Strong Spin-Orbit Torques, Tunable DMI, and Variable Interfacial Spin-Orbit Coupling in Pt-based Spin Hall Metal/Ferromagnet Systems

Recent work finds that the spin Hall effect in Pt is dominated by the intrinsic Berry phase effect. The internal spin Hall ratio θ_SH should be enhanced by alloying Pt with a component that raises the resistivity ρ but does not materially reduce the spin Hall conductivity σ_SH. This talk will report on an extensive study of two fcc Pt alloys which establishes that for Au_1-xPt_x(Pd_1-xPt_x), at the optimal concentration x ≈ 0.25, anti-damping spin-orbit torque efficiency ξ_DL≈ 0.30 (0.26) for ρ = 83 (57.5) μΩ cm at 4 nm thickness [1,2]. This indicates θ_SH ≥ 0.58 (0.47) and σ_SH ≈ 0.7(1)×10⁶ Ω^-1m^-1; considerably larger than predicted by recent first principles-calculations. Moreover, the DMI at the Pt alloy/FM interface is both strong and tunable by composition over a wide range. These results establish Au_1-xPt_x and Pd_1-xPt_x as the most energy-efficient spin current generators for spin-torque manipulation of metallic FM systems and for chiral spintronics applications. The effect of interfacial spin-orbit coupling (ISOC) on the spin transparency of the Pt(alloy)/FM interface has also been examined through controlled variation of its strength through thermal annealing, revealing that ISOC at HM/FM interfaces should be minimized via interfacial passivation to maximize ξ_DL [3,4].
[1] L. Zhu et al. Highly efficient spin current generation by the spin Hall effect in Au_1-xPt_x, Phys. Rev. Applied 10,031001 (2018).
[2] L. Zhu et al. Strong damping-like spin-orbit torque and tunable Dzyaloshinskii-Moriya interaction generated by low-resisitivity Pd_1-xPt_x alloys, submitted 2018.
[3] L. Zhu et al. Spin-orbit torques in heavy metal/ferromagnet bilayer with variable interfacial spin-orbit coupling strength, submitted 2018.
[4] L. Zhu et al. Irrelevance of magnetic proximity effect to spin-orbit torques in heavy metal/ferromagnet bilayers, Phys. Rev. B 98,134406 (2018).