MAGNETISM AND ELECTRONIC-STRUCTURE OF A LOCAL MOMENT FERROMAGNET

We propose a self-consistent approximate solution of the s-f model for describing the exchange coupling of a local moment system with a part ially filled energy band. Induced electronic correlations account for the characteristic quasiparticle band effects which become manifest vi a striking temperature dependencies, band deformations and splittings. For weak s-f exchange interactions a 'Stoner-like' spin splitting of the conduction band proportional to the f magnetization occurs. As soo n as the coupling exceeds a critical value an additional spin splittin g of the quasiparticle dispersion sets in, which is due to different e lementary excitations. One of these appears as a repeated emission and reabsorption of a magnon by the conduction electron, resulting in an effective electron-magnon attraction. This gives rise to a polaronlike quasiparticle (a 'magnetic polaron'). Other elementary processes are connected to magnon emission or absorption by the conduction electron ('scattering states'). The polarization of the conduction band due to the s-f exchange interaction J feeds back to the localized spin system leading to an indirect coupling between the spins. For weak s-f coupl ing the RKKY mechanism dominates (T-c proportional to J(2)), but with remarkable deviations for intermediate and strong couplings. The Curie temperature saturates with increasing J, where the saturation value i s strongly dependent on the band occupation n. The oscillating behavio ur of the effective exchange integral connecting the localized spins r estricts ferromagnetism to special regions for n. The magnetization cu rve, the spin polarization of the itinerant electrons, and f-f as well as s-f spin correlation functions are worked out for a simple cubic l attice and discussed in terms of the band occupation n and the s-f exc hange coupling J.