Abstract

We present a broadband ferromagnetic resonance study of the Gilbert damping enhancement (Delta alpha) due to spin pumping in NiFe/Pt bilayers. The bilayers, which have negligible interfacial spin memory loss, are studied as a function of the Pt layer thickness (t(Pt)) and temperature (100-293 K). Within the framework of diffusive spin pumping theory, we demonstrate that Dyakonov-Perel (DP) or Elliot-Yaffet (EY) spin relaxation mechanisms acting alone are incompatible with our observations. In contrast, if we consider that the relation between spin relaxation characteristic time (tau(s)) and momentum relaxation characteristic time (tau(p)) is determined by a superposition of DP and EY mechanisms, the qualitative and quantitative agreement with experimental results is excellent. Remarkably, we found that tp must be determined by the out-of-plane electrical resistivity (rho) of the Pt film and hence its spin diffusion length (lambda(Pt)) is independent of tPt. Our work settles the controversy regarding the t(Pt) dependence of lambda(Pt) by demonstrating its fundamental connection with rho considered along the same direction of spin current flow.