S. K. Date

Dependence on cation distribution of particle size, lattice parameter, and magnetic properties in nanosize Mn–Zn ferrite

In Mn1−xZnxFe2O4 (x=0 to 1) nanosize particles prepared through hydrothermal precipitation we observe a decrease in particle size from 13 to 4 nm with increasing Zn concentration from 0 to 1. The lattice constant, a, for all Mn/Zn concentrations is found to be less than that for the corresponding bulk values. At specific compositions within x=0.35 and 0.5, the temperature dependence of the magnetization exhibits a cusp-like behavior below the temperature at which the nanoparticles undergo a ferri- to para-magnetic transition (Tc). The Curie temperatures, Tc, of the nanoparticles are in the range of 175–500u200a°C, which are much higher than their corresponding bulk values. To explain these unusual features, the strong preferential occupancy of cations in chemically inequivalent A and B sites and the metastable cation distribution in nanoparticles are invoked.

PREPARATION AND CHARACTERIZATION OF NANOSIZE MN-ZN FERRITE

Abstract Synthesis of nanosize (9–12xa0nm) Mn 0.65 Zn 0.35 Fe 2 O 4 particles from metal chloride solution through a hydrothermal precipitation route using aqueous ammonia, and their characterization by XRD, TEM and VSM are reported. Chloride ion concentration in the solution and the pH of precipitation are shown to play a crucial role in retaining the initial stoichiometry of the solution in the nanoparticles. While at lower pH, precipitation of Mn was incomplete, higher pH of precipitation led to Zn loss in the particles. The optimum pH for stoichiometric precipitation was found to lie around 10. The coercive force, H c and the transition temperature, T c (from ferrimagnetic to paramagnetic state) were high as compared to reported bulk values and confirm the nanosize nature of the particles. The M versus T curve, instead of showing a monotonic drop of magnetization, showed a cusp before T c . This cusp-like feature is shown to arise due to an irreversible phase transition involving cationic redistribution in the unit cell of the nanoparticles.

Mossbauer effect study of Fe57 doped LiNbO3 and LiTaO3

Abstract The Mossbauer effect at dilute Fe3+ and Fe2+ impurities in LiNbO3 and LiTaO3 has been measured. Lattice site substitution and the charge compensating process are discussed.

Moussbauer spectroscopy of BaTiO3 and SrTiO3

Single crystals and ceramics of BaTiO3 and SrTiO3 have been studied by Mossbauer spectroscopy. The materials were investigated as sources and absorbers, doped with Fe57 (Co57) and Sn119. In the preparation of the samples various parameters (temperature, atmosphere, impurity concentration etc.) have been changed. It turned out to be a difficult problem to incorporate Fe into BaTiO3 or SrTiO3 lattices at the Ti4+ site. Thermal doping of single crystals was unsuccessful, in this case a second phase was formed by the addition of Fe57 and Co57. In melt grown BaTiO3 crystals a few hundred ppm Fe could be incorporated in the lattice trivalently. In contrast, Sn is relatively easy to introduce into BaTiO3 and the Mossbauer spectrum of a melt grown single crystal shows a very sharp line. No effects of phase transitions on the hyperfine parameters or the Debye Waller factor could be detected.

Electronic structure, electron density, electric field gradient-, magnetic susceptability- and G-tensor of K3Fe(CN)6

Abstract Molecular orbital calculations are presented for the electronic structure of K 3 Fe(CN) 6 based on clusters of formula K 8 Fe(CN) 5+ 6 , using a semi-empirical molecular orbital method including mixing of low-lying electronic configurations and spin-orbit coupling. The energy splittings obtained are in qualitative agreement with ligand field studies from several workers, excluding those of Merrithew and Modestino. While other authors interpret either Mossbauer data, or ESR results, or susceptibility values only, we obtain from the electron structure calculations charge densities at the iron nucleus, and electric field gradient, magnetic susceptibility and gyromagnetic tensors, which consistently interpret experimental Mossbauer-, EST- and magnetic anisotropy results. From the electronic structure calculations as well as from the reanalysis of experimental quadrupole line intensities (obtained by Oosterhuis and Lang) we derive that orthorhombic polytypism of K 3 Fe(CN) 6 has to be considered for a consistent interpretation of the experimental data. The successful correlation between calculation and experiment in an energy range of about 300 K above the electronic groundstate is a measure for the adequacy of our electronic structure calculations in this low-energy range.

Nuclear electric field gradient determination for Fe2 ions in ferroelectric lithium niobate

Nuclear electric field gradient (EFG) parameters for Fe2+ ions in ferroelectric lithium niobate are determined using both single crystal orientation experiments and magnetic perturbation technique for polycrystalline samples. The results show clearly the presence of an axially symmetric EFG with no drastic change in the surrounding local environment. The different microscopic models, proposed to explain photorefractive processes in iron doped lithium niobate, are compared and critically reviewed with our experimental results.