Hysteresis loops of magnetic thin films with perpendicular anisotropy

We model the magnetization of quasi-two-dimensional systems with easy perpendicular (z) axis anisotropy upon change of the external magnetic field along z. The model is a generalization of the scalar phi-fourth model that considers only the z component of the magnetization, and includes magnetic exchange, dipolar interactions, structural disorder, and an external z-oriented magnetic field. The phase diagram in the disorder/interaction strength plane is presented, and the different qualitative regimes are analyzed, mainly focusing on the existence or not of an abrupt nucleation step in the process of magnetization reversal. The results compare very well with observed experimental hysteresis loops and spatial magnetization patterns, as for instance in the case of Co-Pt multilayers.