Itinerant Theory of Ferromagnetism and Hund's-Rule Coupling

We propose a Zener-like model for narrow bands, which includes the intra-atomic Hund's-rule coupling of $d$ electrons. Each of Hubbard's split bands is then shown to split again into spin-polarized sub-bands, yielding ferromagnetism when the density of Zener electrons is low but yielding antiferromegnetism when the density increases.