Engineering nonequilibrium spin-dependent electronic states via dual optical excitation

While femtosecond laser pulses can manipulate magnetization on its intrinsic timescale, the resulting electronic configurations in metals typically relax within only a few picoseconds through efficient electron–electron, electron–phonon, and spin-flip scattering. Developing approaches that extend the lifetime of nonequilibrium spin-dependent electronic states represents both a fundamental challenge in condensed-matter physics and an important step toward future ultrafast spintronic technologies.

In this presentation, I will show that coherent dual-pulse optical excitation provides a new route for controlling magnetic relaxation beyond the limits of conventional single-pulse excitation. Using time-resolved magneto-optical measurements on metallic GdFeCo ferrimagnetic films, we demonstrate a pronounced reduction of the damping of ferromagnetic-resonance–like precession, with the modified magnetic state persisting for several hundred picoseconds at room temperature. The observed behavior cannot be explained solely by thermal effects or by the spatially periodic excitation profile generated by the two pump pulses, indicating that the excitation pathway itself modifies the transient electronic state governing spin relaxation. I will further present recent results demonstrating that dual optical excitation also modifies the exchange mode of the ferrimagnet. Since the exchange resonance is directly governed by the microscopic exchange interaction between magnetic sublattices, these observations suggest that the light-induced electronic configuration influences not only macroscopic magnetic damping but also fundamental magnetic interactions on the atomic scale. Together, the results point toward the formation of a long-lived nonequilibrium spin-dependent electronic configuration in which spin relaxation and angular momentum transfer are substantially modified. Finally, I will discuss possible microscopic mechanisms, the role of spin–orbit–mediated spin mixing, and future directions toward engineering transient electronic states with tailored magnetic properties.

Séminaire organisé dans le cadre du programme interdisciplinaire MAT-PULSE