  The spin Hall effect (SHE) converts charge current to pure spin currents in
orthogonal directions in materials that have significant spin-orbit
coupling.The efficiency of the conversion is described by the spin Hall Angle
(SHA). The SHA can most readily be inferred by using the generated spin
currents to excite or rotate the magnetization of ferromagnetic films or
nano-elements via spin-transfer torques.Some of the largest spin torque derived
spin Hall angles (ST-SHA) have been reported in platinum. Here we show, using
spin torque ferromagnetic resonance (ST-FMR) measurements, that the
transparency of the Pt-ferromagnet interface to the spin current plays a
central role in determining the magnitude of the ST-SHA. We measure a much
larger ST-SHA in Pt/cobalt (~0.11) compared to Pt/permalloy (~0.05) bilayers
when the interfaces are assumed to be completely transparent. Taking into
account the transparency of these interfaces, as derived from spin-mixing
conductances, we find that the intrinsic SHA in platinum has a much higher
value of 0.19 +- 0.04 as compared to the ST-SHA. The importance of the
interface transparency is further exemplified by the insertion of atomically
thin magnetic layers at the Pt/permalloy interface that we show strongly
modulates the magnitude of the ST-SHA.
