Self‐consistent fluctuation theory for classical spin systems on 3D lattices

The thermodynamic properties of classical vector ferromagnetic models (D‐component ‘‘spins’’ with n components interacting with each other) on 3D lattices are calculated in the 1/z approximation (z is the number of nearest neighbors) self consistently taking into account long‐ranged Gaussian fluctuations of the molecular field. The numerical solution for the magnetization, susceptibility and the internal energy of a ferromagnet is valid in the whole plane (H,T) excluding the narrow region near the critical point (‖τ‖ ≲ 1/z2) and agrees well with available series results. In the spherical limit D→∞ the 1/z approximation becomes exact.