Critical behavior of a model band ferromagnet

We investigate the critical behavior of a band ferromagnet within the framework of the Hubbard model. With the aid of a self-consistent moment method the possibility of para-ferromagnetic phase transitions is inspected with respect to the s-band occupation n (0≤n≤2) and the Coulomb interaction U. For an fcc lattice structure phase transitions appear when the energy band is more than half filled and U exceeds a critical value (U/W≥1; W the Bloch bandwidth). They are of first order for slightly more than half-filled bands (up to n≊1.3), arising as consequences of the competition between two coexisting ferromagnetic solutions, which cancel mutually at the transition point. For n≥1.35 the phase transitions are of second order. The critical phenomena get a strictly universal character, becoming completely n and U independent, with critical exponents of Landau type (α=α’=0; β=0.5; γ=1; δ=3). In band systems with second-order transitions the paramagnetic, static susceptibility exhibits clear Curie-Weiss behavior, with a paramagnetic Curie temperature Θ being in most cases substantially greater than ${\mathit{T}}_{\mathit{C}}$.