  We report the observation of the crossover between the extrinsic and
intrinsic spin Hall effect induced by alloying. We found that the spin Hall
angle, the ratio of the spin Hall conductivity to the electric conductivity,
changes drastically by tuning the composition of Au-Cu alloy. The spin Hall
angle changes the sign only in a limited range of the Cu concentration due to
the extrinsic skew scattering, while the intrinsic contribution becomes
dominant with increasing the Cu concentration. This observation provides
essential information for fundamental understanding of spin-orbit physics.
