The form of the spin-lattice coupling in hydrated salts of the iron-group ions is derived in terms of the lattice vibrational coordinates and the effective spin variable, using general symmetry arguments. The most important coupling terms for octahedral and spherical symmetry are given, and the role of the breathing and rotational modes is discussed, as well as the concept of adiabatic and non-adiabatic coupling. This work forms the basis of the semiclassical theory of phonon radiation processes and phonon multipole fields and interactions to be presented in a subsequent paper. Finally, explicit expressions for the coupling constants in terms of the electronic structure of the ions are also derived, for the case of a nondegenerate ground orbital level.
Role of spin-lattice coupling in the ultrafast demagnetization ofGd1−xTbxalloys