Influence of microstructure on thermal stability of spin-valve multilayers

We investigated the influence of microstructure on the thermal degradation of giant magnetoresistance Delta R 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 degrees C is the interlayer diffusion along the grain boundary, which had more influence on Delta R than the compositional mixing at the multilayer interface. The polycrystal spin-valve multilayers deposited on Ta and CoZrTa underlayers had rapid degradation of Delta R above the annealing temperature of 225 degrees C, resulting in 16% and 20% degradation at 275 degrees 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(lll) substrate brought about a marked reduction of the thermal degradation of Delta R to within 3%, even at 275 degrees C, due to a remarkable reduction of crystallographic defects like the grain boundary. (C) 1998 American Institute of Physics.