Determination of spin pumping as a source of linewidth in sputtered Co90Fe10/Pd multilayers by use of broadband ferromagnetic resonance spectroscopy

Abstract

We performed a systematic study of damping in Co${}_{90}$Fe${}_{10}$/Pd multilayers by use of broadband (1--60 GHz) ferromagnetic resonance (FMR) spectroscopy in the perpendicular geometry. The data were fitted with the conventional Landau-Lifshitz equation in conjunction with an inhomogeneous contribution to linewidth $\ensuremath{\Delta}$${H}_{0}$. Samples were prepared with net perpendicular anisotropy field values ranging from \ensuremath{-}0.5 to $+$1.2 T. $\ensuremath{\Delta}$${H}_{0}$ shows a dependence on the perpendicular anisotropy, though the Landau-Lifshitz damping parameter $\ensuremath{\alpha}$, which ranged from 0.016 to 0.04, exhibits no trend as a function of anisotropy. We explain the wide variation of $\ensuremath{\alpha}$ as a result of spin pumping from Co${}_{90}$Fe${}_{10}$ into adjacent nonmagnetic layers. We use a quantitative model for spin pumping that includes the intrinsic spin-mixing conductance at the Co${}_{90}$Fe${}_{10}$/Pd interface and the spin-diffusion length of Pd, which were experimentally measured at room temperature to be (1.07 $\ifmmode\pm\else\textpm\fi{}$ 0.13) $\ifmmode\times\else\texttimes\fi{}$ 10${}^{19}$ m${}^{\ensuremath{-}2}$ and 8.6 $\ifmmode\pm\else\textpm\fi{}$ 1.0 nm, respectively. We quantitatively show how $\ensuremath{\alpha}$ is enhanced by spin pumping through an FMR investigation of individual Pd/CoFe/Pd, and Pd/CoFe/Pd/CoFe/Pd layer structures.