Specific heat and mean energy of a Heisenberg ferromagnet in the Green's function method

A system of equations that permits calculation of the longitudinal correlation function in terms of the correlation functions of the transverse spin components is proposed. In the Green's function method, the system permits calculation of the susceptibility, mean energy, and specific heat in the same approximation as the magnetization. The experimental data on the specific heat in Gd and EuS agree qualitatively with the conclusions of the theory.