the spinel series Ge_xCu_1?xFe₂O₄ (X = 0 to 0.8) have been investigated by means of Mössbauer spectroscopy. Mössbauer spectra for X = 0.0 to 0.6 suggest the existence of two hyperfine fields, one due to the Fe³⁺ tetrahedral ions (A-sites) and the other due to Fe³⁺ octahedral ions (B-sites), while for X = 0.8 it shows the superposition of hyperfine field split spectra from A- and B-site ions and a broad central line spectrum. For 0.2 ? X ? 0.4, fast electron exchange among octahedral iron ions occurs as in Fe₃O₄. The variations of nuclear magnetic fields at the A- and B-sites are explained on the basis of AB and BB supertransferred hyperfine interactions.     

Mössbauer study of cubic pyrochlores,Bi2−xYxFeSbO7

The solid solutions Bi_2?xY_xFeSbO₇ (x = 0,1,2) which are cubic pyrochlores of the type A₂B₂O₇ have been prepared for the first time and the lattice parameters determined. The ⁵⁷Fe Mössbauer spectra have been recorded both in the paramagnetic and magnetically ordered states. From the values of the Mössbauer parameters it is found that Fe ions are in the high spin trivalent state. The large value for the quadrupole splitting of these compounds is attributed to the highly distorted BO₆ octahedron in these compounds. The quadrupole splitting decreases with increase of Bi concentration and therefore, the BO₆ octahedron is found to have the lowest distortion for the composition Bi₂FeSbO₇. The magnetic transition temperature and the value of the magnetic hyperfine fields have been determined. The distribution of Fe³⁺ and Sb⁵⁺ ions in B sites gives rise to the distribution in magnetic hyperfine fields.     

An investigation of FeCoCrO4 by Mossbauer spectroscopy

The spinel FeCoCrO₄ has been studied between 4.2 and 538°K. Characteristic Mossbauer spectra of paramagnetic, magnetic and electronic relaxation types have been observed. The Mossbauer parameters for Fe³⁺ ions situated at tetrahedral (A) and octahedral (B) sites have been calculated. The cation distribution in magnetic and paramagnetic phases is found to be approximately Fe_0.5³⁺Co_0.5²⁺[Co_0.5²⁺Fe_0.5³⁺Cr³⁺]O₄. The Neel temperature been determined by the temperature scanning method to be 310±5°K.     

Non-collinear spin arrangements in NiZn ferrites studied by the Mössbauer effect

Samples of the nickel-zinc ferrite series Ni_xZn_1-xFe₂O₄ with x = 0.15, 0.20, and 0.25 have been studied by the Mössbauer-effect technique at liquid helium temperature using longitudinal magnetic fields of 16kOe. The hyperfine fields at ⁵⁷Fe nuclei in A and B sites have been determined. Canted spin structures associated with Fe³⁺ ions are observed for all samples. Spectra were fitted using a localized canting model. The ratio of intrasublattice to intersublattice exchange constants resulting in the best fit is J_BB/J_AB = 0.130±0.005.     

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.     

Mössbauer studies and magnetic studies of Ni doped orthoferrites PrFe1−xNixO3 (x≤0.3)

The magnetic properties of polycrystalline PrFe_1−xNi_xO₃ (x≤0.3) system were studied using Mössbauer spectroscopy and magnetization measurements. The Mossbauer spectra exhibit six line spectra which loses its sharpness as the Ni substitution increases within the system. As the Ni concentration in the system increases, the hyperfine field and isomer shift shows decrease, which is vivid from the sluggish nature of the sextets. The small value of quadrupole splitting confirms the octahedral environment of the Fe⁺³ ions. The magnetization curves show the reversible behavior and represent the fall in negative molecular field leading to AFM frustration. From these results, we conclude that sagging in the spectra reveals the change from antiferromagnetic state to ferromagnetic state, which can be attributed to mixed state of Fe⁺³ ions i.e. high spin (HS) and low spin (LS) which is a consequence of progressive collapse of Hund’s rule due to HS→LS transition. These results confirm the weak ferromagnetic component due to canted-AFM spin arrangement of Fe³⁺ magnetic moments.     

Spectral studies of Co substituted Ni-Zn ferrites

The spinel ferrites Zn_0.35Ni_0.65−xCo_xFe₂O₄, 0≤x≤1, have been prepared using the standard ceramic technique. Room temperature Mössbauer, X-ray and infrared IR spectra were used for carrying out this study. X-ray patterns reveal that all the samples have single-phase cubic spinel structure. The Mössbauer spectra of the samples show a paramagnetic phase for x=0 and a six-line magnetic pattern and a central paramagnetic phase for x≥0.1. They are analyzed and attributed to two magnetic subpatterns and two quadrupole doublets due to Fe³⁺ ions at the tetrahedral A-sites and octahedral B-sites. Four absorption bands are observed in IR spectra. They confirm the spinel structure of the samples and existence of Fe³⁺ ions in the sample sublattices. The deduced hyperfine interactions, lattice parameters, absorption band positions and intensities and force constant are found to be dependent on the substitution factor x, where the cation distribution is estimated. The hyperfine magnetic fields, magnetization and lattice resonant frequency are found to be dependent on the interionic distance.     

