Abstract
The theory of excitations in anisotropic magnets is reviewed. It will be shown, by discussion of a simple model system with Heisenberg interaction plus planar single ion anisotropy, that the conventional spinwave theory is treating the single ion anisotropy very poorly. A new Bose operator expansion of tensor operators which is different from the Holstein Primakoff transformation will be discussed. The spin waves fulfil the Goldstone theorem exactly for planar anisotropy. An infinite order perturbation approach shows that in general only renormalized parameters are obtainable from spin wave data of anisotropic magnets. The approach shows clearly the general structure of the ground state corrections due to crystal field effects. The Er data for the cone phase can be analyzed successfully with single ion anisotropy and isotropic exchange interaction, J(q). The Tb data can be understood on the basis of the same model.
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