  We explore the ultrafast generation of spin currents in magnetic multilayer
samples by applying fs laser pulses to one layer and measuring the magnetic
response in the other layer by element-resolved x-ray spectroscopy. In
Ni(5~nm)/Ru(2~nm)/Fe(4~nm), the Ni and Fe magnetization directions couple
antiferromagnetically due to the RKKY interaction, but may be oriented parallel
through an applied magnetic field. After exciting the top Ni layer with a fs
laser pulse, we find that also the Fe layer underneath demagnetizes, with a
$4.1 \pm 1.9$\% amplitude difference between parallel and antiparallel
orientation of the Ni and Fe magnetizations. We attribute this difference to
the influence of a spin current generated by the fs laser pulse that transfers
angular momentum from the Ni into the Fe layer. Our results confirm that
superdiffusive spin transport plays a role in determining the sub-ps
demagnetization dynamics of synthetic antiferromagnetic layers, but also
evidence large depolarization effects due to hot electron dynamics, which are
independent of the relative alignment of the magnetization in Ni and Fe.