Mössbauer study of Fe57 and Co57 in Co1 + xV2−xO4

Mössbauer sources and absorbers, prepared by doping Fe⁵⁷ and radioactive Co⁵⁷ into samples of Co_1?xV_2?xO₄ (0 ? x ? 1), were studied in the temperature range of 80–500 K. The source and absorber spectra are very similar. The absence of any Fe²⁺ at the A site can be understood by partial covalent bond formation with an anion. However, the predominance of Fe³⁺ at the B site (with some Fe²⁺ for x = 1) cannot be explained by simple crystal-field or molecular-orbital theories. The x dependence of the isomer shift and of the Fe³⁺B-site quadrupole interaction can be related to changes in the lattice constant and the oxygen parameter. The temperature dependence of the Fe²⁺B-site quadrupole interaction can be fitted in the motional-averaging model. In the range of 0 ? x ? 0.5 the temperature dependence of the isomer shift shows effects of chemical bonding beyond the second-order Doppler shift.     

Mössbauer study of cobalt ferrite nanocrystals substituted with rare-earth Y ions

Mössbauer spectroscopy is used to characterize the crystallite size and structure of CoFe_2−xY_xO₄ (x=0, 0.1, 0.3, 0.5) ferrite nanocrystallites synthesized by the sol-gel auto-combustion method. The effect of the substitution of Fe³⁺ ions by Y³⁺ ions on the structure of cobalt ferrite nanocrystallites is investigated. The Mössbauer spectra showed two sets of six-line hyperfine patterns for all the samples, indicating the presence of Fe in both A and B-sites. On increasing the concentration of doped Y, the hyperfine field strength and the isomer shift first increase and then decrease, whereas the quadrupole splitting continuously increases. The superparamagnetism was observed for all the samples and the change of ratio of the superparamagnetism component reflects the size of crystal grain.     

Influence of 50 MeV Lithium Ion Irradiation on Hyperfine Interactions of Cr0.5 Li0.5 Fe2O4

Spinel oxide Cr_0.5 Li_0.5 Fe₂O₄ has been irradiated at Nuclear Science Centre, New Delhi, by 50 MeV lithium ions of fluence 5*10¹³ ions/cm² and irradiation effect on hyperfine interactions has been investigated by Mossbauer spectroscopy. The Mossbauer spectrum of irradiated sample shows no paramagnetic doublet contribution and the hyperfine fields corresponding to the Fe³⁺ in the octahedral (B) and the tetrahedral (A) sites are very well separated. That is the observed superimposed A and B sites in unirradiated sample are split into separate lines after Li irradiation. Further an increase of the intensity of the lines (2)–(5) with respect to (1)–(6) signals an orientation of the hyperfine magnetic field towards a direction perpendicular to the ion path due to the irradiation induced strain by the latent tracks. The computer simulation of Mossbauer spectra indicated that the irradiated Fe³⁺-site occupancy of the A-site hyperfine field increased from 43% to 55% whereas the B-site hyperfine field decreased from 57% to 45% compared to unirradiated sample.     

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.     

Mossbauer study of ferrite systems Co x Mn1−x Fe2O4 and Ni x Mn1−x Fe2O4

Mössbauer spectra of Co _x Mn_1?x Fe₂O₄ and Ni _x Mn_1?x Fe₂O₄ ferrites withx values ranging from 0·1 to 0·8 in steps of 0·1 have been recorded at room temperature. All spectra exhibit well-defined Zeeman hyperfine patterns. It has been observed that hyperfine field at Fe³⁺ nucleus increases more rapidly by nickel substitution than by cobalt substitution. This has been explained in terms of exchange interactions and cation distribution in the spinels. Hyperfine fields, isomer shifts and quadrupole splittings have been determined.     

