Effect of Ru thickness on spin pumping in Ru/Py bilayer

We report the effect of Ru thickness (tRu) on ferromagnetic resonance (FMR) line-width of Ru(tRu)/Py(23 nm) bilayer samples grown on Si(100)/SiO2 substrates at room temperature by magnetron sputtering. The FMR line-width is found to vary linearly with frequency for all thicknesses of Ru, indicating intrinsic origin of damping. For Ru thicknesses below 15 nm, Gilbert-damping parameter, α is almost constant. We ascribe this behavior to spin back flow that is operative for Ru thicknesses lower than the spin diffusion length in Ru, λsd. For thicknesses >15 nm (>λsd), the damping constant increases with Ru thickness, indicating spin pumping from Py into Ru.