Application of the functional integration method to the Heisenberg model of ferromagnetism

The partition function of a Heisenberg ferromagnet is represented as a functional integral over the stochastic fields. The integrand describing the partition function of the system of spins without interaction is expanded in a series of powers of the stochastic fields. It is shown that this leads to the usual expansions of the original expression in powers of the interaction. A functional integral representation of the partition function in the Ising model is considered as a special case. By means of a cumulant expansion of the integrand a diagram technique is constructed for the calculation of the partition function, the magnetization, and the Green's functions of an Ising ferromagnet.