Precise control of spin order on femtosecond timescales is a cornerstone for the development of next-generation spintronics and quantum information technology. Recently, a research team led by Profs. MENG Sheng and WANG Yaxian from the Institute of Physics, Chinese Academy of Sciences (IOP CAS) has demonstrated the optical manipulation pathways of 2D itinerant ferromagnets. Their work, titled "Photoinduced femtosecond spin-flip assisted by a single-mode linear phonon," was published in Science Advances.

Utilizing real-time time-dependent density functional theory (rt-TDDFT) and Ehrenfest molecular dynamics, the team reveals that a monolayer of Fe₃GeTe₂ can undergo a complete spin reversal within approximately 300 femtoseconds when driven by an intense near-infrared laser pulse, and such ultrafast spin-flip is not a purely thermal process but is driven by a complex synergetic coupling between electrons and the crystal lattice.

Specifically, the laser pulse triggers the displacive excitation of coherent A_1g phonons, which effectively lower the energy barrier between opposite spin polarizations. Simultaneously, the nonequilibrium electron occupation created by the laser breaks 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.

Beyond the structural dynamics, the team highlights a profound 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 are being switched alongside its magnetic order.

By systematically varying the laser fluence, the team 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 offer a promising theoretical foundation for advancing future THz-frequency spintronics and quantum information technologies.

This research was supported by the Ministry of Science and Technology of China, the National Natural Science Foundation of China, and the Chinese Academy of Sciences.

Fig. 1. Schematics for the phonon-induced ultrafast spin-flip in monolayer FGT.

Fig. 2. The structural and magnetization (Mz) dynamics of monolayer Fe₃GeTe₂ upon optical excitation.

Fig. 3. Optical manipulation of the ultrafast spin-flip.

Fig. 4. Transient Berry curvature during the spin-flip.

Fig. 5. The PES illustration of light-induced spin-flip.

Contact:
Institute of Physics
MENG Sheng
Email：smeng@iphy.ac.cn

Key words:
Spin-flip; 2D magnetism; optical manipulation;

Abstract:
Light-driven 300-fs spin-flip in Fe₃GeTe₂, via optically excited A_1g phonons and nonequilibrium electrons, enables ultrafast spintronic control.