Magnetic history dependence of metastable states in systems with dipolar interactions

Abstract

We present the results of a Monte Carlo simulation of the ground state and magnetic relaxation of a model of a thin film consisting on a two-dimensional square lattice of Heisenberg spins with perpendicular anisotropy K, exchange J and long-range dipolar interactions g. We have studied the ground state configurations of this system for a wide range of the interaction parameters J/g, K/g by means of the simulated annealing procedure, showing that the model is able to reproduce the different magnetic configurations found in real samples. We have found the existence of a certain range of K/g, J/g values for which in-plane and out-of-plane configurations are quasi-degenerated in energy. We show that when a system in this region of parameters is perturbed by an external force that is subsequently removed different kinds of ordering may be induced depending on the followed procedure. In particular, simulations of relaxations from saturation under an a.c. demagnetizing field or in zero field are in qualitative agreement with recent experiments on epitaxial and granular alloy thin films, which show a wide variety of magnetic patterns depending on their magnetic history.Comment: Invited paper to 3rd EuroConference on Magnetic Properties of Fine Nanoparticles, Barcelona, October 99. To be published in JMMM (References included 22-Dec-1999