We have studied the magnetic properties of ${[\mathrm{Ag}\left(t_{\mathrm{Ag}}\right)∕\mathrm{Co}\left(1.2\mathrm{nm}\right)]}_{60}$ multilayers grown in an oxygen atmosphere. Partial oxidation of the Co layers results in the appearance of $t_{\mathrm{Ag}}$ dependent exchange-bias properties. A strong increase in the exchange-bias field $H_E$ together with a significant reduction in the coercivity $H_C$ are observed when $t_{\mathrm{Ag}}$ is decreased below $t_{\mathrm{Ag}}^{*}=4\mathrm{nm}$, whereas these two parameters adopt approximately constant values for $t_{\mathrm{Ag}}>t_{\mathrm{Ag}}^{*}$. At $t_{\mathrm{Ag}}^{*}$, a transition from continuous to islandlike silver layers (on reducing $t_{\mathrm{Ag}}$) is signaled by electrical resistivity and x-ray reflectivity results. From magnetic hysteresis loops recorded at room temperature and magnetization curves, it is concluded that this transition induces a granular morphology in the Co regions, which are previously $(t_{\mathrm{Ag}}>t_{\mathrm{Ag}}^{*})$ plateletlike, and enhances the oxidized fraction of Co $\left(f_{\mathrm{Co}\mathrm{O}}\right)$. The increase (decrease) in $H_E$ $\left(H_C\right)$ with reducing $t_{\mathrm{Ag}}$ below $t_{\mathrm{Ag}}^{*}$ is correlated to the increase in both the electrical resistivity and $f_{\mathrm{Co}\mathrm{O}}$. From the latter correlation, we infer that the higher degree of oxidation in the granular Co layers is associated with effectively thicker antiferromagnetic (CoO) regions than in the $t_{\mathrm{Ag}}>t_{\mathrm{Ag}}^{*}$ (continuous multilayer) case—with correspondingly higher magnetic anisotropy energies—which may account for both the enhancement in $H_E$ and the reduction in $H_C$. In addition, our study provides information on the surfactant effect of ${\mathrm{O}}_2$ in Ag sputter growth since the continuity thickness value $(t_{\mathrm{Ag}}^{*}=4\mathrm{nm})$ is found to be lower than those previously reported in nonreactive sputtering of Ag.

• Received 5 December 2006

DOI:https://doi.org/10.1103/PhysRevB.76.104430