Ferromagnetic material dependence of spin-orbit torque in PtMn/ferromagnet bilayer

Spin Hall effect (SHE) in heavy metals generates a spin-orbit torque (SOT) on a ferromagnet. Research has been focused on the material properties of heavy metals to enhance SOT efficiency, and promising alloys for large SHE have been reported. In this study, we optically investigated the SOT of a PtMn alloy with Mn concentrations ranging from 0 to 75%. We observed that the critical Mn concentration for the maximum SOT depends on the ferromagnet: with Co as a ferromagnet, a peak spin Hall angle of 0.11 was obtained at an Mn concentration of 25%; with CoFeB as a ferromagnet, a peak spin Hall angle of 0.18 was obtained at an Mn concentration of 50%. Such a strong ferromagnetic dependence may originate from the structural matching between the poorly crystalline structure of PtMn and amorphous structure of CoFeB.