▎ 摘　　要

Control of Gilbert damping parameter is imperative for various spintronic and magnonic devices, and various schemes have been attempted to achieve that. We report a large tunability of Gilbert damping by varying the underlayer of CoFeB thin film from few-layer graphene (FLG) to graphite layer. We measured the ultrafast magnetization dynamics of CoFeB, FLG/CoFeB, and graphite/CoFeB by using time-resolved magneto-optical Kerr effect (TR-MOKE) magnetometry. While the magnetization precession frequency remained independent of the underlayer, a very large variation (similar to 200%) in the value of the Gilbert damping coefficient alpha is observed from FLG/CoFeB (alpha approximate to 0.035 +/- 0.005) to graphite/CoFeB (alpha approximate to 0.008 +/- 0.001). This large variation of the damping coefficient is understood in terms of the extrinsic spin-orbit interaction of FLG and graphite films, which is very large in FLG due to the presence of large amount of surface defects in it. A faster demagnetization time and fast relaxation time (tau(1)) were noted for graphite/CoFeB bilayer system than that of FLG/CoFeB. In general, we infer that interfacial spin physics is primarily governed by the growth of CoFeB layer from our bilayer systems. This finding suggests a new direction toward the control of precessional magnetization dynamics, leading to applications in miniaturized high-speed magnetic devices.