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 O-2 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.