Spin-Fermion Model of Magnetism: Quasi-particle Many-Body Dynamics

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Theoretical foundations and applications of the generalized spin-fermion (sp-d) exchange lattice model to various magnetic systems, e.g. rare-earth metals and compounds and magnetic semiconductors are discussed. The capabilities of the model to describe spin quasi-particle spectra are investigated. The main emphasis is made on the dynamical behavior of two interacting subsystems, the localized spins and spin density of itinerant carriers. A nonperturbative many-body approach, the Irreducible Green Functions (IGF) method, is used to describe the quasi-particle dynamics. Scattering states are investigated and three branches of magnetic excitations are calculated in the regime characteristic of a magnetic semiconductor. For a simplified version of the model (Kondo lattice model) we study the spectra of quasi-particle excitations with special attention given to diluted magnetic semiconductors in simple approximation, to demonstrate the possibilities of the IGF approach. For this, to include the effects of disorder, a modified mean fields are determined self-consistently. The approach permits to investigate and clarify the role of various interactions and disorder effects in unified and coherent fashion.