Manipulation of field-like torque in obital-torque-induced magnetiztion swtching for Cr/CoFeB/MgO multilayers

The orbital Hall effect and its experimental detection in the weakly spin-orbit coupled 3d metals, such as Cr, have attracted considerable research attention due to the gigantic orbital Hall conductivity and the novel orbital degree of freedom. Here, we investigate damping-like torque (DLT) and field-like torque (FLT) efficiencies of Cr-based CoFeB/MgO layered structures with perpendicular magnetic anisotropy by varying the thickness of the MgO layer. Our studies indicate that the MgO thickness can influence the oxidation degree of CoFeB, and hence the effective magnetic anisotropy energy density and torque efficiency. The DLT and FLT efficiencies depend very differently on the MgO thickness. The DLT efficiency remains virtually unchanged with MgO thickness, while the FLT efficiency decreases significantly with MgO thickness. The sample with the maximum FLT efficiency has the lowest critical switching current density and the largest switching ratio, suggesting a close relationship between the magnetization switching and the FLT efficiency. Furthermore, the FLT efficiency vs 1/Hk shows opposite signs of the slope in the under- and over-oxidized samples, respectively, which can potentially be used to distinguish between interface reflection and bulk scattering mechanisms in the generation of FLT.