The equation of state of a three-dimensional Ising-like system

The free energy F and the order parameter M are calculated near the phase transition point for a 3D Ising-like system in the temperature region T<Tc. An explicit expression for free energy as a function of temperature and field is obtained at arbitrary values of the latter. Hamiltonian parameters dependence of coefficients of the free energy expression is found. Expression for the model's magnetization as a function of temperature and field is derived by direct differentiation of F with respect of external field.