The quadratic-layer antiferromagnet

Abstract

This paper discusses the statistical mechanics of the isotropic and the weakly anisotropic quadratic-layer antiferromagnet. This system is of current interest since it appears to be quite a good model for the less anisotropic members of two recently discovered isomorphic series of antiferromagnetic layer crystals. The antiferromagnetic phase, depending for its very existence on anisotropy, is discussed using spin-wave and Green's function methods, and comparisons are made with a typical three-dimensional (simple-cubic) system. The paramagnetic phase is investigated by Green's function and high temperature series expansion techniques, with particular attention paid to magnetic susceptibility and spin correlations. By expanding the Green's function results as a high temperature series, a test of the extrapolation techniques, which are frequently used in conjunction with this type of series and are particularly controversial for the two-dimensional Heisenberg system, can be made in the Green's function context.