Cluster variational theory of spin (3)/(2) Ising models

A cluster variational method for spin 3/2 Ising models on regular lattices is presented that leads to results that are exact for Bethe lattices of the same coordination number. The method is applied to both the Blume-Capel (B C) and the isotropic Blume-Emery-Griffiths model (BEG). In particular, the first-order phase line separating the two low-temperature ferromagnetic pha ses in the BC model, and the ferrimagnetic phase boundary in the BEG model are studied. Results are compared with those of other theories whose qualit ative predictions have been in conflict. (C) 2000 Elsevier Science B.V. All rights reserved.