Spin-Filter Specular Spin Valves with Higher MR Ratio and Thinner Free Layers

Abstract

Spin-valves with higher magnetoresistance (MR) ratio and thinner free layers are required for applications of nanometer-sized reply heads. In traditional spin valve structures, reducing the free layer thickness below 5 nm normally results in a reduction in MR ratio. In this work we have developed a spin-filter specular spin-valve with structure “Ta 3.5 nm/NiFe 2 nm/IrMn 6 nm/CoFe 1.5 nm/NOLl/CoFe 2 nm/Cu 2.2 nm/CoFe t F/Cu t SF/Nol2/Ta 3 nm, which is demonstrated to maintain MR ratio higher than 12% even when the CoFe free layer is reduced to 1 nm. A semi-classical Boltzmann transport equation was used to simulate MR ratio. Results will be presented in comparison with the experimental measurement. An optimized MR ratio of 15% was obtained when t F was about 1.5 nm and t SF about 1.0 nm as a result of the balance between the increase in electron mean free path difference and current shunting through conducting layer. MR ratio of up to 20% is obtainable for such a structure if Cu conductor spacer is reduced to 1.8 nm. It is found that the Cu enhancing layer not only enhances the MR ratio but also improves soft magnetic properties of CoFe free layer due to the low atomic intermixing observed between Co and Cu. The CoFe free layer of 1–4 nm exhibits a low coercivity of ~3 Oe even after annealing at 270 °C for 7 h in a field of 1 kOe. HRTEM cross sectional images showed that the NOL1 introduced from oxidation of the original bottom-pinned CoFe layer is actually a mixture of oxides and ferromagnetic metals. Un-oxidized CoFe grains epitaxially grown right across the nano-oxide layer are present. This led us to believe that the specular reflection of spin-polarized electrons is achieved by these oxidised regions and that the remained un-oxidised ferromagnetic CoFe in NOL1 shall decrease the specular scattering on the NOL1 surface, but it provides the direct exchange paths between the IrMn and pinned CoFe layer.