FERROMAGNETISM OF ANDERSON-LOCALIZED-ELECTRONS - APPLICATION TO CLUSTER COMPOUNDS

A study of the electrical transport and magnetic properties of a serie s of cluster compounds with the generic formula A(0.5)M(2)X(4) suggest s that the electrons at the Fermi surface are localized, and the ferro magnetism seen in these compounds arises from these electrons. The mag netism of these compounds shows some features characteristic of itiner ant 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 v ia direct exchange interaction which is ferromagnetic. Using a mean-fi eld approximation we calculate the various magnetic properties, which are in qualitative accord with the observed behavior. In particular, w e find that the single-particle excitations play a dominant role in th e magnetism of these compounds, even though the electrons are localize d. We also analyze the spin-wave excitations in this model and discuss their effect on low-temperature thermodynamics.