  We report very efficient spin current generation by the spin Hall effect in
the alloy Au0.25Pt0.75, which, as determined by two different direct spin-orbit
torque measurements, exhibits a giant internal spin Hall ratio of > 0.58
(anti-damping spin-orbit torque efficiency of ~ 0.35 in bilayers with Co), a
relatively low resistivity of ~ 83 uOhm cm, an exceptionally large spin Hall
conductivity of > 7.0x10^5 ohm^-1 m^-1, and a spin diffusion length of 1.7 nm.
This work establishes Au0.25Pt0.75 as a milestone spin current generator that
provides greater energy efficiency than that yet obtained with other heavy
metals or with the topological insulators Bi2Se3 and (Bi,Se)2Te3. Our findings
should advance spin-orbit torque-based fundamental research and benefit the
development of new fast, efficient spin-orbit torque-driven magnetic memories,
skyrmion and chiral domain wall devices, and microwave and terahertz emitters.
