Thermal relaxation of the free layer anisotropy in spin valves

Abstract

While the thermal stability of the pinned layer magnetization in spin valves (SVs) has been studied extensively, less attention has been paid to the stability of the free layer anisotropy. Control of this anisotropy is important since it contributes significantly to the sensitivity and magnetic stability of the device. The kinetics of anisotropy relaxation in SVs are studied through isothermal hard axis anneals at temperatures ranging from 120 °C to 200 °C. A general quantitative method is applied to extract the anisotropy in a complete SV in the presence of interlayer coupling and pinned layer rotation. For a free layer consisting of a Ni81Fe19/Co90Fe10 bilayer, it is found that the anisotropy relaxation is proportional to T ln(t/t1), i.e., linear in temperature T and logarithmic in time t. The characteristic onset time t1 is described by t1_0 exp(Ea/kT). These results are compared with previously reported data for single NiFe films.