  The generation of large spin currents, and the associated spin torques, which
are at the heart of modern spintronics, have long been achieved by
charge-to-spin conversion mechanisms, i.e. the spin Hall effect and/or the
Rashba effect, intrinsically linked to a strong spin-orbit coupling. Recently,
a novel path has been predicted and observed for achieving significant
current-induced torques originating from light elements, hence possessing a
weak spin-orbit interaction. These findings point out to the potential
involvement of the orbital counterpart of electrons, namely the orbital Hall
and orbital Rashba effects. In this study, we aim at quantifying these
orbital-related contributions to the effective torques acting on a thin Co
layer in different systems. First, in Pt|Co|Cu|AlOx stacking, we demonstrate a
comparable torque strength coming from the conversion due to the orbital Rashba
effect at the Cu|AlOx interface and the one from the effective spin Hall effect
in bottom Pt|Co system. Secondly, in order to amplify the orbital-to-spin
conversion, we investigate the impact of an intermediate Pt layer in
Co|Pt|Cu|CuOx. From the Pt thickness dependence of the effective torques
determined by harmonic Hall measurements complemented by spin Hall
magneto-resistance and THz spectroscopy experiments, we demonstrate that a
large orbital Rashba effect is present at the Cu|CuOx interface, leading to a
twofold enhancement of the net torques on Co for the optimal Pt thickness. Our
findings not only demonstrate the crucial role that orbital currents can play
in low-dimensional systems with weak spin-orbit coupling, but also reveal that
they enable more energy efficient manipulation of magnetization in spintronic
devices.
