Gap Layer Effect on Performances of Differential Dual Spin Valve

Several materials (Ta, Cu, Al, Ru) are investigated for the gap layer (GL) in a differential dual spin valve (DDSV) as a function of GL thickness in terms of the differential effect and interlayer coupling. High field measurements in current in the plane geometry show that similar GMR effect can be obtained for two spin valves (SVs) with Ta, Ru, and Cu GLs, while for Al GL, GMR ratio of the upper SV decreases as Al thickness increases. For Ta GL, the two free layers (FLs) may switch either simultaneously or separately, depending on which FL switches first due to the weak ferromagnetic (FM) Neel coupling between the FLs. For Ru GL, the switching behavior depends on the thickness of Ru, due to the oscillation of interlayer exchange coupling. Cu GL causes additional GMR resulted from the FL/GL/FL structure. Detailed investigation on thickness dependence of Ru GL shows that FM interlayer coupling is achieved with Ru thickness of 1.6, 2.4, and 4 nm. The good differential effect can be obtained till Ru = 8 nm, above which MR of the upper SV decreases. Patterned DDSV sensors with Ru GL = 2.4 nm shows an overall FM coupling between the FLs, implying FM exchange coupling is larger than the antiferromagnetic magnetostatic coupling at edges.