Order parameters and the free energy of the three-dimensional ising model below the transition temperature

A method is suggested for calculating free energy of the three-dimensional Ising model below the phase transition point. It is shown that the long-wave fluctuations produce the main contribution into the non-analytic part of the free energy. They can be conveniently described by the Gaussian density measure with nonanalytic dependence of the dispersion upon temperature. The order parameter is calculated by means of a self-consistent procedure which makes it possible to obtain both the critical index for the ever age spin moment and the critical amplitude.