Interfacial and bulk contributions to spin-orbit torque and magnetization switching in non-magnet/ferromagnet structures

Abstract

Spin-orbit torque (SOT) allows for the efficient switching of magnetization via the in-plane current in perpendicularly magnetized nonmagnetic-metal (NM)/ ferromagnetic-metal (FM) nanostructures, representing various advantages such as high reliability and fast speed. Studies on spin Hall effect, which is one of possible mechanisms, have been greatly reported. The efficiency of SOT-induced switching has been known to be determined by NM and its spin-Hall angle. However, the interfacial spin-orbit coupling contribution to switching has been largely ignored due to the difficulties in identifying it. Here, I focus on the interfacial contribution to SOT by using insertion of Ti layer between NM and FM layers. Six types of metals were used as NM layers (Mg, Al, Cr, Cu, Ta, Pt) in NM/Ti/CoFeB/MgO structures. First, I examined the perpendicular magnetic anisotropy (PMA) properties of the samples, which is a prerequisite for the measurement of SOT. Among them, structures with four types of metals (Cr, Cu, Ta, Pt) showed strong PMA. I measured the considerable magnitude of SOT in structures with Cr, Ta, Pt. I further explored the interfacial effect by inserting various thicknesses of Ti layers. I report significant interfacial effects on SOT in Pt/CoFeB/MgO structures. I observe that the switching efficiency and the SOT are enhanced by 1.8 and 2.5 times, respectively, when the Pt/CoFeB interface is engineered with 1 nm of Ti. In comparison, Ta/CoFeB/MgO structures show a monotonous degradation of their switching performances upon the insertion of Ti. The results here experimentally suggest that interface modulation enables progress in SOT-induced switching and enriches our understanding of SOT.