  The spin Hall effect (SHE) is found to be strong in heavy transition metals
(HM), such as Ta and W, in their amorphous and/or high resistivity form. In
this work, we show that by employing a Cu-Ta binary alloy as buffer layer in an
amorphous Cu$_{100-x}$Ta$_{x}$-based magnetic heterostructure with
perpendicular magnetic anisotropy (PMA), the SHE-induced damping-like
spin-orbit torque (DL-SOT) efficiency $|\xi_{DL}|$ can be linearly tuned by
adjusting the buffer layer resistivity. Current-induced SOT switching can also
be achieved in these Cu$_{100-x}$Ta$_{x}$-based magnetic heterostructures, and
we find the switching behavior better explained by a SOT-assisted domain wall
propagation picture. Through systematic studies on Cu$_{100-x}$Ta$_{x}$-based
samples with various compositions, we determine the lower bound of spin Hall
conductivity
$|\sigma_{SH}|\approx2.02\times10^{4}[\hbar/2e]\Omega^{-1}\cdot\operatorname{m}^{-1}$
in the Ta-rich regime. Based on the idea of resistivity tuning, we further
demonstrate that $|\xi_{DL}|$ can be enhanced from 0.087 for pure Ta to 0.152
by employing a resistive TaN buffer layer.
