Time and temperature dependence of high thermal stability in NiO/Co/Cu/Co/M spin valves

The thermal stability of Cu/Co 'bottom' spin valve structures deposited on NiO has been assessed as a function of temperature, time, and cap layer compositions including Au, Ta and Ta/sub x/O/sub y/. Samples with 5.0 nm thick cap layers of Au showed a reduction in giant magnetoresistance (GMR) from 9.3% to 3.6% after a 0.5 hr anneals at 250/spl deg/C. As a function of time, the sample resistance, R(t), showed an exponential saturation with a time constant, /spl tau/, which follows an Arrhenius law, /spl tau/=/spl tau//sub 0/ exp[/spl Delta/E/kT], with /spl Delta/E=1.29/spl plusmn/0.03 eV and /spl tau//sub 0/ on the order of 10/sup -8/ s to 10/sup -9/ s. These parameters, which fit R(t), correlate with the time and anneal temperature dependence of GMR decay in the Au-capped spin valves. In contrast, samples with Ta or Ta/sub x/O/sub y/ cap layers showed good stability for 0.5 hr anneals up to 325/spl deg/C. Decreases in GMR from 12.7% to 3.9% following a 20 h anneal at 325/spl deg/C are accompanied by significant changes in the hysteresis loops which appear to be associated with the pinned layer. With a 5.0 nm Ta cap, R(t) follows a logarithmic-like time dependence characteristic of a distribution of activation energies.