Magnetic structures of Ising and vector spins monolayers by Monte-Carlo simulations

We derive the optimal magnetic structures for monolayers of either square or triangular lattice symmetry with evidence for morphological differences. The interplay between short-range exchange and long-range dipolar forces leads to quite different results for Ising spins and vector spins. For the Ising model, spin domains with parallel stripes, chevron patterns and labyrinths at different scales and with thermal disorder are deduced. For the vector model with a weak perpendicular anisotropy, the spins are planar and form a lattice of vortices of both signs. Such a structure remains stable even under a large perpendicular magnetic field, whereas a weak in-plane magnetic field is sufficient to obtain a uniform magnetic domain. For a sufficiently large perpendicular anisotropy, a mixed structure appears that includes spin vortex areas surrounding spin-up and spin-down areas. (C) 1998 Elsevier Science B.V. All rights reserved.