  We study the ultrafast demagnetization of Ni/NiMn and Co/NiMn
ferromagnetic/antiferromagnetic bilayer systems after excitation by a laser
pulse. We probe the ferromagnetic order of Ni and Co using magnetic circular
dichroism in time-resolved pump--probe resonant X-ray reflectivity. Tuning the
sample temperature across the antiferromagnetic ordering temperature of the
NiMn layer allows to investigate effects induced by the magnetic order of the
latter. The presence of antiferromagnetic order in NiMn speeds up the
demagnetization of the ferromagnetic layer, which is attributed to
bidirectional laser-induced superdiffusive spin currents between the
ferromagnetic and the antiferromagnetic layer.
