Thickness dependent magnetic reversal process and hysteresis loops in exchange-coupled hard-soft trilayers

Abstract

The demagnetization process of a hard/soft/hard sandwich has been investigated systematically within a self-contained micromagnetic model, with particular attention on the cases with small hard layer thickness. The microscopic and macroscopic hysteresis loops, as well as the angular distributions of the magnetization between nucleation and pinning have been obtained numerically, with the formula for the nucleation field derived. It is found that both nucleation and pinning fields, as well as the gap in between decrease as the hard layer thickness reduces. The hard layer thickness has great effect on the hysteresis loops only when the thickness is very small, where the hysteresis loop is nearly square and the dominant coercivity mechanism is the nucleation. The thickness regions at which the theoretical and practical giant energy products can be achieved have been discussed. In most cases, the hard layer can be taken as sufficiently thick so that the magnetization at its surface obeys a simple coherent rotation model. In these cases, the calculation can be simplified significantly, with only the influence of the soft layer thickness accounted.