Transition Temperature of the Two-Dimensional Heisenberg Ferromagnet with S=½

Abstract

The transition temperatures of a group of copper compounds of the general formula Cu(C_nH_2n+1NH₃)₂Cl₄ have been obtained from ac susceptibility data. These compounds contain layers of CuCl₂, separated by two layers of (C_nH_2n+1NH₃)Cl. Also two compounds with the Cl⁻‐ion replaced by Br⁻ have been investigated. In the Cl‐series, n was varied from 1–6, whereby the copper‐copper distance within a layer remains practically constant, the distance between copper ions from adjacent layers increasing from 10 to about 20 Å. Values for the exchange parameter J have been derived from paramagnetic Curie temperatures and heat‐capacity data at low temperatures. Whereas both the interaction between neighboring CuCl₂‐layers and the anisotropy clearly vary through the series of compounds, the values of J/kT_c we obtained agree rather well to J/kT_c=1.7. This suggests that the transition temperature is indeed characteristic for the layer‐structure.