Direct versus indirect excitation of ultrafast magnetization dynamics in FeNi alloys

Ultrafast demagnetization can be induced either coherently by direct interaction with the optical light field or indirectly via excitation of the electron system, which subsequently couples to the magnetization. An intriguing direct process is optical intersite spin transfer (OISTR), which changes the local magnetic moments during the optical absorption process itself and leads to a potentially coherent net spin transport between two magnetic subsystems. In this paper, we find that both direct and indirect excitation of an FeNi alloy result in identical experimental signatures in transverse magneto-optical measurements in the extreme ultraviolet spectral range, which has been previously interpreted as evidence for the OISTR effect: a delayed onset of the ultrafast response of Ni with respect to Fe as well as an increasing signal for photons probing energies below the Ni resonance. Our findings align with recent theoretical and experimental investigations, which propose alternative explanations of these experimental observations and do not rely on a direct and coherent interaction between light and spin. Instead, the distinctly different magnetization dynamics of Fe and Ni may be governed by intersite spin transfer driven by electron scattering or be the result of element-specific microscopic properties such as inhomogeneous spin-orbit coupling or electron magnon scattering