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.     

Study on dielectric and magnetic properties of Ho3Fe5O12 ceramics

Ho₃Fe₅O₁₂ ceramics with garnet structure were prepared by the solid-state reaction method. The results revealed the existence of Fe²⁺ ions have intensive influence on dielectric and magnetic properties of Ho₃Fe₅O₁₂ ceramics, which could be further confirmed by oxygen treatment. With a magnetic field lower than 10 kOe, the ME coefficient reaches 33 ps m⁻¹ at room temperature. And the ME coupling was further verified by dielectric anomaly near Néel temperature.     

Mössbauer effect study of nonstoichiometric CoNiZn ferrites

The existence of fast electron exchange between pairs of Fe³⁺?Fe²⁺ ions was detected in Mössbauer spectra of non-stoichiometric CoNiZn ferrites containing an excess of Fe₂O₃. Analysis of spectra from a number of samples of various preparations showed correlation of bulk magnetic properties with hyperfine parameters of ferric ions at octahedral sites in the spinel structure.     

Mössbauer effect and electron spin resonance study of CdxMg1−xFe2O4 system

This work deals with the application of Mossbauer Effect (ME) in studying the crystal electric field and the cation distribution among tetrahedral (A) and the octahedral (B) sites of the spinal structure in the ferrite system Cd_x Mg_1–x Fe₂O₄ (x=0, 0.2,...1). The electron spin resonance technique (ESR) was also applied for studying this ferrite system. It was possible to characterize the ESR spectra of ferrite through the combination with the ME spectra.The ESR spectra of magnesium ferrite showed two resonance positions of Fe³⁺ ions and indicated that a strong exchange interaction is dominant in the pure Mg-ferrite. For high Cd concentration ferrites only single resonance line was observed. These results could be interpretted on the basis of the ME results where it indicated that Cd²⁺ ions prefer tetrahedral positions, forcing the Fe³⁺ ions from these positions to join those in the octahedral sites. The complete site occupation with different types of cations was successfully achieved from the ME spectra. The values of the quadrupole splitting indicated that for each ferrite in the system there exists an electric field gradient surrounding the Fe³⁺ ions in each of the octahedral and tetrahedral sites. The increase in the Mg-concentration increases the symmetry of the electric field at these sites.     

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.     

Magnetoelectric and Mössbauer studies of Fe2 TeO6

The magnetic symmetry of antiferromagnetic Fe₂TeO₆ indicates that this material should exhibit magnetoelectricity. This prediction has been confirmed by the observation of the electrically induced magnetoelectric (ME) effect in powder samples. Both parallel and perpendicular ME susceptibilities were measured as a function of increasing temperature. The ME effect vanishes at 209°K which is identified as the Néel point of the compound. The single crystal ME susceptibilities are derived from the powder results. The maximum value of the axial ME susceptibility α₃₃ = M₃/E₃ is (in Gaussian units) 3 × 10⁻⁵ at T = 175°K. In addition to magnetoelectricity the magnetic symmetry of Fe₂ TeO₆ indicates that no hyperfine field should exist at the Te⁶⁺ sites. This prediction was confirmed by Mössbauer studies on I¹²⁹ produced in Te⁶⁺ sites by irradiating samples of Fe₂ TeO₆ in a reactor.     

57Fe Mössbauer spectroscopy on disordered crystalline media with Ca-gallogermanate type structure. II. Below the magnetic ordering temperature

Magnetic phase transitions in disordered crystalline iron germanates of the type Sr₂LnFe₃Ge₃O₁₄ (Ln=La; Nd) were observed at low temperatures by means of⁵⁷Fe Mössbauer spectroscopy. The hyperfine interaction of Fe³⁺ magnetic moments ordered in one octahedral and two tetrahedral magnetic sublattices is discussed. A local environment computation on the octahedral sites confirms the assumption that the wide distribution found for the 4.2 K octahedral hyperfine fields is due to the random occupation by Fe³⁺ and Ge⁴⁺ ions of the nearest cation environment of the octahedral iron. The effect of substituting Ln rare earth ions in the Thompson cubes on the magnetic behaviour of these compounds is pointed out.     

