Feromagnetism in the single-band Hubbard model in the limit of strong correlations

A thermodynamic potential of the single-band strong correlation Hubbard model is calculated by means of diagram technique in a two-loop approximation. Some features of diagram technique for Hubbard $X$-operators are discussed. It is shown that, in the case of almost half-filled band, the system tends to be paramagnetically unstable. The dependence of extrapolation Curie temperatures on hole concentration is given.