Precise control of spin order on femtosecond timescales is important for the development of next-generation spintronics and quantum information technology. In a recent study published in Science Advances, a research team led by Profs. MENG Sheng and WANG Yaxian from the Institute of Physics of the Chinese Academy of Sciences demonstrated the optical manipulation pathways of two-dimensional (2D) itinerant ferromagnets.

Based on real-time time-dependent density functional theory and Ehrenfest molecular dynamics, researchers revealed that a monolayer of Fe₃GeTe₂ underwent a complete spin reversal within approximately 300 femtoseconds when driven by an intense near-infrared laser pulse. Such ultrafast spin-flip was not a purely thermal process but driven by a complex synergetic coupling between electrons and the crystal lattice.

Specifically, the laser pulse triggered the displacive excitation of coherent A_1g phonons, which effectively lowered the energy barrier between opposite spin polarizations. Meanwhile, the nonequilibrium electron occupation created by the laser broke the energy degeneracy of the spin-up and spin-down states, providing the necessary driving force to overcome the remaining barrier and stabilize the new spin configuration.

Besides structural dynamics, researchers highlighted a connection between magnetism and band topology by resolving an abrupt sign reversal in the Berry curvature and the instantaneous Chern number accompanying the spin-flip, indicating that the topological properties of the material were being switched alongside its magnetic order.

By systematically varying the laser fluence, researchers constructed a comprehensive phase diagram identifying three distinct dynamical regimes: demagnetization, spin-flip, and ferromagnetic spin-melting. Their findings provide a general protocol for the optical manipulation of spin orders, and lay a foundation for advancing THz-frequency spintronics and quantum information technologies.