NEAREST-NEIGHBOR ISING ANTIFERROMAGNET ON THE FCC LATTICE - EVIDENCE FOR MULTICRITICAL BEHAVIOR

The phase behavior of the Ising model with nearest-neighbor antiferrom agnetic interactions on the fee lattice in a homogeneous magnetic fiel d is studied by means of large-scale Monte Carlo simulations. In accor dance with the most recent of the previous investigations, but with si gnificantly higher accuracy, it is found that the ''triple'' point at which the disordered phase coexists with both the AB phase as well as with the A(3)B phase (corresponding to the model's lattice gas interpr etation as a binary alloy A(x)B(1-x) such as CuxAu1-x) occurs at a non zero temperature. However, there is numerical evidence that the first- order jumps on the three associated phase coexistence lines tend to ze ro when approaching this point, which means that it is, in fact, a mul ticritical point. Since the Landau theory does not support this pictur e, and the simulation data do not definitely exclude a usual triple po int with small jumps, the question about the nature of the point must be considered as unresolved.