  Ni$_{80}$Fe$_{20}$ (Py) and Py-Cu exhibit intriguing ultrafast
demagnetization behavior, where the Ni magnetic moment shows a delayed response
relative to the Fe [S. Mathias et al., PNAS {\bf 109}, 4792 (2012)]. To unravel
the mechanism responsible for this behavior, we have studied Py-Cu alloys for a
wide range of Cu concentrations using X-ray magnetic circular dichroism (XMCD).
The magnetic moments of Fe and Ni are found to respond very differently to Cu
alloying: Fe becomes a strong ferromagnet in Py, with the magnetic moment
largely unaffected by Cu alloying. In contrast, the Ni magnetic moment
decreases continuously with increasing Cu concentration. Our results are
corroborated by ab-initio calculations of the electronic structure, which we
discuss in the framework of virtual bound states (VBSs). For high Cu
concentrations, Ni exhibits VBSs below the Fermi level, which are likely
responsible for an increased orbital/spin magnetic ratio at high Cu
concentrations. Fe exhibits VBSs in the minority band, approximately 1 eV above
the Fermi level in pure Py, that move closer to the Fermi level upon Cu
alloying. A strong interaction between the VBSs and excited electrons above the
Fermi level enhances the formation of localized magnons at Fe sites, which
explains the different behavior between Fe and Ni during ultrafast
demagnetization.
