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

Abstract

Giant magnetoresistance (MR) and effective exchange coupling field (Hex) 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(=tP2-tP1), 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 (Δteff) caused by intermixing has to be considered for designing B–SV structures. Annealing also induced changes in subpeak profiles, Hex, and MR ratios resulting from interdiffusion. In overall, a B–SV consisting of a NiFe seedlayer exhibited higher thermal stability than others.