J. De Sitter

On the Mossbauer parameters in BiFeO3

The Mossbauerspectrum of BiFeO3 is shown to consist of two super-imposed six-line spectra. The main distinction between the two iron sites is made up by the different ΔEQ values, even having opposite sign. A qualitative explanation is proposed.

On the cation distribution in the spinel systemyMg2TiO4−(1−y)MgFe2O4

The cation distribution and the oxygen parameter for the system Mg1+yFe2(1−y) TiyO4 have been determined using the intensities of X-ray reflections. Results are given for slowly cooled and quanched samples. An explanation is proposed to account for the behaviour of the lattice parameter and the oxygen parameter.

A Mössbauer investigation of the quadrupole splitting in β-FeOOH

Abstract High resolution 57Fe Mossbauer effect measurements at room temperature show that the spectra of pure, synthetic β-FeOOH in its paramagnetic state consist of a superposition of two discrete doublets, due to the existence of two different electric field gradients (EFG) which are proven to be related to the presence of halogen ions in this compound.

On the effective magnetic hyperfine field in naturally occuring goethite

Abstract The asymmetric broadening of the Mossbauer absorption lines frequently observed in goethite is shown to be due to relaxation effects in the surface layers rather than being an intrinsic property of this material. The particle size determines entirely the shape of the observed M-spectrum at room temperature which can vary from a magnetically ordered spectrum to a pure paramagnetic one.

Incomplete magnetic ordering in Fe2(1−y)Mg1+y Ti y O4 spinels with intermediate compositiony

Static magnetization measurements on the ferrimagnetic spinels Fe2(1−y)Mg1+yTiyO4 withy=0.3, 0.4, 0.5, and 0.6 show that these compounds have no well-defined orderdisorder transition temperature and that their ferrimagnetism may not be described in terms of the Néel theory. From the Mössbauer spectra we conclude that a temperature dependent number of the ferric ions does not participate in the ferrimagnetism of those compounds with compositiony≧0.4. The explanation of the observed magnetic and Mössbauer properties is based on the assumption that each ferric ion must have at least two magnetic linkages of the type FeA3+−O2−−FeB3+ in order to couple its magnetic moment to the neighbouring ones over the entire temperature interval between 0 K and the respective Néel temperature.

Temperature dependence of the Mössbauer parameters of Fe0.2Mg1.9Ti0.9O4 and Fe0.4Mg1.8Ti0.8O4

Abstract The temperature dependence of the Mossbauer spectra of two compounds of the magnesium ferrite-magnesium titanate solid-solution series is investigated. The Debije temperatures θD of 57Fe nuclei in octahedral (B) and tetrahedral (A) sites are determined from the isomer shift versus temperature curves. From these Debije temperatures, the ratio of recoilles fractions f B f A is calculated. The obtained values of θD are found to be consistent with the observed areas of the Mossbauer absorption lines.

On the magnetic properties of the basic compounds in the Fe2O3-Bi2O3 system

Abstract The Mossbauer spectrum of BiFeO3 shows the presence of two non-equivalent octahedral sites for the ferric ion, having different quadrupole splittings below the Neel temperature. The hyperfine parameters in Bi2Fe4O9 point to a greater degree of covalency for the ferric ions in tetrahedral coordination than in octahedral coordination.

On the bulk and surface contributions to the magnetic hyperfine field of small particles of goethite (α-FeOOH)

Abstract The principle of the analysis of the asymmetrically broadened Mossbauer lines of goethite is described. The asymmetric shapes can be described by superposition of two contributions. The first one arises from the bulk part of the grain and the second one is due to the continuous decrease of the magnetic field in the outer layers.

Frequency dependent conductivity tensor for polarons in a magnetic field

Abstract In this contribution the magneto-optical response properties of a conduction electron are studied, starting from the Kubo formula. The frequency dependent resistivity tensor ϱ is evaluated to second order in perturbation theory, for arbitrary temperature and external magnetic field. Inverting ϱ, the frequency dependent conductivity tensor σ, which contains the information about the magneto-optical and transport properties of the system, is obtained. An explicit calculation is made for polarons. As a test for the calculation different limiting cases, which have been studied before, are investigated in detail. The magneto phonon effect is obtained in the zero frequency limit. The pinning effect and the anomalous broadening of the cyclotron resonance line for ωc ≈ ωLO follow in a natural way. The phonon assisted cyclotron resonances are studied for different configurations.