Time-Resolved Magnetization Dynamics and Damping Constant of Sputtered Co/Ni Multilayers

Co/Ni multilayers with a stack of Ta (2 nm)/[Co (t_Co)/Ni (t_Ni)] N/Ta (30 nm) were prepared by dc magnetron sputtering, and their magnetization dynamics were measured by time-resolved magneto-optical Kerr effect (TRMOKE). The total thickness of the multilayer and perpendicular anisotropy were varied by changing the bilayer period d = t_Co+ t_Ni and number of repeats N while t_Ni/t_Co was kept at a constant of 2.5. The TRMOKE measurements show clear damped oscillation of the magnetization of Co/Ni multilayers after the pump pulse illumination, and the damping constant α of the Co/Ni multilayers was estimated from the TRMOKE waveform. The estimated α was found to be independent both on total thickness and anisotropy field of the multilayer and was estimated to be ~ 0.035 for all the multilayers. This means that the use of Ta capping and buffer layers is effective to evaluate intrinsic damping constant of the Co/Ni multilayer, and that independent control of α and perpendicular anisotropy are possible for the magnetic multilayers.