Interfacial effects on magnetic interlayer coupling between perpendicular Co/Ni multilayers across Ru spacer

The interlayer exchange coupling properties in synthetic antiferromagnetic (AF) structures based on Co/Ni perpendicular multilayers were investigated by varying the Ru spacer thickness (t_Ru) and the ferromagnetic (FM) layer material at the FM/Ru interfaces. Samples of two Co layers adjacent to Ru own much stronger AF coupling field (H_AF) and show two H_AF peaks located at t_Ru=0.47 and 1.08nm. If the interfacial Co layers are substituted by Ni, the H_AF at the 1st peak decreases much more considerably, becomes lower than the 2nd one and even disappears due to strong pinhole-induced FM coupling. Such pinhole effect makes H_AF of t_Ru<0.85nm remains unchanged or even decrease with decreasing temperatures. By tuning the interfacial Co layer thickness, pinhole effect has been suppressed as a result of greatly enhanced H_AF, leading to a normal temperature dependence of Ruderman-Kittel-Kasuya-Yosida-type H_AF even at thin t_Ru cases, i.e., increase in H_AF with temperature decreasing.