Enhancing Spin-Hall Spin–orbit Torque Efficiency by Bulk Spin Scattering Modulation in Ferromagnets with Ruthenium Impurities

We report that the efficiency of the spin-Hall spin–orbit torque (SOT) in perpendicularly magnetized Pt/Co/MgO films can be engineered by introducing ruthenium (Ru) impurities into the bulk of the Co layer. As the concentration of the Ru impurities increases, the effective field of the damping-like SOT is increased by a factor of 2.3, leading to SOT efficiency per unit current density enhancing from 0.069 to 0.155. However, the Ru incorporation shows less impact on the field-like SOT efficiency. First-principles calculations reveal that the Ru incorporation can significantly increase the density of states of the majority spin at the Fermi level but has less influence on the minority spin case. We suspect that such an electronic-structure modulation may reduce the scattering of the spin-Hall spin currents flowing across the Co-Ru layer, resulting in SOT efficiency enhancement. The SOT enhancement by Ru incorporation is also observed in Ta/CoFeB/MgO films, indicating that modulating the spin scattering inside ferromagnets with judiciously selected impurities is a promising strategy to construct low-power-dissipation SOT spintronic devices.