A comparison of calculated and measured critical temperatures in Heisenberg magnetic materials

Critical temperatures are calculated for the ferromagnets EuO and EuS and for the antiferromagnets, RbMnF3, MnF2, NiF2, KMnF3, NiO and MnO using high-temperature series techniques and exchange constants as derived from the measurements of spin-wave dispersion relations. After correcting for thermal expansion and other small effects the predicted transition temperature should be good to between 0.6% in the most favourable case and 6% in the least favourable one. There is satisfactory agreement with the observed critical temperature for EuO, EuS, NiF2 and NiO. Discrepancies of 5.6+or-1.1 and 16+or-2% in KMnF3 and MnO respectively may be related to lattice distortion effects accompanying the transitions; a discrepancy of -4.7% in MnF2 could be largely accounted for by dipolar interactions, but a discrepancy of 2.5+or-0.6% in RbMnF3 remains quite unexplained. There is no overall evidence for inconsistencies between current theories and measurements. Further measurements on EuO and RbMnF3 could provide more exacting tests.