Gilbert damping in asymmetric MgO(Ta)/CoFeB/Ta(MgO) structures with perpendicular magnetic anisotropy

We investigate the ultrafast magnetization dynamics of perpendicularly magnetized MgO/CoFeB/Ta and Ta/CoFeB/MgO stack structures using an all-optical time-resolved magneto-optical Kerr measurement technique. We find that the effective Gilbert damping parameter αeff converges to a low value of ∼0.02 in the high external field regime, and it stays unchanged at ∼0.02 regardless of the pump fluence. Two asymmetric stack structures of CoFeB show identical dynamic behavior and Gilbert damping, implying that the influence of the under and top interfaces is not distinct.