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 (xi(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, sigma(SH). We find that the (001) films do exhibit exceptionally high xi(DL) ranging from 0.45 at 293 K to 0.65 at 30 K, which sets the lower bounds of sigma(SH) to be 1.9 x 10(5 )and 3.75 x 10(5) Omega(-1) m(-1), respectively, 10 times higher and of opposite sign than the theoretical prediction. Furthermore, xi(DL) and sigma(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 sigma(SH) near the DNLs.