Spin injection across the ferrimagnetic insulator (YIG)/normal metal (Au) interface was studied by ferromagnetic resonance. The spin mixing conductance was determined by comparing the Gilbert damping in bare YIG films with those covered by a $\mathrm{Au}/\mathrm{Fe}/\mathrm{Au}$ structure. The Fe layer in $\mathrm{Au}/\mathrm{Fe}/\mathrm{Au}$ acted as a spin sink as displayed by an increased Gilbert damping parameter $\ensuremath{\alpha}$ compared to that in the bare YIG. In particular, for the $9.0\text{ }\text{ }\mathrm{nm}\text{ }\text{ }\mathrm{YIG}/2.0\text{ }\text{ }\mathrm{nm}\text{ }\text{ }\mathrm{Au}/4.3\text{ }\text{ }\mathrm{nm}\text{ }\text{ }\mathrm{Fe}/6.1\text{ }\text{ }\mathrm{nm}\text{ }\text{ }\mathrm{Au}$ structure, the YIG and Fe films were coupled by an interlayer exchange coupling, and the exchange coupled YIG exhibited an increased Gilbert damping compared to the bare YIG. This relationship between static and dynamic coupling provides direct evidence for spin pumping. The transfer of spin momentum across the YIG interface is surprisingly efficient with the spin mixing conductance ${g}_{\ensuremath{\uparrow}\ensuremath{\downarrow}}\ensuremath{\simeq}1.2\ifmmode\times\else\texttimes\fi{}{10}^{14}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}2}$.

Spin Pumping at the Magnetic Insulator (YIG)/Normal Metal (Au) Interfaces