Influences of a Uniform External Magnetic Ffiled on the Magnetic Properties of the Dipolar Heisenberg System

The effects of a uniform external parallel magnetic field, with strength h, on the magnetic properties of the Heisenberg system on a square lattice of size 32 X 32 have been investigated using Monte Carlo (MC) method. The model consists of three dimensional classical spin vectors in which the spins interact through the dipolar interaction, the magnetic surface anisotropy, the antiferromagnetic exchange interaction and a uniform external magnetic field along x-axis of the square lattice. The relative exchange parameter J/g, where g is the strength parameter of the dipolar parameter and J is the strength parameter of the exchange interaction, is fixed at the value -10 (i.e., J/g= -10). From a series of MC simulations, the magnetic phase diagrams for this system have been determined as a function of both the relative magnetic surface anisotropy parameter κ/g, where κ is the strength parameter of the magnetic surface anisotropy, and temperature T/g for three selected values h/g (i.e., h/g = 10, 20 and 27). At low temperatures and for k/g -4 the phase diagrams show perpendicular antiferromagnetic phase; while for k/g -4 the equilibrium phase is planer antiferromagnetic phase. The MC results also show that the phase boundary separating the two ordered phases appears to be first order with very small latent heat. Moreover, the MC results indicate that the phase boundary separating the two ordered states from the paramagnetic phase is second order. While the sequence of phases observed for deferent values of h is similar, at very low temperature the results show that the line of the first order transitions between the two ordered phases shifts towards the negative values of κ/g, with decreasing in its slope as the external field is increased. In addition, the MC results show that, both the perpendicular and planer phases are shrink as the applied field is increased.