PHASE-TRANSITIONS IN THE SPIN-3/2 BEG MODEL

The spin-3/2 Ising model with a biquadratic exchange interaction (K) and a single ion anisotropy (D) is studied using two approaches. By means of a variational method we obtain the mean field phase diagram of the model. Besides a second order transition line occurring at high temperatures, the phase diagram exhibits two low temperature lines, which have not been reported previously. In the (T, D)-plane diagram a first order transition line separating the phases (3/2, 3/2) and (1/2, 1/2) occurs between the points (T = 0, D = -1.0) and (T almost-equal-to 0.76, D almost-equal-to -1.0). In the (T, K)-plane a line running in the non-zero temperature region from (T = 0, K = -1/3) to (T = 0, K = -1.0) separates the region of equal sublattice magnetizations from that of unequal sublattice magnetizations. On the second approach, Monte Carlo simulations have been performed confirming the main qualitative predictions of the mean field calculations.