Phase transitions in the mixed Ashkin-Teller model

By using a mean-field approximation (MFA) and Monte-Carlo (MC) simulations, we have studied the effect on the phase diagrams of mixed spins (sigma = 1 /2 and S = 1) in the Ashkin-Teller model (ATM) on a hypercubic lattice. By varying the strength describing the four spin interaction and the single io n potential, we have obtained by these two methods quite rich phase diagram s with several multicritical points. This model exhibits a new partially or dered phase [S] which does not exist neither in the spin-1/2 ATM nor in the spin-1 ATM. While MFA yields phase diagrams which are sometimes qualitativ ely incorrect, accurate results are obtained from MC simulations. From the critical exponents which have been calculated using finite-size scaling ide as, we have shown that all phase transitions are Ising-like except for the paramagnetic-Baxter critical surface on which the critical exponents vary c ontinuously, by varying only the strength of the coupling interaction indep endently of the value of the single ion potential.