Magnetic impurity in an anisotropic ferromagnet

Abstract

Abstract There has been much interest in the effect of impurities on the continuous energy spectrum of elementary excitations in solids. The perturbation produced by the impurity is characterized by its spatial extent and, for sufficiently small perturbations, the energy levels of the impurity-host system may be solved for exactly. The impurity and perturbed regions of the host form a complex with discrete energy levels that may lie within or outside the energy band of the host, virtual and localized states, respectively. The virtual levels decay into the band with a finite lifetime and may have a significant effect on the thermodynamic properties of the system. Localized levels have a long lifetime and, in general, are of less importance to the thermodynamic properties, but may have an important role in magneto-optical transitions. The literature on the effects of impurities in simple metals, crystals, and ferromagnetics is very extensive.