Long-lived nonequilibrium spin configuration induced by dual optical excitation

Sergii Parchenko; Arata Tsukamoto; Andreas Scherz

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arXiv · 2610.25236

Long-lived nonequilibrium spin configuration induced by dual optical excitation

Sergii Parchenko·Arata Tsukamoto·Andreas Scherz

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Abstract

The ability to create and control nonequilibrium spin states is central to understanding ultrafast magnetism and to developing optical routes for manipulating magnetic materials beyond conventional thermal pathways.In metallic systems, photoexcited electronic distributions rapidly evolve toward quasi-thermal states through efficient electron-electron and electron-phonon scattering, making long-lived modification of spin-dependent interactions difficult to achieve. Here, we investigate coherent magnetization precession in an amorphous GdFeCo thin film driven by simultaneous excitation with two femtosecond laser pulses.Using a symmetric dual-pump geometry that continuously evolves from single-pulse to spatially periodic excitation, we separate effects associated with the excitation pattern from those arising from simultaneous dual-pump excitation. We identify a narrow excitation window in which the lifetime of the ferromagnetic-resonance-like mode increases strongly while its frequency exhibits a small but reproducible increase.Time-window analysis shows that the enhanced lifetime persists for several hundred picoseconds, substantially longer than transient magnetostatic effects associated with the periodic excitation pattern.The combined frequency and decay time changes cannot be explained by spatially structured heating alone and instead point to a long-lived nonequilibrium spin-dependent electronic configuration. Within a phenomenological Elliott-Yafet framework, the simultaneous increase in FMR decay time and frequency is consistent with a transient reduction of the effective spin-mixing parameter governing spin-flip scattering. Our results demonstrate that the excitation pathway, rather than excitation energy alone, can serve as a control parameter for spin relaxation and provide access to nonequilibrium spin states that are not readily reached by conventional single-pulse excitation.

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Sergii Parchenko, Arata Tsukamoto, Andreas Scherz. 2026-10-18. Long-lived nonequilibrium spin configuration induced by dual optical excitation. https://arxiv.org/abs/2610.25236

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