Inhomogeneous magnon scattering during ultrafast demagnetization

TL;DR

This study investigates how Cu alloying affects ultrafast demagnetization in NiFe alloys, revealing that magnon diffusion and exchange interactions play key roles in the observed delayed magnetic response.

Contribution

It demonstrates that magnon diffusion explains the delayed Ni response and shows how Cu alloying modifies exchange interactions and magnon scattering in NiFe alloys.

Findings

Cu alloying enhances magnon generation in Fe sublattice.

Magnon diffusion accounts for the delayed Ni demagnetization.

Cu modifies RKKY-like exchange interactions between Fe and Ni.

Abstract

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.