Critical thickness effects of NiFeCr-CoFe seed layers for spin valve multilayers

In this article, we present data on the critical dependence of the magnetic, electrical and microstructural properties of spin-valves (SV) on seed-layer thicknesses. The SV structure is: seed-layer/PtMn 140 Å/CoFe 16 /Ru 8.5 Å /CoFe 21 Å /Cu 20 Å/CoFe 12 Å/NiFe 30 Å/Ta 30 Å, where the seed layer is NiFeCr-CoFe or NiFeCr/NiFe. As the thickness of the bilayer seed layer is varied, it is found that a critical thickness boundary exists across which the film properties are radically different. The GMR ratio increased from 7% to 14% (a 100% change), the sheet resistance decreased by about 4 ohms/square and the crystalline texture transitioned from weak to extremely strong (111) texture. The critical thickness boundary is at a combined thickness of 37 Å to 40 Å. These results suggest a mechanism at the boundary between NiFeCr and CoFe during film growth. A better lattice match between NiFeCr-CoFe, for example, NiFeCr 33 Å and CoFe 7 Å, generates a strong (111) texture, which enhances the MR% as compared to NiFeCr 33 Å/CoFe 6 Å. The H₅₀ (the field at 50% MR) also exceeds 2000 Oe. This also indicates enhancement of the PtMn fcc to fct transition based on the specifically combined thicknesses of NiFeCr-CoFe. With the NiFeCr-NiFe seed layer, the critical thickness effect is not observed within these thickness ranges.