  Ni$_{0.8}$Fe$_{0.2}$ (Py) and Py alloyed with Cu exhibit intriguing ultrafast
demagnetization behavior, where the Ni magnetic moment shows a delayed response
relative to the Fe, an effect which is strongly enhanced by Cu alloying. We
have studied a broad range of Cu concentrations to elucidate the effects of Cu
alloying in Py. The orbital/spin magnetic moment ratios are largely unaffected
by Cu alloying, signifying that Cu-induced changes in the ultrafast
demagnetization are not related to spin-orbit interactions. We show that magnon
diffusion can explain the delayed Ni response, which we attribute to an
enhanced magnon generation rate in the Fe sublattice relative to the Ni
sublattice. Furthermore, Py exhibits prominent RKKY-like exchange interactions,
which are strongly enhanced between Fe atoms and diminished between Ni atoms by
Cu alloying. An increased Fe magnon scattering rate is expected to occur
concurrently with this increased Fe-Fe exchange interaction, supporting the
results obtained from the magnon diffusion model.
