  The magnitude of spin-orbit torque (SOT), exerted to a ferromagnet (FM) from
an adjacent heavy metal (HM), strongly depends on the amount of spin currents
absorbed in the FM. We exploit the large spin absorption at the Ru interface to
manipulate the SOTs in HM/FM/Ru multilayers. While the FM thickness is smaller
than its spin dephasing length of 1.2 nm, the top Ru layer largely boosts the
absorption of spin currents into the FM layer and substantially enhances the
strength of SOT acting on the FM. Spin-pumping experiments induced by
ferromagnetic resonance support our conclusions that the observed increase in
the SOT efficiency can be attributed to an enhancement of the spin-current
absorption. A theoretical model that considers both reflected and transmitted
mixing conductances at the two interfaces of FM is developed to explain the
results.
