A Green's Function Treatment of a Ferromagnetic Layered System

Abstract

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 T, is given in terms of the ratio a of the interlayer exchange interaction to the intralayer one and of the exchange anisotropy 71, 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 increases as the system becomes XY-like and two-dimensional. For the nearly two-dimensional Heisenberg system the magnetization behaves like lv1,oc1-cT'12 in the vast intermediate temperature region, which amounts to (0.19~0.72) T, in the case of K 2CuF4 with a=0.00066 and the XY-like anisotropy of one percent, and the present theory IS m satisfactory agreement with the experimental results. § l. Introduction Magnetic layered systems with an anisotropic interaction present great interest­ mg problems not only experimentally but also theoretically, because the changes in the dimensionalities of the lattice and of the spin are connected closely with the