57Fe Mössbauer and infrared studies of the system Co1−x Cd x Fe2O4

Mössbauer and infrared studies were made on samples of the ferrite system Co_1–xCd_xFe₂O₄ x=0.0, 0.1, 0.3, 0.5, 0.7, 0.9 and 1. Mössbauer spectra were taken at room temperature. The spectra of the samples withx0.7 showed well defined Zeeman patterns and they have been analyzed with two components, one due to A-site Fe³⁺ ions, and the other due to B-site Fe³⁺ and Fe²⁺ ions. The pattern due to B-site appeared to be composite and an explanation is given. The spectra withx=0.9 and 1 showed only a quadrupole splitting. The effect of cadmium substitution on the various hyperfine interactions has been discussed and the cationic distribution has been deduced for all values ofx. Far infrared spectra of the ferrite samples in the range 200–700 cm^–1 were reported. Four bands were observed: the high frequency bandv ₁ is assigned to tetrahedral complexes, and the low frequency bandv ₂ to octahedral complexes, a small bandv ₃ is due to Co²⁺-O^2– complexes andv ₄ is assigned to the lattice vibration of the system. The splitting occurred in thev ₁ andv ₂ bands atx=0.9 and inv ₂ atx=1, indicating the presence of Fe²⁺ ions in octahedral sites.     

Mössbauer study of Fe1 + xV2−xO4 spinels for the determination of cation distributions and magnetic structure

Mössbauer experiments were carried out in the temperature range of 80–500 K on the spinel series of Fe_{1 + x}V_2?xO₄. The determined cation distribution has Fe²⁺ and Fe³⁺ at both A and B spinel sublattice sites for t whole series except for x = 0. Charge hopping was observed at the B lattice sites. The present cation distribution reasonably predicts several experimentally observed constants such as the lattice constant, the oxygen parameter, the saturation magnetic moment and the Curie constant. The magnetic structure in best agreement with the experimental values shows the AA spins to be antiparallel for Fe, the BB spins to be parallel for identical ions and antiparallel for different ions, and shows a strongly antiparallel AB interaction between trivalent iron ions. Crystal distortions due to the Jahn-Teller effect were observed at 80 K for compositions with 0 ? x ? 0.75.     

Mössbauer spectroscopy of 57Fe in high Tc superconductors YBa2Fe3xCu3(1−x)O7−δ

Mossbauer spectroscopy of ⁵⁷Fe in both tetragonal and othorhombic phases of YBa₂(Fe_xCu_1−x)₃O_7−δ, with x = 0.01, 0.02 and 0.10, at temperatures 4.2 K, 75 K, 90 K, and 300 K have been performed. In all samples three major subspectra corresponding to iron in different local environments are observed. It is concluded that Fe substitutes mainly Cul. At 4.2 K, samples with x=0.01 in the “quenched” tetragonal phase exhibit magnetic hyperfine structure, due to slow spin relaxation rates, whereas in the orthorhombic superconducting phase, only samples with x=0.1 exhibit magnetic hyperfine structure, in this case probably due to spin glass magnetic order.     

Mossbauer, infrared and magnetic susceptibility studies of iron sodium borate glasses doped by sulphur

The affect of sulphur on the structural properties of iron sodium diborate glasses having the composition {(100−x)Na₂B₄O₇+xFe₂O₃}+yS, where x=0.05, 0.15 and 0.25 mol% and Y=0, 2.5 and 5 wt% was studied by infrared, Mossbauer spectroscopy and magnetic susceptibility measurements. It was found that, for samples having 5 mol% Fe₂O₃ and free from sulphur, the iron ions are present in both Fe²⁺ and Fe³⁺ states and also 92% of the total iron enters the glass network as a glass former. The ratio of Fe³⁺/Fe²⁺ increases with increasing the iron content for sulphur-free samples and others containing sulphur. This ratio also decreases with increasing the sulphur content. The magnetic susceptibility was found to decrease with increasing the sulphur content. Also, the increase of Fe₂O₃ content led to a less symmetrical environment of Fe³⁺ ions and vice versa for the Fe²⁺ environment.     

Mixed hyperfine interactions and defect structure in magnesiowüstite Fe−Mg−O

An analysis of the 4.2 K spectra of Fe_xO (x∼0.91) and (Fe_0.4Mg_0.6)_xO is presented in which it is considered that because of the large electric field gradient at Fe²⁺ defect sites, the spectra cannot be described by Lorentzian sextets. It is assumed that the magnetic hyperfine field vector is oriented at random in the coordinate system defined by the EFG main axis, and that the EFG coordinate system is also distributed randomly. The simplifying assumption of the asymmetry parameter η=0 allows an analytical formula to be used to describe the complex spectra. Distributions of both magnetic hyperfine field and quadrupole interaction were progressively refined resulting in reasonable fits to the spectra with the main features being well reproduced. The magnetic hyperfine field distribution is rather broad with several features present while distinct values were obtained in the distribution of quadrupole interactions. These latter values are considered to correspond to the defect configurations around the Fe²⁺ sites. The distribution of hyperfine fields is considered to reflect the varying strengths of superexchange due to the high defect concentration as well as the effects of magnetic relaxation.