  Element specific ultrafast demagnetization was studied in Fe$_{1-x}$Ni$_{x}$
alloys, covering the concentration range between $0.1<x<0.9$. For all
compositions, we observe a delay in the onset of Ni demagnetization relative to
the Fe demagnetization. We find that the delay is correlated to the Curie
temperature and hence also the exchange interaction. The temporal evolution of
demagnetization is fitted to a magnon diffusion model based on the
presupposition of enhanced ultrafast magnon generation in the Fe sublattice.
The spin wave stiffness extracted from this model correspond well to known
experimental values.
