Strong enhancement of spin-orbit torques in ferrimagnetic Ptx(Si3N4)1–x /CoTb bilayers by Si3N4 doping

Abstract

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 ${\mathrm{Pt}}_{x}{({\mathrm{Si}}_{3}{\mathrm{N}}_{4})}_{1--x}/{\mathrm{Co}}_{0.65}{\mathrm{Tb}}_{0.35}$ 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 ${\mathrm{Pt}}_{0.7}{({\mathrm{Si}}_{3}{\mathrm{N}}_{4})}_{0.3}$ with a high spin Hall ratio of 0.8 and a high charge conductivity of $1.2\ifmmode\times\else\texttimes\fi{}{10}^{6}\phantom{\rule{0.16em}{0ex}}{\mathrm{\ensuremath{\Omega}}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}$ 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.