Field cooling dependence of the anisotropy in exchange biased FeF$_{2}$/Co films*

Using polarized neutron reflectometry we have studied the magnetization reversal in exchange biased single-crystal FeF$_{2} $/Co films grown on MgF$_{2}$. A recent study showed that the anisotropy of the antiferromagnetic FeF$_{2}$ plays an important role in determining the magnitude and effective direction of the exchange bias field H$_{E}$[1]. After field cooling perpendicular to the c-axis (easy axis of FeF$_{2}$), the magnetization curve determined with the applied field parallel to the c-axis, shows a double loop, one with positive bias, and one with negative bias. This behavior suggests that the antiferromagnet is simply split into two types of domains, inducing opposite H$_{E}$ along the c-axis. However, our observation of spin-flip reflectivity in the field regions separating the two loops indicates that some rotation of the Co magnetization also occurs during reversal, implying that H$_{E}$ does not lie exclusively along the c-axis. \\[4pt] [1] Hongtao Shi, David Lederman, Phys. Rev. B 66, 094426 (2002).