Extrinsic Spin–Orbit Coupling-Induced Large Modulation of Gilbert Damping Coefficient in CoFeB Thin Film on the Graphene Stack with Different Defect Density

Abstract

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 (∼200%) in the value of the Gilbert damping coefficient α is observed from FLG/CoFeB (α ≈ 0.035 ± 0.005) to graphite/CoFeB (α ≈ 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 (τ1) 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.