Scaling dependence and tailoring of the pinning field in FePt-based exchange coupled composite media

We studied exchange coupled composite (ECC) media with an out-of-plane easy axis consisting of hard magnetic L10 chemically ordered FePtCu alloy films and magnetically softer [Co/Pt]N multilayer stacks. By tailoring the structural properties of the ternary FePtCu alloy and [Co/Pt]N multilayers, we can tune the magnetic parameters of the composite in a wide range. This allowed us to address experimentally one of the most crucial properties determining the performance of ECC media, namely the pinning field of the magnetic domain wall present at the interface between the hard and soft layers. We demonstrate that the pinning field is proportional to the difference of the magnetic anisotropy constants of the hard and soft layers, which confirms the theoretical predictions. Furthermore, we show that the pinning field can be efficiently decreased after an additional annealing step. Transmission electron microscopy investigations indicated that the origin of the observed effect is due to a heat-induced phase transformation of iron oxide present at the interface between the hard and soft layers. This study reveals that tailoring the properties of the hard/soft interface is another efficient tuning knob for optimization of the performance of ECC media.