  The ability of spin-orbit interactions to convert charge current into spin
current, most often in the bulk of heavy metal thin films, has been the
hallmark of spintronics in the last decade. In this study, we demonstrate how
the insertion of light metal element interface profoundly affects both the
nature of spin-orbit torque and its efficiency in terms of damping-like
($H_{\text{DL}}$) and field-like ($H_{\text{FL}}$) effective fields in
ultrathin Co ferromagnet. Indeed, we measure unexpectedly large
$H_{\text{FL}}$/$H_{\text{DL}}$ ratio ($\sim$2.5) upon inserting a 1.4 nm thin
Al layer in Pt|Co|Al|Pt as compared to a similar stacking including Cu instead
of Al. From our modelling, these results strongly evidence the presence of
large Rashba interaction at Co|Al interface producing a giant $H_{\text{FL}}$,
which was not expected from a metallic interface. The occurrence of such
enhanced torques from an interfacial origin is further validated by
demonstrating current-induced magnetization reversal showing a significant
decrease of the critical current for switching.
