Onsager reaction-field theory for magnetic models on diamond and hcp lattices

The Onsager reaction-field (ORF) theory is extended so that it applies to any three-dimensional Bravais lattice with a basis. The ORF calculation is used to predict the critical temperature for classical Ising, XY, and Heisenberg magnetic models, in particular, on diamond and hexagonal close packed lattices. Results are compared with series extrapolations and other theoretical approaches where available. For the hcp lattice the ORF calculation is seen to be exactly equivalent to a Green’s function approach.