Enhanced spin-torque efficiency by metal insertion in the Pt/Co/MgO system

Developing energy-efficient magnetoresistive random-access memory based on spin-orbit torques (SOTs) faces challenges due to the limited effective SOTs exerted on neighboring ferromagnets (FMs) by spin polarizations from the heavy metals (HMs). Over the past decade, extensive research has sought new materials with heightened spin-orbit coupling (SOC) and robust intrinsic spin Hall angles. Here, we investigate the effect of metal insertion (MI) on spin-torque efficiency in the Pt/Co/MgO system. Notably, the Mg insertion between Co and MgO augments the effective dampinglike SOT efficiency to 0.487, while Mo insertion results in an efficiency of 0.26, compared to 0.10 without any insertion. This efficiency enhancement is achieved by increasing spin-orbital scattering in Co through more specular reflection at the MI/MgO interface and spin-transparent transmission at the Co/MI interface. We conducted an element-resolved spin-pumping experiment using the x-ray ferromagnetic resonance technique to observe spin transport behavior in the metal-inserted stack with a ferromagnetic detector layer, and we confirmed greater spin transparency in Co/Mg than in Co/Mo. Without altering the intrinsic spin Hall angle of HM or the interfacial spin transparency of the HM/FM interface, we significantly enhanced the spin-torque efficiency by inserting metal between FM and MgO. This approach of enhancing spin-torque efficiency holds great promise for advancing SOT technologies and facilitating efficient spin generation within spintronics research.