Metallic ferromagnetism without exchange splitting

In the band theory of ferromagnetism there is a relative shift in the posit ion of majority and minority spin bands due to the self-consistent field du e to opposite spin electrons. In the simplest realization, the Stoner model , the majority and minority spin bands are rigidly shifted with respect to each other. Here we consider models at the opposite extreme, where there is no overall shift of the energy bands. Instead, upon spin polarization one of the bands broadens relative to the other. Ferromagnetism is driven by th e resulting gain in kinetic energy. A signature of this class of mechanisms is that a transfer of spectral weight in optical absorption from high to l ow frequencies occurs upon spin polarization. We show that such models aris e from generalized tight binding models that include off-diagonal matrix el ements of the Coulomb interaction. For certain parameter ranges it is also found that reentrant ferromagnetism occurs. We examine properties of these models at zero and finite temperatures, and discuss their possible relevanc e to real materials. [S0163-1829(99)04709-5].