Local moment systems: magnetism and electronic correlations

摘要

We describe local-moment systems by the (multiband) s-f model (ferromagnetic Kondo-lattice model) which traces back the characteristic properties of such materials to an interband exchange coupling between itinerant conduction electrons and localized magnetic moments. We first present a many-body approach to the electronic and magnetic properties of the single-band model. The exchange coupling leads, on the one hand, to a distinct temperature-dependence of the electronic quasiparticle spectrum and, on the other hand, to magnetic properties, as e. g. the Curie temperature TC or the magnon dispersion, which are strongly influenced by the band electron selfenergy and therewith in particular by the carrier density. Results for the electronic part are given in terms of quasiparticle densities of states and quasiparticle band structures and for the magnetic part in terms of the selfconsistently derived Curie temperature and spin wave spectra. The transition from weak-coupling (RKKY) to strong-coupling (double exchange) behaviour is worked out. The multiband model is combined with an ab-initio bandstructure calculation to describe real magnetic materials. The proposed method avoids the double counting of relevant interactions and takes into account the correct symmetry of atomic orbitals. For the ferromagnetic metal Gd we get a selfconsistently derived Curie temperature of 301.5 K and a T = 0-moment of 7.81μ _B, very close to the experimental values. Furthermore a striking induced temperature-dependence of the 5d conduction bands explains respective photoemission data. For the ferromagnetic semiconductors EuO and EuS we present results for electronic and magnetic bulk properties as well as for thin films.