Localized Impurity Spin Excitations in a Ferromagnetic Metal

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Abstract
The structure of the theory of impurity spin excitations in a one-band model of a ferromagnetic metal is examined. The existence of a local magnetic moment and an off-diagonal spin correlation in the impurity ground state is shown to be closely related to the occurrence of localized spin excitations (localized magnons). Expressions for both the transverse and longitudinal reduced susceptibility functions are obtained in terms of single-particle Green's functions. Poles of the reduced transverse susceptibility function are shown to correspond to local magnon states, and poles of the reduced longitudinal susceptibility are identified with fluctuations in the magnitude of the impurity moment. The role of localized electronic states is briefly discussed. Some of the well-known results for the unperturbed ferromagnet are derived in the Appendix.