Phase behavior of Ising mixtures

We present phase diagrams that were calculated both in mean-field theory and via Monte Carlo (MC) simulations for binary mixtures of a ferromagnetic Ising fluid and a nonmagnetic fluid (Ising mixtures) in the absence of an external field. We look at both the simple ideal Ising mixture, consisting of an ideal Ising fluid and a hard-sphere fluid, as well as at the general case with one component being a nonideal Ising fluid and the other a van der Waals fluid. It is shown that the mean-field phase diagram of the ideal Ising mixture in the limit of infinite pressure is identical to that of the Blume-Capel model for He-3-He-4 mixtures. The MC phase diagrams were obtained using the Gibbs ensemble, the cumulant intersection technique, and the multihistogram reweighting method, adapted to the semigrand ensemble. The results are qualitatively compared with mean-field theory, and both types of tricritical lines occurring there are verified in the computer simulations.