  We report on a systematic comparative study of the spin Hall efficiency
between highly face-centered cubic (fcc)-textured Pt-Al alloy films grown on
MgO(001) and poorly-crystallized Pt-Al alloy films grown on SiO$_2$. Using
CoFeB as the detector, we show that for Al compositions centering around $x =
25$, mainly L1$_{2}$ ordered Pt$_{100-x}$Al$_x$ alloy films grown on MgO
exhibit outstanding charge-spin conversion efficiency. For
Pt$_{78}$Al$_{22}$/CoFeB bilayer on MgO, we obtain damping-like spin Hall
efficiency as high as $\xi_\textrm{DL} \sim +0.20$ and expect up to seven-fold
reduction of power consumption compared to the polycrystalline bilayer of the
same Al composition on SiO$_2$. This work demonstrates that improving the
crystallinity of fcc Pt-based alloys is a crucial step for achieving large spin
Hall efficiency and low power consumption in this material class.
