We obtain the phase diagram of a ferromagnetic mixed Ising system, consisting of spin-1/2 and spin-S variables, on a Bethe lattice of coordination number z, with nearest-neighbor exchange interactions and single-ion terms. The problem is formulated as a discrete nonlinear map. There is a tricritical point for S integer and z≥5. In the infinite-coordination-number limit, we regain the results of an exact calculation for a Curie-Weiss version of the model.

• Received 15 January 1991

DOI:https://doi.org/10.1103/PhysRevB.44.852