Effect of Orbital Degeneracy and Intra-Atomic Exchange on the Occurrence of Ferromagnetism

Abstract

The role of orbital degeneracy and intra-atomic exchange for the possible origin of ferromagnetism in transition metals is investigated assuming doubly degenerate bands with intra-atomic Coulomb and exchange interactions. Roth showed in the narrow band limit with one electron per atom that the ground state is ferromagnetic with a superlattice ordering with respect to the orbital states. It is found here that there exist two phase transitions at finite temperatures. At the higher transition temperature only the superlattice ordering of the orbital states appears. The spin susceptibility, which obeys at higher temperatures the antiferromagnetic Curie-Weiss law, is modified by the appearance of this orbital ordering and finally it goes over into a ferromagnetic behavior with the help of intra-atomic exchange. Ferromagnetism thus occurs at the lower transition temperature. This conclusion is suggested in a two-atom system and is confirmed within a molecular field approximation.