Broadband in-plane ferromagnetic resonance measurements were performed over a frequency range from 7 to 40 GHz on various Co56Fe24B20 systems with adjacent thin non-magnetic layers of Ru, Ta and Cu. Co56Fe24B20 samples bounded by either Ru or Ta layers exhibit a contribution to the Gilbert damping constant inversely proportional to the thickness of the Co56Fe24B20 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 Co56Fe24B20 thickness, which can be understood based on the far larger spin diffusion length of Cu in comparison with Ru or Ta.

The magnetization relaxation of thin ferromagnetic Ni 81 Fe 19 films in close proximity and in direct contact with antiferromagnetic FeMn films was investigated using broadband ferromagnetic resonance. For unbiased films, a contribution to the effective-damping parameter α eff inversely proportional to the ferromagnetic film thickness is found, consistent with the presence of spin pumping. For biased films, we find a unidirectional contribution to the effective-damping parameter, which does not scale with the inverse film thickness. The unidirectional character of the magnetization relaxation is described in terms of two-magnon scattering.