Interface and microstructure evolutions in synthetic ferrimagnet-based spin valves upon exposure to postdeposition annealing

Abstract

Synthetic ferrimagnet-based spin-valve (SV) films comprising Ta 5/NiFe 2/IrMn 9/CoFe (P1) 1.5/Ru 0.7/CoFe (P2) 3/Cu 2.8/CoFe 1.6/NiFe 3.2/Ta 5 (in nanometers) were examined with an emphasis given on understanding the interface evolution of IrMn/CoFe (P1) upon postdeposition annealing. A 300 °C, 5 h annealing resulted in 12% and 42% decreases in effective pinning field and 29% and 53% decreases in MR ratio for the bottom (B) and top (T) SV, respectively. Because both structures exhibit different thermal degradation behaviors, we were curious to uncover the underlying causes by employing various analytical methods. The column sizes of the B–SV and T–SV were about 30 and 15 nm, respectively. No major difference in the (111) crystal texture was observed. We found that the main cause was due to the interlayer diffusion of atomic constituents such as Mn, most likely through grain boundaries and IrMn/CoFe interface, where the degree of the diffusion was considerably higher for the T–SV. Due to the surface free energy differences depending on the deposition sequence, two types of SV structures behaved differently upon exposure to annealing. The B–SV featuring less grain boundaries and nonabrupt IrMn/CoFe (P1) interface could effectively impede interlayer diffusion and, as a result, exhibited less thermal degradation in MR properties.