METALLIC FERROMAGNETISM IN A SINGLE-BAND MODEL - EFFECT OF BAND FILLING AND COULOMB INTERACTIONS

A single-band tight-binding model with on-site repulsion and nearest-n eighbor exchange interaction has been proposed as a simple model to de scribe metallic ferromagnetism. Here we extend previously obtained exa ct-diagonalization studies for a one-dimensional 1/2-filled band syste m to other band fillings, and consider the effect of including various other Coulomb matrix elements in the Hamiltonian that are expected to be of appreciable magnitude in real materials. Results of exact diago nalization and mean-field theory for the one-dimensional case are comp ared. As the band filling decreases from 1/2, the tendency to ferromag netism is found to decrease in exact diagonalization, while mean-field theory predicts the opposite behavior. A nearest-neighbor Coulomb rep ulsion term is found to suppress the tendency to ferromagnetism; howev er, the effect becomes small for large on-site repulsion. A pair hoppi ng interaction enhances the tendency to ferromagnetism. A nearest-neig hbor hybrid Coulomb matrix element breaks electron-hole symmetry and c auses metallic ferromagnetism to occur preferentially for more than ha lf-filled rather than less-than-half-filled bands in this model. Mean- field theory is found to yield qualitatively incorrect results for the effect of these interactions on the tendency to ferromagnetism. The i mplications of these results for the understanding of ferromagnetism i n real materials is discussed.