  Since the discovery of a 'giant' spin Hall effect (SHE) in certain heavy
metal elements there has been an intense effort to identify and develop new and
technologically viable, heavy-metal-based thin film materials that could
generate spin currents with even greater efficiency to exert spin-orbit torques
(SOT) on adjacent ferromagnetic nanostructures. In parallel, there have been
wide ranging fundamental studies of the spin currents that can arise from
robust, intrinsic spin-orbit interaction (SOI) effects in more exotic systems
including topological insulators, transition metal dichalcogenides with broken
crystalline symmetry, Weyl and Dirac semimetals where gapless electronic
excitations are protected by topology and symmetry. Here we experimentally
study strong SOT from the topological semimetal IrO2 in (001) and (110) normal
films, which exhibit distinctly different SHE strengths. Angle resolved
photoemission spectroscopy studies have shown IrO2 exhibits Dirac nodal lines
(DNL) in the band structure, which could enable a very high spin Hall
conductivity (SHC). The (001) films exhibit exceptionally high damping like
torque efficiency ranging from 0.45 at 293 K to 0.65 at 30 K which sets the
lower bound of SHC that is ten times higher and of opposite sign than the
theoretical prediction. We observe a substantial reduction of SHC in
anisotropically strained (110) films, which suggests that the DNLs that are
present in the (001) films and contribute to SHC, are disrupted and gapped due
to the large anisotropic strain in (110) films, which in turn significantly
lowers SHC. Very large value of SHC at room temperature of this Dirac semimetal
could be very promising for the practical application.
