Role of the Antiferromagnetic Bulk Spin Structure on Exchange Bias

TLDR

Exchange bias in FM/AF multilayers depends on the cooling field, indicating that the bulk antiferromagnet spin structure is crucial. The authors employed FM/AF/FM trilayers without interlayer coupling and varied the cooling field to orient the AF below its Néel temperature while saturating the FM layers either parallel or antiparallel. The differing exchange bias fields for the two cooling configurations demonstrate that bulk AF moments, not just interfacial effects, pin uncompensated spins, while coercivity enhancement originates from a separate mechanism.

Abstract

The cooling field dependence of the exchange bias field in ferromagnet/antiferromagnet (FM/AF) multilayers demonstrates that the bulk AF spin structure plays a crucial role on the origin of exchange bias. FM/AF/FM trilayers were designed to eliminate any interlayer exchange coupling between the FM slabs. By choosing the magnetic cooling field, the AF is ordered below its Néel temperature with the FM layers fully saturated either parallel or antiparallel to each other. The significant difference in the exchange bias field between these two cooling configurations confirms that exchange bias cannot be a purely interfacial effect and that the bulk AF moments play a significant role in pinning the uncompensated spins at the AF/FM interface. This experiment also demonstrates that the mechanism responsible for coercivity enhancement has a different origin and is independent of the process that gives rise to exchange bias.