  Using element-specific measurements of the ultrafast demagnetization of
Ru/Fe$_{65}$Co$_{35}$ heterostructures, we show that Ru can exhibit a
significant magnetic contrast (3% asymmetry) resulting from ultrafast spin
currents emanating from the demagnetization process of the FeCo layer. We use
this magnetic contrast to investigate how superdiffusive spin currents are
affected by the doping of heavy elements in the FeCo layer. We find that the
spin currents are strongly suppressed, and that the recovery process in Ru
slows down, by Re doping. This is in accordance with a change in interface
reflectivity of spin currents as found by the superdiffusive spin transport
model.
