Ferromagnetism of anderson localized electrons: application to cluster compounds

Abstract

A study of the electrical transport and magnetic properties of a series of cluster compounds with the generic formula A_0.5M₂X₄ suggests that the electrons at the Fermi surface are localized, and the ferromagnetism seen in these compounds arises from these electrons. The magnetism of these compounds shows some features characteristic of itinerant models and others which are characteristic of localized models. We construct a model which has a nondegenerate band of localized states with on-site repulsion. Further, the singly occupied states interact via direct exchange interaction which is ferromagnetic. Using a mean-field approximation we calculate the various magnetic properties, which are in qualitative accord with the observed behavior. In particular, we find that the single-particle excitations play a dominant role in the magnetism of these compounds, even though the electrons are localized. We also analyze the spin-wave excitations in this model and discuss their effect on low-temperature thermodynamics.