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 $\alpha$ values obtained for both stacked films was also discussed. The $\alpha$ values increased linearly with increasing inverse CoFeB thickness ($t_{\mathrm{CoFeB}}$). The slope of the $\alpha$ vs $1/t_{\mathrm{CoFeB}}$ characteristic for Ta/CoFeB/MgO films was smaller than that for Ta/CoFeB/Ta films, implying that the enhancement of $\alpha$ was caused by the CoFeB/Ta interface. Comparison of the annealing temperature dependence of $\alpha$ and the perpendicular magnetic anisotropy constant $K_{\mathrm{u}}$ revealed no correlation between $\alpha$ and $K_{\mathrm{u}}$.

• Received 24 March 2014

DOI:https://doi.org/10.1103/PhysRevB.89.174416