Mössbauer studies of spin reorientations in oxides

Abstract

Mössbauer spectroscopy has been remarkably useful in understanding several properties of the mixed oxides. The investigations relating to spin arrangements form an important part of the successful applications of this method. In mixed spinel ferrites and garnets, Mössbauer spectroscopy unambiguously showed the non-collinearity of spins in the same sublattice in several cases. The detailed investigations showed features predicted by the localized canting model (LCM), viz., the simultaneous presence of non-collinearity at the two non-equivalent sites in certain cases, ’spin reversal’ of ions without magnetic ions on the nearest non-equivalent sites. However, it has not been possible to observe other details of the model. Another striking result obtained is that non-collinearity decreases rapidly as the temperature increases, disappears at temperatures below 80 K. These studies have not, however, succeeded in determining the exchange constants unambiguously. In orthoferrites, the complications due to the variations in the environments of ions in the same sublattice are not present. Consequently, studies of spin reorientations (SR) relative to the crystal axes and other magnetic properties have provided accurate results, ideal for theoretical analyses. SR is due to the anisotropic-symmetric and antisymmetric exchange interactions between the rare earths and iron group ions, which are much smaller than the isotropic part of the exchange interaction. Thus, a detailed theoretical analysis expresses SR parameters in terms of these smaller interactions and the external magnetic field, if present. The number of parameters involved is, however, large. Analyses of the experimental data are made using a simpler two-sublattice model involving a smaller number of parameters. This, nevertheless, makes comparison between similar orthoferrites possible. There are other oxides as well which show SRs due to changes in the signs and magnitudes of the crystal field anisotropic constants. Such studies are useful for investigating crystal field interactions.