Theory of magnetic properties of rare earth compounds (Localized moments and hybridization effects)

Abstract

The authors study a system of itinerant, uncorrelated, electronic states forming a set of conduction (and valence) bands weakly hybridized with a system of highly correlated, ionic-like, electronic states. The latter are localized around lattice sites and correlated so as to give ionic configurations with a well defined total angular momentum J. The effective coupling that arises between the localized moments as a consequence of hybridization is studied. The result, in fourth order perturbation theory, shows that two 'different' interactions occur: a Ruderman-Kittel-Kasuya-Yosida interaction, characteristic of metals, and an Anderson type superexchange, characteristic of insulators. Which of the two dominates depends on the number of conduction electrons, position of the Fermi level and position of the localized levels with respect to the Fermi energy. In general both have to be included. Three simple models are examined: a free-electron-like conduction band (with an application to metallic Ce), free-electron-free-hole-like conduction bands, and an s-like BCC tight binding band. For this last example the equilibrium configuration of the moments is computed from the classical 'Heisenberg' energy. Although no specific application is made to magnetic semiconductors, it seems likely that many compounds in this group (Eu chalcogenides, for instance) could serve as examples of the electronic system treated.