Gilbert damping constants of Ta/CoFeB/MgO(Ta) thin films measured by optical detection of precessional magnetization dynamics

The magnetization dynamics of both Ta/CoFeB/MgO and Ta/CoFeB/Ta films were investigated using an all-optical pump-probe method. The magnetic field strength and the applied field direction dependencies of the precession frequency and the relaxation time were explained well by the Landau-Lifshitz-Gilbert equation when taking the magnetic anisotropy distribution in the film into account. The thickness dependence of the α values obtained for both stacked films was also discussed. The α values increased linearly with increasing inverse CoFeB thickness (tCoFeB). The slope of the α vs 1/tCoFeB characteristic for Ta/CoFeB/MgO films was smaller than that for Ta/CoFeB/Ta films, implying that the enhancement of α was caused by the CoFeB/Ta interface. Comparison of the annealing temperature dependence of α and the perpendicular magnetic anisotropy constant Ku revealed no correlation between α and Ku.