Variational approach to finite-temperature magnetism. II. Antiferromagnetism.

Abstract

The overall features of a variational theory for finite-temperature magnetism, which has recently been proposed, are investigated numerically for a half-filled Hubbard model. The phase diagram, sublattice magnetization, staggered susceptibility, amplitude of local moments, charge fluctuations, and electronic contributions to the internal energy, entropy, and the thermal expansion are calculated as functions of temperature T and the Coulomb interaction U. It is found that the local electron correlations, which cannot be described by the static approximation to the functional integral, act so as to reduce the effective Coulomb interaction for the magnetization, the N\'eel temperature, the internal energy, the specific heat, and the entropy, while they enhance the atomic character for the amplitude of local moments, the charge fluctuation, and the reduced-magnetization-- versus--temperature curves. Electron correlations reduce the thermal expansion, in contrast to the case of ferromagnets.