Large interfacial Rashba interaction and resultant dominating field- like torque in atomically thin metallic heterostructures

The hallmark of spintronics in the last decade has been the ability of spin-orbit interactions to convert a charge current into the spin current, and vice versa, most often in the bulk of heavy metal thin films. Here, we demonstrate how the insertion of a light metal element interface profoundly affects both the nature of spin-orbit torque and its efficiency in terms of damping-like (HDL) and field-like (HFL) effective fields in an ultrathin Co film. Indeed, we measure unexpectedly an HFL/HDL ratio much larger than 1 (up to 2.5) by inserting a nanometer-thin Al metallic 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 a large Rashba interaction at Co|Al interface generating a giant HFL, which is 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 in the critical current for switching.