Variational approach to finite-temperature magnetism.

Abstract

A theory is presented for magnetism at finite temperatures which includes local electron correlations. It goes beyond the static approximation to the functional-integral method. The theory is based on variational methods. At T=0 it reduces to a correlated ground state of the form proposed by Gutzwiller [Phys. Rev. 134, A293 (1964); 137, A1726 (1965)]. In the high-temperature limit the static approximation is recovered. A single-site approximation is made in order to make numerical calculations possible. The theory is applied to Fe and Ni. A large reduction of the Curie temperature of Fe is found due to correlations. The amplitude of the local moment is increased by the electron correlations. It hardly changes with temperature in contrast to the results of the static approximation. We also discuss the magnetization-versus-temperature curves, the paramagnetic susceptibility, and the charge fluctuations.