DEPENDENCE OF MAGNETIZATION REVERSAL ON THE CRYSTALLITE SIZE IN MNBI THIN-FILMS - EXPERIMENT, THEORY, AND COMPUTER-SIMULATION

A micromagnetic model is proposed to explain the magnetization reversa l of thermally evaporated MnBi thin films. The model assumes that the films consist of grains on a square lattice with perpendicular magneti c anisotropy. In order to describe the magnetic reversal of a particul ar grain, dipole coupling, wall energy, Zeeman energy, and an energy b arrier for the reversal of a single grain are taken into account. Diso rder is included by random fluctuations of the lateral grain size in t he Zeeman-energy term. The simulation is carried out by using a Monte Carlo method. The experimentally observed decrease in coercivity with increasing him thickness is accurately described by the model and can be explained by an increase in lateral grain size. The asymmetry of th e slope of the hysteresis curve of thicker MnBi films can be understoo d in terms of disorder. In the presence of fluctuations in grain size, large grains reverse more easily than small ones leading to an asymme try of the slope of the hysteresis curve.