We report strong enhancement of spin-orbit torques by incorporating ${\mathrm{Si}}_3{\mathrm{N}}_4$ impurities into the dirty metal Pt. We find that the ${\mathrm{Si}}_3{\mathrm{N}}_4$ impurities lower the spin Hall conductivity and the charge conductivity of the Pt host at different rates, leading to a twofold increase in the dampinglike spin-orbit torque per unit current density for ferrimagnetic ${\mathrm{Pt}}_x{\left({\mathrm{Si}}_3{\mathrm{N}}_4\right)}_{1–x}/{\mathrm{Co}}_{0.65}{\mathrm{Tb}}_{0.35}$ bilayers. This torque enhancement is attributed to the optimized trade-off between the intrinsic spin Hall conductivity and the spin carrier lifetime in the dirty limit. We also find that only 58% of the angular momentum of the spin current entering the ferrimagnetic ${\mathrm{Co}}_{0.65}{\mathrm{Tb}}_{0.35}$ relaxes via exchange interaction and thus makes a contribution to spin torque generation. This work establishes ${\mathrm{Pt}}_{0.7}{\left({\mathrm{Si}}_3{\mathrm{N}}_4\right)}_{0.3}$ with a high spin Hall ratio of 0.8 and a high charge conductivity of $1.2\times {10}^6{\mathrm{\Omega }}^{-1}{\mathrm{m}}^{-1}$ as a compelling spin Hall metal for spin-orbitronics. This work also reaffirms the variation of the spin-orbit torque with relative spin relaxation rates within the magnetic layer.

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• Cellular bilayers
• Spin Hall effect
• Spin-orbit torque
• Ferrimagnets