Thermodynamics of itinerant magnets in a classical spin fluctuation model

Thermodynamics of itinerant magnets is studied using a classical model with one parameter characterizing the degree of itinerancy. Monte Carlo simulations for bcc and fcc lattices are compared with the mean-field approximation and with the Onsager cavity field approximation extended to itinerant systems. The qualitative features of thermodynamics are similar to the known results of the functional integral method. It is found that magnetic short-range order is weak and almost independent on the degree of itinerancy, and the mean-field approximation describes the thermodynamics reasonably well. Ambiguity of the phase space measure for classical models is emphasized. The Onsager cavity field method is extended to itinerant systems, which involves the renormalization of both the Weiss field and the on-site exchange interaction. The predictions of this approximation are in excellent agreement with Monte Carlo results.