On the magnetic phase transition of some layered copper compounds: II. The effect of anisotropic and dipolar interaction terms

Abstract

In part I of this paper we presented an analysis of the dependence of the reduced critical temperature ( t c) on the ratio R≡ H′ E/ H E of the interlayer ( H′ E and interlayer ( H E) exchange coupling, as observed for a number of layer-type copper compounds. In the previous analysis — aiming at a determination of the critical χ parameters of the ideal ( R = 0), two-dimensional (quadratic), S = 1 2 , Heisenberg ferromagnet — the consequences of the occurence of residual anistropic and dipolar terms in the spin-spin interactions were not considered in detail. The present paper is devoted to a discussion of the additional anistropy- and dipolar-induced t c shifts. We present a compilation of the relevant experimental data of the thusfar investigated ferromagnetic, nearly two-dimensional copper compounds. Most of these compounds show a dominant XY type of anisotropy, with a relative magnitude of about 3 × 10 −3. The anisotropy-induced t c shift will be treated in a semi-quantitative way through the introduction of the “ t c-shift cross-over effect”. The dipolar interactions are acting mainly via an enhancement of the XY-like character of the anisotropy and will be dealt with only qualitatively. The corrections obtained for the anisotropy + dipolar-induced t c shifts are compared with recent calculations of Yamaji and Kondo, who have taken account of these effects in a first-order Green's-function model.