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 reversal of thermally evaporated MnBi thin films. The model assumes that the films consist of grains on a square lattice with perpendicular magnetic anisotropy. In order to describe the magnetic reversal of a particular grain, dipole coupling, wall energy, Zeeman energy, and an energy barrier for the reversal of a single grain are taken into account. Disorder is included by random fluctuations of the lateral grain size in the Zeeman-energy term. The simulation is carried out by using a Monte Carlo method. The experimentally observed decrease in coercivity with increasing film thickness is accurately described by the model and can be explained by an increase in lateral grain size. The asymmetry of the slope of the hysteresis curve of thicker MnBi films can be understood in terms of disorder. In the presence of fluctuations in grain size, large grains reverse more easily than small ones leading to an asymmetry of the slope of the hysteresis curve.