Maximizing Spin-Orbit-Torque Efficiency of Pt/Ti Multilayers: Trade-Off Between Intrinsic Spin Hall Conductivity and Carrier Lifetime

We report a comprehensive study of the maximization of the spin Hall ratio (theta(SH)) in Pt thin films by the insertion of submonolayer layers of Ti to decrease carrier lifetime while minimizing the concurrent reduction in the spin Hall conductivity (sigma(SH)). We establish that the intrinsic sigma(SH) and carrier lifetime sets a practical upper bound of theta(SH) of Pt, while robust against the strain and the moderate interruption of crystal order caused by these insertions, begins to decrease rapidly at high resistivity level because of the shortening carrier lifetime. The unavoidable trade-off between the intrinsic sigma(SH) and carrier lifetime sets a practical upper bound of theta(SH) >= 0.8 for heterogeneous materials where the crystalline Pt component is the source of the spin Hall effect and the resistivity is increased by shortening the carrier lifetime. This work also establishes a very promising spin Hall metal of [Pt 0.75 nm/Ti 0.2 nm](7)/Pt 0.75 nm for energy-efficient, high-endurance spin-orbit-torque technologies (e.g., memories, oscillators, and logic) due to its combination of a giant theta(SH) approximate to 0.8, or equivalently, a dampinglike spin-torque efficiency per unit current density xi(j)(DL) approximate to 0.35, with a relatively low resistivity (90 mu Omega cm) and high suitability for practical technology integration.