Influence of microstructure on thermal stability of spin-valve multilayers

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Abstract
We investigated the influence of microstructure on the thermal degradation of giant magnetoresistance ΔR on spin-valve multilayers constructed by IrMn/CoFe/Cu/CoFe/NiFe, using transmission electron microscopy and energy dispersive x-ray spectroscopy. We found that the main cause of thermal degradation at about 250 °C is the interlayer diffusion along the grain boundary, which had more influence on ΔR than the compositional mixing at the multilayer interface. The polycrystal spin-valve multilayers deposited on Ta and CoZrTa underlayers had rapid degradation of ΔR above the annealing temperature of 225 °C, resulting in 16% and 20% degradation at 275 °C, respectively. A Ta underlayer with amorphous structure has the effect of reducing the density of the grain boundary by improving the surface smoothness, as compared with a crystalline CoZrTa underlayer, leading to slightly less degradation. By contrast, a spin-valve epitaxial layer deposited on a MgO(111) substrate brought about a marked reduction of the thermal degradation of ΔR to within 3%, even at 275 °C, due to a remarkable reduction of crystallographic defects like the grain boundary.