Effects of Anisotropic Strain on Spin-Orbit Torque Produced by the Dirac Nodal Line Semimetal IrO₂

We report spin-torque ferromagnetic resonance studies of the efficiency of the damping-like (ξ_DL) spin-orbit torque exerted on an adjacent ferromagnet film by current flowing in epitaxial (001) and (110) IrO₂ thin films. IrO₂ possesses Dirac nodal lines (DNLs) in the band structure that are gapped by spin-orbit coupling, which could enable a very high spin Hall conductivity, σ_SH. We find that the (001) films do exhibit exceptionally high ξ_DL ranging from 0.45 at 293 K to 0.65 at 30 K, which sets the lower bounds of σ_SH to be 1.9 × 10⁵ and 3.75 × 10⁵ Ω^-1 m^-1, respectively, 10 times higher and of opposite sign than the theoretical prediction. Furthermore, ξ_DL and σ_SH are substantially reduced in anisotropically strained (110) films. We suggest that this high sensitivity to anisotropic strain is because of changes in contributions to σ_SH near the DNLs.