Enhanced spin-pumping efficiency in hcp Co polycrystalline films obtained by sputtering deposition with high substrate temperature

The variation of the resonance field and the Gilbert damping parameter (G) with hcp Co film thickness was evaluated by measuring the X- and Qband ferromagnetic resonance (FMR) for buffer-layer/Co (2-16 nm)/Cu (1 nm)/Pt (15 nm) films. The two types of hcp Co films were deposited at a substrate temperature (T_sub) of either 400 °C or room temperature using a magnetron sputtering system. The G values for the Co films with T_sub = 400 °C exhibited a more rapid increase with decreasing thickness. In addition, the effective linewidth of FMR spectrum exhibited more rapid increase in the Q-band FMR for T_sub = 400 °C. The experimental results suggest that the spin-pumping is enhanced to a greater extent in the Co films fabricated at T_sub = 400 °C. Judging from the structural analysis of the Co films, it is possible that grain growth involving atomically flat surfaces of Co films might enhance the spin-pumping efficiency.