The exchange bias of polycrystalline Co films grown on epitaxial, 67-nm-thick single-crystalline films of (110) ${\mathrm{Fe}}_x{\mathrm{Zn}}_{1-x}{\mathrm{F}}_2$ films was measured as a function of Fe concentration. A set of samples was grown with a pure, 1.0-nm-thick ${\mathrm{FeF}}_2$ layer at the interface, and another set was grown without the interface layer. Unlike previous measurements of twinned ${\mathrm{Fe}}_x{\mathrm{Zn}}_{1-x}{\mathrm{F}}_2$ films, the exchange bias of samples with the pure interface layer remained relatively constant as the Fe concentration was decreased from $x=1.0$ to 0.35. A decrease in $H_E$ with increasing $x$ in samples without the pure interface layer was also observed, which can be explained by a weakening of the ${\mathrm{Fe}}_x{\mathrm{Zn}}_{1-x}{\mathrm{F}}_2∕\mathrm{Co}$ exchange interaction as the Fe concentration is decreased. Evidence for the creation of frozen domains in the antiferromagnet was obtained from vertical shifts in the hysteresis loops at low temperatures for samples with $x⩽0.75$, which agrees with the critical concentration $x_C=0.76$ above which antiferromagnetic domains are not expected to form due to lack of percolation of nonmagnetic impurities. The large enhancement of exchange bias expected from the domain state model is not observed in these samples because of the existence of the critical concentration in ${\mathrm{Fe}}_x{\mathrm{Zn}}_{1-x}{\mathrm{F}}_2$.

• Received 13 April 2005

DOI:https://doi.org/10.1103/PhysRevB.72.224417