Monte Carlo simulation has been used to determine the phase diagram of a metamagnet Ising model in the presence of a random and uniform magnetic field. The model consists of a spin-1/2 metamagnet in which the nearest neighbor and next nearest neighbor spin interactions are antiferromagnetic (J 1 < 0) and ferromagnetic (J 2 > 0), respectively. We used a bimodal probability distribution for the random magnetic field. We have calculated the staggered magnetization and the fourth-order Binder cumulants in order to obtain the critical points. The phase diagram in the uniform field versus temperature plane presents continuous and first-order transition lines. The phase transition lines, together with the critical and tricritical points, have been obtained for several random field values.
A set of single domain particles has been studied through Monte Carlo simulations on three different lattices. A simple cubic lattice, a face centered cubic lattice and a liquid-like structure. The particles are coupled by long-range dipolar forces and present a single ion uniaxial anisotropy, whose magnitude is chosen from a Gaussian distribution, and whose easy magnetization axes are oriented randomly in the three-dimensional space. We determined the blocking temperature and the hysteresis curves as a function of the ratio between the magnitude of dipolar coupling and uniaxial anisotropy. We show that the remanence and coercive field depend strongly on the nature of the magnetic arrangement. These results are compared with those found for a system of noninteracting particles.
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