Exchange coupling characteristics of bottom-type synthetic ferrimagnet based spin valves

Giant magnetoresistance (MR) and effective exchange coupling field (H _ex) variations in bottom-type synthetic ferrimagnet based spin-valve (B-SV) films consisting of sputter-deposited Ta 5/seedlayer (NiFe, Cu, or Ru) 2/IrMn 9/CoFe (P1) 1.0-1.5/Ru 0.7/CoFe (P2) 3/Cu 2.8/CoFe 1.6/NiFe 3.2/Ta 5 (in nanometers) multilayers have been investigated. In particular, a focus was given on understanding the magnetotransport changes when the thickness difference between P2 and P1 layers, Δt(=t _P2-t _P1), was varied. These results were compared with those from similarly structured top spin-valve (T-SV) samples. When characterizing the subpeak profiles of MR transfer curves, we found that the subpeak shape of a B-SV with Δt=1.5 (=3.0-1.5)nm was different from that of a T-SV with the same Δt but was remarkably similar to that of a T-SV with Δt=2.0 (=3.0-1.0)nm. The degree of interfacial mixing (estimated to be in the order of 0.5 nm) during deposition at IrMn/CoFe (P1) appears higher for the B-SV: this resulted in an increase in effective Δt (or a decrease in effective P1 thickness). Hence, the effective thickness difference (Δt _eff) caused by intermixing has to be considered for designing B-SV structures. Annealing also induced changes in subpeak profiles, H _ex, and MR ratios resulting from interdiffusion. In overall, a B-SV consisting of a NiFe seedlayer exhibited higher thermal stability than others.