  We investigate the impact of pinned antiferromagnetic order on the
decoherence of spin current in polycrystalline IrMn. In NiFe/Cu/IrMn/CoFe
multilayers, we coherently pump an electronic spin current from NiFe into IrMn,
whose antiferromagnetic order is globally pinned by static exchange-bias
coupling with CoFe. We observe no anisotropic spin decoherence with respect to
the orientation of the pinned antiferromagnetic order. We also observe no
difference in spin decoherence for samples with and without pinned
antiferromagnetic order. Moreover, although there is a pronounced resonance
linewidth increase in NiFe that coincides with the switching of IrMn/CoFe, we
show that this is not indicative of anisotropic spin decoherence in IrMn. Our
results demonstrate that the decoherence of electron-mediated spin current is
remarkably insensitive to the magnetization state of the antiferromagnetic IrMn
spin sink.
