Spin pumping in Co₅₆Fe₂₄B₂₀multilayer systems

Broadband in-plane ferromagnetic resonance measurements were performed over a frequency range from 7 to 40 GHz on various Co 56 Fe 24 B 20 systems with adjacent thin non-magnetic layers of Ru, Ta and Cu. Co 56 Fe 24 B 20 samples bounded by either Ru or Ta layers exhibit a contribution to the Gilbert damping constant inversely proportional to the thickness of the Co 56 Fe 24 B 20 layer, consistent with spin-pumping theory. In contrast, samples with 20 nm thick Cu bounding layers did not show a significant dependence of the Gilbert damping constant on the Co 56 Fe 24 B 20 thickness, which can be understood based on the far larger spin diffusion length of Cu in comparison with Ru or Ta.