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Evidence of an exchange-based origin of demagnetization from the fluence dependent delay of Ni in a disordered Fe0.5Ni0.5 alloy

Somnath Jana1,*, Ronny Knut2, Puloma Singh1, Kelvin Yao1, Christian Tzschaschel1, Johanna Richter1, Daniel Schick1, Denny Sommer1, Dieter Engel1 et al.

Olof Karis2, Clemens von Korff Schmising1, and Stefan Eisebitt1,3

  • *Contact author: sj.phys@gmail.com
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Phys. Rev. B 112, 214412 – Published 5 December, 2025

DOI: https://doi.org/10.1103/f39s-kvy8

Abstract

The delayed demagnetization in Ni relative to Fe in the ultrafast demagnetization studies in FeNi alloy has led to two competing theoretical explanations: The Inhomogeneous Magnon Generation (IMG) and the Optically Induced Spin Transfer (OISTR) model. The IMG attributes the delay to the preferential magnon generation at the Fe sites and its subsequent propagation to Ni, while OISTR proposes direct spin transfer from Ni to Fe. In this study, we employ element-resolved extreme ultraviolet spectroscopy to investigate the effect of excitation strength on this delay, aiming to resolve the controversy. The data indicate a significant reduction in the delay with increasing fluence, which is inconsistent with the theoretical predictions of OISTR. These findings, in conjunction with the observation of a saturation of Fe demagnetization at the onset of Ni demagnetization, indicate that a spin-wave instability within the IMG framework may provide a potential explanation for the experimental results.

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Phys. Rev. B 112, 214412– Published 5 December, 2025
  • Received 12 March 2025
  • Accepted 31 October 2025
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