  A spin-torque ferromagnetic resonance study is performed in epitaxial
$\mathrm{Fe / Ir_{15}Mn_{85}}$ bilayers with different Fe thicknesses. We
measure a negative spin-Hall angle of a few percent in the antiferromagnetic
IrMn in contrast to previously reported positive values. A large spin-orbit
field with Rashba symmetry opposing the Oersted field is also present.
Magnitudes of measured spin-orbit torques depend on the crystallographic
direction of current and are correlated with the exchange bias direction set
during growth. We suggest that the uncompensated moments at the Fe / IrMn
interface are responsible for the observed anisotropy. Our findings highlight
the importance of crystalline and magnetic structures for the spin-Hall effect
in antiferromagnets.
