Effectively manipulating magnetism in ferromagnet (FM) thin-film nanostructures with an in-plane current has become feasible since the determination of a "giant" spin Hall effect (SHE) in certain heavy metal/FM systems. Recently, both theoretical and experimental reports indicate that metallic antiferromagnet materials can have both a large anomalous Hall effect and a strong SHE. Here we report a systematic study of the SHE in PtMn with several PtMn/FM systems. By using interface engineering to reduce the "spin memory loss" we obtain, in the best instance, a spin-torque efficiency ξDLPtMn≡TintθSHPtMn≃0.24, where Tint is the effective interface spin transparency. This is more than twice the previously reported spin-torque efficiency for PtMn. We also find that the apparent spin diffusion length in PtMn is surprisingly long, λsPtMn≈2.3nm. © 2016 American Physical Society.