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

We report spin-torque ferromagnetic resonance studies of the efficiency of the damping-like (ξ) spin-orbit torque exerted on an adjacent ferromagnet film by current flowing in epitaxial (001) and (110) IrO thin films. IrO 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. We find that the (001) films do exhibit exceptionally high ξ ranging from 0.45 at 293 K to 0.65 at 30 K, which sets the lower bounds of σ to be 1.9 × 10 and 3.75 × 10 Ω m, respectively, 10 times higher and of opposite sign than the theoretical prediction. Furthermore, ξ and σ 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 near the DNLs.