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_xO_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°C. As a function of time, the sample resistance, R(t), showed an exponential saturation with a time constant, τ, which follows an Arrhenius law, τ=τ₀ exp[ΔE/kT], with ΔE=1.29±0.03 eV and τ₀ on the order of 10 ^-8 s to 10^-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_x O_y cap layers showed good stability for 0.5 hr anneals up to 325°C. Decreases in GMR from 12.7% to 3.9% following a 20 h anneal at 325°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