Photoinduced femtosecond spin-flip assisted by a single-mode linear phonon

Optical manipulation of spin-flip on a picosecond to femtosecond timescale has long been pursued to innovate next-generation electronic devices. However, understanding the ultrafast spin-electron-lattice coupled dynamics remains challenging, especially when the system is driven far from equilibrium. Here, we demonstrate an ultrafast light-induced spin-flip within 300 fs in Fe₃GeTe₂, a prototypical two-dimensional itinerant ferromagnet. Notably, by varying the laser fluence, we identify three distinct regimes that emerge assisted by a photoinduced single linear phonon mode, namely demagnetization, spin-flip, and spin-melting. We resolve the dominant role of displacively excited A_1g phonons, while nonequilibrium electron occupation is essential to break the degeneracy of the spin up and down states and lower the spin-flip energy barrier. Accompanying the spin-flip, we also observe a sign change of the Berry curvature, implying involvement of nontrivial band topology. Our results provide a general guidance for optical manipulation of spin orders, holding promises for advancing future spintronics and information technology.