Micromagnetic simulation of hysteresis characteristics for thin magnetic films

Current magnetic recording technology, employing a longitudinally oriented recording layer, is facing a superparamagnetic limit that will severely impede further rapid increase in density. This makes perpendicular recording more attractive owing to the smaller impact of thermal fluctuations on a grain's thermal stability. Quantitative prediction of the macroscopic magnetic properties of the perpendicular thin films from measurable microscopic inputs becomes essential, but is challenging.; In this work, micromagnetic models with use of realistic grain configurations are developed and applied to study the magnetic properties of Co/Pd (Co/Pt) superlattices. The total system energy consists of crystalline anisotropy, intergranular exchange coupling, magneto-static interaction, Zeemann energy and thermal fluctuations. The magnetostatic interaction is efficiently calculated for the irregular grains by combining analytical derivation and numerical integration. Based on the fluctuation dissipation theorem, a fluctuation formalism is applied to the magnetic grains to include thermal effects in the system. For the first time, a scaling technique is proposed, based on the Arrhenius-Neel law, to study thermally assisted magnetization reversal over long-time scales micro-magnetically. The micromagnetic models are successfully applied to hysteresis simulation of Co/Pd superlattices. The effects of grain irregularity and grain boundary on magnetic properties of the superlattice are studied. It is found that the nucleation field of the thin film greatly depends on the grain size uniformity. Domain size is increased and pronounced collective behavior is observed when the inter-granular exchange coupling becomes strong. A bit's thermal stability can be greatly improved by introducing a weak intergranular exchange energy.; An incoherent rotation model is also developed to study non-uniform switching in Co/Pt superlattices with large grain size. The macroscopic magnetic properties of the superlattice are studied by the model with use of only experimental data and electronic structure calculation as input. Calculation shows that larger thermal fluctuations experienced by the spins along grain boundaries act as nucleation sites in the cobalt layers, which consequently cause domain propagation and the whole grain switching. This result quantitatively explains the experimental phenomenon that the VSM coercivities are significantly smaller than the anisotropy fields.