Application of the Method of the Two-Time Green's Function to the Heisenberg Ferromagnet. II

Abstract

The method of the two-time Green's function is applied to the Heisenberg ferromagnet. The equations of motion for the lowest order and the next higher order Green's functions are solved in an approximation which leads to the first nonvanishing contribution of the spin-wave interaction to the spontaneous magnetization at low temperatures. It is shown that this contribution is of order $T^4$ and is the same as Dyson's result.