Determination of the Spin-Hall-Effect-Induced and the Wedged-Structure-Induced Spin Torque Efficiencies in Heterostructures with Perpendicular Magnetic Anisotropy

Abstract: We report that by measuring current-induced hysteresis loop shift versus in-plane bias magnetic field, the spin Hall effect (SHE) contribution of the current-induced effective field per current density, $\chi_{SHE}$, can be estimated for Pt and Ta-based magnetic heterostructures with perpendicular magnetic anisotropy (PMA). We apply this technique to a Pt-based sample with its ferromagnetic (FM) layer being wedged-deposited and discover an extra effective field contribution, $\chi_{Wedged}$, due to the asymmetric nature of the deposited FM layer. We confirm the correlation between $\chi_{Wedged}$ and the asymmetric depinning process in FM layer during magnetization switching by magneto-optical Kerr (MOKE) microscopy. These results indicate the possibility of engineering deterministic spin-orbit torque (SOT) switching by controlling the symmetry of domain expansion through the materials growth process.