Scaling of spin relaxation and angular momentum dissipation in permalloy nanowires

We study the relationship between the damping (α) and the nonadiabaticity of the spin transport (β) in permalloy nanowires. α is engineered by Ho doping, and from the characteristics of the current-induced domain-wall velocity, determined by high-resolution x-ray magnetic circular-dichroism photoemission electron microscopy, β due to spin relaxation is measured. We find that β scales with α and conclude that the spin relaxation that leads to nonadiabatic spin torque originates from the same underlying mechanism as the angular momentum dissipation that causes viscous damping.