Combined magnetic X-ray and polarized neutron reflectivity study of the origins of exchange bias in the Co / FeF2 system

We discuss studies of the magnetic specular reflectivity of neutrons and X-rays from the exchange bias system consisting of a single crystal film of antiferromagnetic FeF2 capped with a ferromagnetic Co film cooled in an applied magnetic field below the TNTN of the FeF2. This system exhibits a shift of the magnetic hysteresis loop along the direction of the cooling field HcHc (positive exchange bias) or in the opposite direction (negative exchange bias) depending on the magnitude of the cooling field. The use of neutrons with polarization analysis enables the spatial distribution of different vector components of the magnetization to be determined, and the use of resonant magnetic X-ray scattering enables magnetization in a compound system to be determined element-selectively. Our results show that the coupling across the interface between the relatively few uncompensated Fe spins and the more numerous Co spins is antiferromagnetic. In a large cooling field which overrides this coupling, the Fe spins are oriented along HcHc and some get pinned in this direction, in turn pinning the Co spins above the interface oppositely and thus creating positive exchange bias. For sall values of HcHc the Fe spins get locked in the opposite direction producing negative exchange bias. In addition a significant fraction of the Fe spins at the interface are unpinned and always align opposite to the Co magnetization.