Theory of Ferromagnetism in Narrow-Band Solids, with Application to Experiments onCoxFe1−xS2

Abstract

In a previous publication, ferromagnetism of the Hubbard model was studied in the infiniteinteraction limit for a nearly half-filled band, using a new diagrammatic approach based on the work of Nagaoka. In this paper, the results of this previous work are shown to apply away from the nearly half-filled-band limit, in which this system was shown to be rigorously ferromagnetic in a paper by Nagaoka. As was done previously, the approach due to Nagaoka, in which one considers the motion of "holes" (i.e., sites not containing an electron) is used to obtain an expansion of the partition function in powers of inverse temperature. The approximation of keeping only contributions from those "hole" paths having electrons of one spin only is shown to be valid over a wide range of electron concentration. Using this method, the Curie temperature and magnetization are calculated and compared in detail with some transition-metal disulfide experimental systems which are simple two-band itinerant-electron systems to which this model can be shown to be applicable. To understand these systems, a discussion of their disordered nature is introduced. Comparison is also made with other narrow-band theories, including Hubbard's theory and the work of Harris and Lange.