Mössbauer spectroscopy of the new iron oxide Fe3B7O13(OH)

In order to investigate the electronic state, the local structure, and the magnetic structure of a new ion oxide Fe₃B₇O₁₃(OH), we have applied ⁵⁷Fe Mössbauer spectroscopy. The room-temperature values of isomer shift and quadrupole splitting are 1.16 mm/s and 3.21 mm/s, respectively, which indicate that the Fe ions are in high spin Fe^2?+? state. The spectrum at 4.2 K is composed of a well-resolved hyperfine sextet with the hyperfine field of 3.6 T. In a trimer, each Fe^2?+? magnetic moment is supposed to be directed from Fe^2?+? to OH^???.     

Fe57 Mössbauer study in cobalt substituted magnetite

The Mössbauer effect has been studied in the mixed ferrites Co _x Fe_3–x O₄ (forx=0.8, 0.9 and 1) with the spinel structure in the temperature range between 78 and 380 K. The composition withx=1, showed an expected Zeeman spectrum with two overlapping magnetic hyperfine patterns related to the Fe³⁺ ions in tetrahedral and octahedral sites. While for samples withx=0.8 and 0.9 the Mössbauer spectrum for each compound was successfully analysed into three different patterns corresponding to the ferric ions placed at the tetrahedral and octahedral sites and ferrous ions at the octahedral sites, indicating no electron transfer between Fe³⁺ and Fe²⁺, where the quantity of cobalt is sufficiently large to be located at the six nearest neighbours to ferrous ions. The Mössbauer effect parameters were calculated for these observed sites and their variation with temperature reported. The reduced hyperfine magnetic fields of the Fe³⁺ (B) ions were found to follow the Brillouin curve forS=5/2 and one third power law. The magnetic ordering temperature was determined to be 815 K and the possible magnetic interactions were discussed.     

Synthesis and characterization of Cu2+ substituted magnetite

Samples of magnetite, both pure and doped with divalent copper, Fe_3???xCu_xO₄, with x?=?0, 0.05, 0.10 and 0.20 atm.%, were synthesized hydrothermally. The samples were characterized by Atomic Absorption Spectroscopy, Mössbauer Spectroscopy, X-ray diffraction, Scanning Electron Microscopy and SQUID magnetometry. The analyses made by the above techniques showed that as the Cu²⁺ concentration increases, a simultaneous reduction in the magnetic and structural parameters takes place, namely: magnetic hyperfine interactions at octahedral sites, particle size and lattice constant. Degradation in the particles morphology as well as a distribution of their size were also observed. Our study points two important effects of Cu²⁺ in magnetite, the first one is its incorporation within the structure, replacing Fe²⁺ ions and decreasing both the magnetic hyperfine interactions at octahedral sites and the bulk magnetization, the second one is the contraction of the crystalline lattice of magnetite, because incorporation of Cu²⁺ within the structure, generation of vacancies or both simultaneous effects.     

An investigation of the spin glass behaviour in iron antimonate by iron-57 and antimony-121 Mössbauer spectroscopy

Iron antimonate, which contains a superlattice composed of an ordered array of cations in the rutile-type structure, has been shown to undergo a spin glass transition atca. 20K which is induced by antisite atomic ordering. The iron-57 Mössbauer spectra recorded at 298 and 77K provide information on the relaxation times of short range magnetically ordered clusters identified by magnetic susceptibility measurements. The spectrum of 4K is tentatively interpreted in terms of the existence, at temperatures below the spin glass transition temperature, of a hyperfine magnetic field distribution at Fe³⁺ ions with a mean value of 487 kOe and which contains a supertransferred contribution from the nearest neighbour Fe³⁺ ions. The¹²¹Sb Mössbauer spectra are characteristic of Sb⁵⁺ and the increasing linewidths at lower temperatures are consistent with the presence of a supertransferred hyperfine magnetic field at the Sb⁵⁺ species ofca. 16^kOe.     

