Exchange Bias of Single-Crystalline Fe$_x$Zn$_{1-x}$F$_2$/Co Bilayers

The exchange bias of polycrystalline Co films grown on epitaxial, 67 nm thick single-crystalline films of (110) Fe$_x$Zn$_{1-x}$F$_2$ was measured as a function of Fe concentration. A set of samples was grown with a pure, 1.0~nm thick FeF$_2$ layer at the interface, and another set was grown without the interface layer. Unlike previous measurements of \textit{twinned} Fe$_x$Zn$_{1-x}$F$_2$ films, the exchange bias of samples with the pure interface layer remains constant as the Fe concentration is decreased from $x=1.0$ to $x=0.35$. A decrease in samples without the pure interface layer was also observed, which can be explained by a weakening of the Co/Fe$_x$Zn$_{1-x}$F$_2$ exchange interaction as the Fe concentration is decreased. These results imply that, at least in highly anisotropic systems like FeF$_2$, the proposed domain state model does not explain the experimental data.\\ \\ Work supported by NSF grant 0400578.