Phase Transition of Nearly Two-Dimensional Heisenberg Ferromagnets with Weak Perturbations

The double-time Green's function method incorporated with Tyablikov's decoupling is applied to the nearly two-dimensional ferromagnets of layer-structure copper compounds. Besides the strong intralayer ferromagnetic exchange interaction we take account of weak perturbations, i.e., interlayer exchange interaction, intralayer exchange anisotropy, and dipolar interaction. We obtain the leading two in the inverse transition temperature T c -1 , i.e., terms logarithmically singular and constant with respect to the weak perturbation. Spin contraction at zero temperature is very small. Outside the very vicinity of zero temperature the equation for spontaneous magnetization σ as a function of k B T /2 J is reduced to a much simpler one containing only one parameter k B T c c a l c /2 J . Comparison of the calculated T c and temperature dependence of σ with experimental deta seems to show the soundness of applying the RPA theory to these systems. The contributions of dipolar interaction are examined.