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.     

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.     

Relaxation effects in Mössbauer spectra of bridged seven-coordinate Fe3+ pairs

The compound [Fe₂L(H₂O)₄] (ClO₄)₄.H₂O which contains pairs of Fe³⁺ ions within a binucleating macrocycle derived from Schiff base condensation of 2,6-diacetylpyridine and 1,3-diamino-2-hydroxy-propane has been studied by magnetic susceptibility measurements and⁵⁷Fe Mössbauer spectroscopy between 4.2 K and 300 K, and its crystal structure determined. The spectra show relaxation effects at all temperatures. Spectra taken at 4.2 K in applied fields of about 3 T showed thatV _zz is positive and η～0. The spectra were fitted using a stochastic model of a magnetic hyperfine field relaxing parallel to thez axis, giving relaxation times of 10^?9?10^?10 s.     

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.     

Magnetic properties of the mixed spinel Co1+xSnxFe2−2xO4

The structural and magnetic properties of the mixed spinel Co_1+xSn_xFe_2?2xO₄ system for 0.1≤x≤0.5 have been studied by means of X‐ray diffraction, magnetization, a.c. susceptibility and Mössbauer effect measurements. X‐ray intensity calculations indicate that Sn⁴⁺ ions occupy only octahedral (B) sites replacing Fe³⁺ ions and the added Co²⁺ ions substitute for A‐site Fe³⁺ ions. The lattice constants are determined and the applicability of Vegard's law has been tested. The Mössbauer spectra at 300 K have been fitted with two sextets in the ferrimagnetic state corresponding to Fe³⁺ at tetrahedral (A) and octahedral (B) sites for x≤0.4. The Mössbauer intensity data show that Sn possesses a preference for the B‐site of the spinel. As expected, the hyperfine field and Curie temperature determined from a.c. susceptibility decreases with increasing Sn content. The variation of the saturation magnetic moment per formula unit measured at 77 and 300 K with Sn content is satisfactorily explained on the basis of Néel's collinear spin ordering model for x=0.1–0.4.     

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.     

Mössbauer effect study of the MnxZnyNizFe2O4

A⁵⁷Fe Mössbauer effect study of the triangular Mn_xZn_yNi_zFe₂O₄ system at 78 K has been undertaken. The mean values of the hyperfine magnetic field, the values of standard deviations and the asymmetry parameters have been derived from the distribution diagrams of the hyperfine magnetic field. The dilution process affects only slightly the A-site hyperfine field but it is responsible for a wide distribution of the B-site field. For the highly diluted samples, relaxation processes play an important role. The results of Mössbauer effect investigations can be understood in terms of the lack of the Zn₂₊-O-Fe₃₊ magnetic coupling and the weaker Mn₂₊-O-Fe₃₊ and Ni₂₊-O-Fe₃₊ superexchange interactions as compared to the Fe₃₊-O-Fe₃₊ interactions.     

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 effect study of Ni1−xCuxMnyFe2−yO4 system

The Mössbauer effect technique has been employed for the study of magnetic properties of spinel series Ni_1?xCu_xMn_yFe_2?yO₄ with 0.0≤x≤1.0, and y=0.6. The substitution of Mn³⁺ and Cu²⁺ ions results in a slight decrease of the hyperfine field at B‐ as well as A‐sites. The area ratio of Fe³⁺ ions at the A‐ and B‐site at 77 K indicates that Cu²⁺, Ni²⁺ and Mn³⁺ ions occupy the octahedral sites in an evidence for complete inverse spinel in this system. The temperature dependence of the hyperfine parameters has been studied for composition with x=0.5 where Nèel point T_N and Debye temperature θ_D are found to be 650 and 679 K, respectively. The temperature dependence of the sublattice magnetization σ(T) obeys a one‐third‐power law in the range 0.5N<0.99.