Influence of spacer layer morphology on the exchange-bias properties of reactively sputtered Co∕Ag multilayers

We have studied the magnetic properties of [Ag(t_Ag)∕Co(1.2 nm)]₆₀ multilayers grown in an oxygen atmosphere. Partial oxidation of the Co layers results in the appearance of t_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_Ag is decreased below t_Ag^*=4 nm, whereas these two parameters adopt approximately constant values for t_Ag>t_Ag^*. At t_Ag^*, a transition from continuous to islandlike silver layers (on reducing t_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_Ag>t_Ag^*) plateletlike, and enhances the oxidized fraction of Co (f_CoO). The increase (decrease) in H_E (H_C) with reducing t_Ag below t_Ag^* is correlated to the increase in both the electrical resistivity and f_CoO. 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_Ag>t_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 O₂ in Ag sputter growth since the continuity thickness value (t_Ag^*=4 nm) is found to be lower than those previously reported in nonreactive sputtering of Ag.