Localization of Fe d-states in Ni-Fe-Cu alloys and implications for ultrafast demagnetization

Ni$_{0.8}$Fe$_{0.2}$ (Py) and Py-Cu exhibit intriguing ultrafast demagnetization behavior, where the Ni magnetic moment shows a delayed response relative to the Fe. 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. Ab-initio calculations corroborate these results and we discuss the electronic structure in the framework of virtual bound states (VBSs). Fe exhibits VBSs in the minority band that lie approximately 1 eV above the Fermi level in pure Py, and which move closer to the Fermi level upon Cu alloying. A strong interaction between the VBSs and electrons above the Fermi level enhances the formation of magnons at Fe sites. This mechanism is consistent with a demagnetization delay between Fe and Ni, as found experimentally.