Influence of the antiferromagnet spin structure on exchange bias.

The exchange bias phenomenon (EB) is known by a shift of the hysteresis loop along the magnetic field axis ($H_{EB})$. In ferromagnet/antiferromagnet (FM/AF) thin films EB originates from the exchange interaction between FM and AF spins at the interface. Therefore, some theoretical models consider the EB as a purely interfacial phenomenon, but some experimental results are incompatible with these models. In this work we investigate the influence of an internal AF spin structure on EB. FM1/AF/FM2 trilayers have been prepared with disimilar FM1 and FM2 coercivities, so the sample can be cooled down below the AF N\'{e}el temperature with parallel or antiparallel FM magnetizations. It has been observed that $H_{EB}$ changes drastically from the parallel to the antiparallel cooling configuration for AF thicknesses $t_{AF} \quad <$200nm while it hardly varies for $t_{AF} \quad >$300nm. This result demonstrates that the internal spin structure of the AF is a key point to the understanding of EB. Work supported by US-DOE and European Marie-Curie-OIF.