The theory of paramagnetic relaxation

The spin-lattice relaxation of paramagnetic ions in ionic crystals takes place via thermal modulations of the complex consisting of the ion and its immediate neighbours. The standard theory is that due to Van Vleck, who used a crystal-field model to describe the complex. In recent years there have been extensive developments in crystal-field techniques, which have removed many of the difficulties. It has therefore seemed appropriate to attempt to recast the relaxation theory so as to take full advantage of these developments. At the same time it becomes possible to see more clearly how a molecular-field theory might be formulated. It is assumed that the reader is reasonably conversant with crystal-field theory, but that he is less likely to be familiar with lattice dynamics. As the only really new feature which needs to be added to crystal-field theory is that of the vibrating lattice, particular attention is paid to this. The opportunity is also taken to give a glossary of the jargon used in spin-lattice relaxation theory.