  Light transition metals have recently emerged as a sustainable material class
for efficient spin-charge interconversion. We report measurements of
current-induced spin-orbit torques generated by Ni$_{1-x}$Cu$_{x}$ alloys in
perpendicularly magnetized ferrimagnetic Gd/Fe multilayers. We show that the
spin-orbit torque efficiency of Ni$_{1-x}$Cu$_{x}$ increases with the Ni/Cu
atomic ratio, reaching values comparable to those of Pt for Ni$_{55}$Cu$_{45}$.
Furthermore, we demonstrate magnetization switching of a 20-nm-thick Gd/Fe
multilayer with a threshold current that decreases with increasing Ni
concentration, similar to the spin-orbit torque efficiency. Our findings show
that Ni$_{1-x}$Cu$_{x}$$-$based magnetic heterostructures allow for efficient
control of the magnetization by electric currents.
