Transmission electron microscopy analysis of crystallographic transition from fcc to fct on PtMn spin valves

The structural changes during the crystallographic transition from fcc to fct of antiferromagnetic PtMn layers with postdeposition annealing, in spin-valve multilayers constructed by Ta/PtMn/CoFe/Cu/CoFe/NiFe/Ta/Si, were investigated using transmission electron microscopy and energy dispersive x-ray spectroscopy. We determined that the crystallographic orientation of the PtMn layers after postdeposition annealing depends on the initial size of the columnar grains in as-deposited spin valves. For spin valves with an initial grain size of 10 nm, the fcc-->fct transition of PtMn layers was completed with the crystallographic orientation on the (111) texture. In contrast, spin valves with an initial grain size of 50 nm brought about a crystallographic reconstruction of the PtMn layer due to the tensile stress from neighboring grains during the fcc-->fct transition. This results in a transformation from a fcc(111) to a fct(101) texture and a recombination of grains to form giant grains of 300-500 nm in the PtMn layer. This dynamic reconstruction promotes the interlayer diffusion of constituent atoms in spin-valve multilayers, leading to an increase in the interlayer coupling field between the pinned and free layers (H-in) and a promotion of thermal degradation of magnetoresistance (Delta R). (C) 2000 American Institute of Physics. [S0021-8979(00)06320-9].