A Green's Function Treatment of a Ferromagnetic Layered System

A ferromagnetic layered spin-S system with an anisotropic exchange interaction is studied by the RPA Green's function method and the validity of the approximation is ascertained for a nearly two-dimensional Heisenberg system. The Curie temperature Tc is given in terms of the ratio α of the interlayer exchange interaction to the intralayer one and of the exchange anisotropy η, and is shown to be raised abruptly by a slight additional XY-like exchange anisotropy for the nearly two-dimensional isotropic Heisenberg model. The zero-point spin contraction increase as the system becomes XY-like and two-dimensional. For the nearly two-dimensional Heisenberg system the magnetization behaves like Ms ∝ 1 - cT3/2 in the vast intermediate temperature region, which amounts to (0.19∼0.72)Tc in the case of K2CuF4 with α = 0.00066 and the XY-like anisotropy of one percent, and the present theory is in satisfactory agreement with the experimental results.