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

The transition temperatures of a group of copper compounds of the general formula Cu(CnH2n+1NH3)2Cl4 have been obtained from ac susceptibility data. These compounds contain layers of CuCl2, separated by two layers of (CnH2n+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/kTc we obtained agree rather well to J/kTc=1.7. This suggests that the transition temperature is indeed characteristic for the layer-structure.