Enhanced spin orbit torque efficiency induced by large skew scattering in perpendicular Pt/Co/Ta multilayers with superlattice/alloying Nb (Ir) insertion

Abstract

Perpendicular magnetization switching driven by spin–orbit torque (SOT) facilitates great potential applications in high-efficient memory and logic. However, SOT-based devices suffer from a relatively low SOT efficiency and ultrahigh current density in the conventional heavy metal/ferromagnet bilayer structure. Here, we report that the SOT behavior can be effectively tuned by inserting the ultrathin Nb superlattice into heavy metal Pt layer compared with Ir insertion or the PtNb alloying layer. A slight change of critical current density (Jc) can be found in the multilayers with Ir insertion. The Jc value for the Pt/Co/Ta multilayer with [Pt/Nb]7 insertion is decreased to 1.4 × 107 A/cm2, approximately 60% lower than that in pure Pt/Co/Ta multilayers. Furthermore, the SOT efficiency is significantly enhanced with increasing the period number due to the tunable spin Hall angle (θSH). Compared with pure Pt layer, the θSH value is increased 47% for the sample with [Pt/Nb]5, which is also significantly larger than that in the one with the PtNb alloying layer. Enhanced skew scattering induced by Nb superlattice plays the main role in these tunable SOT properties. Our findings provide a feasible scheme to engineer high-efficiency SOT-based logic-in-memory.