Nearest-neighbor Ising antiferromagnet on the fcc lattice: Evidence for multicritical behavior

The phase behavior of the Ising model with nearest-neighbor antiferromagnetic interactions on the fcc lattice in a homogeneous magnetic field is studied by means of large-scale Monte Carlo simulations. In accordance with the most recent of the previous investigations, but with significantly 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 ${\mathit{A}}_3$B phase (corresponding to the model’s lattice gas interpretation as a binary alloy ${\mathit{A}}_{\mathit{xB}1\mathrm{-}\mathit{x}}$ such as ${\mathrm{Cu}}_{\mathit{x}}$ ${\mathrm{Au}}_{1\mathrm{-}\mathit{x}}$) occurs at a nonzero temperature. However, there is numerical evidence that the first-order jumps on the three associated phase coexistence lines tend to zero when approaching this point, which means that it is, in fact, a multicritical point. Since the Landau theory does not support this picture, and the simulation data do not definitely exclude a usual triple point with small jumps, the question about the nature of the point must be considered as unresolved. © 1996 The American Physical Society.