Variational approach to finite-temperature magnetism in the degenerate-band Hubbard model

The variational approach (VA), which goes beyond the static approximation to the functional-integral method by taking into account the local electron correlations of the Gutzwiller type, has been extended to the degenerate bands for the purpose of a more realistic description of the finite-temperature magnetism in narrow-band systems. The magnetic properties of Fe, Co, and Ni are investigated numerically within a single-site approximation to the VA. The magnetization-versus-temperature curve, susceptibility, specific heat, the amplitude of local moment, and the charge fluctuations are calculated. The results are compared with those in the static approximation and the experimental data. The Curie temperatures $T_C$ in the VA are found to be between the observed values $T_C$(obs) and 2$T_C$(obs). It is shown that $T_C$ in Fe and Ni are reduced by a factor of 2 because of electron correlations even in the degenerate-band case. The other strong-correlation effects at finite temperatures are found in particular in Ni; the sign of ∂$T_C$/∂P is changed, and the temperature-independent charge fluctuation is suppressed by a factor of 3.