Na Wu
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 Fe$_3$GeTe$_2$, a prototypical two-dimensional itinerant ferromagnet. Notably, by varying the laser fluence, we identify three distinct regimes arising from a photoinduced single linear phonon mode, namely demagnetization, spin-flip, and spin-melting.
We resolve the dominant role of the displacive excitation of A${\rm 1g}$ phonon modes by decoupling the nonadiabatic dynamics of photoexcited electrons and coherent lattice vibrations. Surprisingly, the A${\rm 1g}$ phonon itself does not lead to spin-flip without electron excitation, while spin-flip in photoexcited states can be well captured by changes in the potential energy landscape.
Accompanying the spin-flip, we also identify a sign change of the Berry curvature of the electronic states, implying the critical involvement of nontrivial band topology. Our results highlight the role of nonequilibrium electron–phonon interactions in light-induced spin dynamics and provide general guidance for optical manipulation of spin orders, with promising implications for future spintronics and information technology.