Coercivity mechanisms in positive exchange-biased Co films and Co/Pt multilayers

Abstract

We describe magnetization measurements on a simple model system that mimics the behavior of positive exchange bias. The system consists of an antiferromagnetic CoO layer exchange coupled to antiferromagnetically coupled Co/Ru/Co or [Co/Pt]N/Co/Ru/[Co/Pt]M multilayer films with in-plane and out-of-plane anisotropy, respectively. In both cases, the two ferromagnetic layers were chosen to have different thickness with the thinner layer coupled to the CoO layer. When cooled below the blocking temperature of the CoO layer the thinner ferromagnetic layer is pinned and the low-field response is governed by the reversal of the thicker ferromagnetic layer with an effective exchange bias that is mediated via the Ru interlayer. For all the films studied we observe a transition in the effective bias from negative to positive with increasing cooling field. However, when varying the cooling field we find three distinct coercivity behaviors during the transition from positive to negative bias that depends on the anisotropy and microstructure of the ferromagnetic layers.