Spin transfer torque ferromagnetic resonance induced spin pumping in the Fe/Pd bilayer system

Abstract

Inconsistencies in estimates of the spin Hall angle (${\ensuremath{\theta}}_{SH}$) and spin diffusion length (${\ensuremath{\lambda}}_{SD}$) of nonmagnetic (NM) layers using the spin transfer torque ferromagnetic resonance (ST-FMR) in ferromagnetic FM/NM bilayer structures are attributed to the inverse spin Hall effect (ISHE) and interfacial parameter contributions, interface spin transparency, interfacial anisotropic magnetoresistance, and effective spin-mixing conductance. These contributions in Fe(10 nm)/Pd(2--8 nm) bilayer structures have been probed employing the simultaneous detection of ST-FMR and ISHE in conjunction with in-plane FMR measurements. The interfacial contributions are found to increase with an increase in Pd layer thickness (${t}_{NM}$), which can be linked to the spin pumping effect in conjunction with spin backflow. Correcting the ${t}_{NM}$ dependence of the ST-FMR spectra for the interfacial and ISHE contributions prior to estimating ${\ensuremath{\theta}}_{SH}$ and ${\ensuremath{\lambda}}_{SD}$ of the Pd layer, the estimated values are found to be $0.10\ifmmode\pm\else\textpm\fi{}0.03$ and $5.4\ifmmode\pm\else\textpm\fi{}1.2\phantom{\rule{0.28em}{0ex}}\mathrm{nm}$, respectively.