Very high spin Hall conductivities and spin Hall ratios in epitaxial Iridium di-oxide films

New metallic materials with exceptionally high spin Hall conductivities and accompanying high spin Hall ratios are desirable both to produce more efficient systems for spin-orbit torque applications and to further test the fundamental understanding of intrinsic spin-orbit interactions. A particularly interesting candidate for such research is the metallic rutile oxide Iridium di-oxide which angle resolved photoemission spectroscopy studies have shown exhibits Dirac nodal lines in the band structure, a feature that could enable a very high . Here we report spin-torque ferromagnetic resonance studies of the damping-like and field-like torques exerted on an adjacent ferromagnetic layer as the result of current flowing in epitaxial (110) and (001) IrO₂ films. The (110) films exhibit a damping-like torque efficiency ≈ 0.18 at 293 K, which sets a lower bound for the spin Hall conductivity . The higher resistivity (~ 300 µΩ-cm) (001) films exhibit even stronger spin-orbit torques, with ranging from ~0.45 at 293K to 0.8 at 30 K as decreases, behavior indicative of the dirty metal regime. The very high value for (001) IrO₂, ≥ , is both a challenge for current theoretical understanding and an exciting prospect for more efficient SOT applications.