Enhanced ferromagnetic damping in Permalloy/Cu bilayers

We have investigated the enhancement of ferromagnetic damping for thin Permalloy (Ni80Fe20) films grown with Cu capping layers of variable thickness (5-1000 nm). The measurements were performed with a pulsed inductive microwave magnetometer in the frequency range between 2.3 and 2.7 GHz. The damping was enhanced if the Cu layers were thicker than the spin-diffusion length of l(s)approximate to 250 nm. For example, the damping was enhanced by 30% for a Permalloy (3 nm)/Cu (1000 nm) bilayer relative to the damping for Permalloy with a 5-nm-thick Cu capping layer. Existing theory for spin pumping from the Permalloy layer into the Cu layer was used to model the additional contribution to damping for these bilayer systems. Additional experiments on Permalloy (5 nm)/Cu(x nm)/Ta (5 nm) provided indirect evidence for spin accumulation inside the Cu layer.