Application of the Môssbauer Effect to the Study of Ferrites and Dielectrics

The present paper reviews the results of studies on the Moessbauer effect in ferrites with garnet and spinel structures, and also in various dielectrics. Using ferroelectrics and ferro magnetics as examples, it is shown that the Moessbauer effect may be successfully used in phase transition studies. In the phase transition region the probability of the Moessbauer effect shows anomalous characteristics connected with the anomalous temperature dependence of the frequency of the optical branch of the crystal lattice vibrational spectrum. Along with quadrupole splitting, a magnetic hyperfine structure of the Moessbauer spectra is observed in ferro magnetics, which disappears on transition to the paramagnetic state. In the region of the Curie ferroelectric temperature a sharp change occurs in the chemical shift in BaTiO₃ and Pb(Fe_0.5Nb_0.5)O₃, indicating a change in the ionic character of the chemical bond. The review quotes the results of studies which do not fit in with these mechanisms. Considering as a concrete example single crystals of tourmaline, opportunities are cited for application of the Moessbauer effect in elucidating structural problems. An important aspect of ferromagnetism is the nature of the magnetic fields at the nuclei. The review gives the results of investigations of Moessbauer spectra in iron garnets and spinels. Analysis of experimental data on the magnetic fields at the nuclei of these compounds shows that the strength of the magnetic field depends on the magnetic moment of the ions, the local environment, and the number and nature of the exchange bonds. In iron garnets account must also be taken of the degree of covalency of the chemical bond and of the deviation of the lattice symmetry from the cubic. The occurrence of magnetic fields at diamagnetic atoms of tin in the lattice of an yttrium iron garnet is described. Magnetic fields at Fe iron nuclei are normally due to the ion's own electrons. In the case of tin, however, the magnetic field is created by polarization of the electron shell of a diamagnetic atom by the magnetic moment of iron atoms, through indirect exchange interaction. (authors)