Photoexcitation-induced spin dynamics in 1T-VSe₂ investigated by ab initio nonadiabatic molecular dynamics

Using photoexcitation to manipulate the magnetic moment in two-dimensional (2D) materials paves the way for the design of opto-spintronic devices. In this work, using ab initio nonadiabatic molecular dynamics simulation, we studied how photoexcitation changed the magnetic moment in the 2D ferromagnetic metal VSe₂. The spin-orbit coupling and phonon excitation lead to the loss of the original spin orientation in both the spin-up and spin-down orbitals, forming a spin-mixing region approximately 1.0 eV above the Fermi level. When spin-up or spin-down electrons pass through this region during relaxation, they lose their original spin orientation. However, spin-down electrons relax approximately an order of magnitude faster than spin-up electrons, as the relaxation for spin-down is primarily intraband, while spin-up electrons undergo interband relaxation. Such different dynamical behaviors for spin-up and spin-down electrons result in the magnetic moment of VSe₂ initially rising within approximately 10 fs after optical excitation, corresponding to the loss of the original spin orientation for spin-down electrons. Subsequently, it decreases by approximately 100 fs, corresponding to the loss of spin orientation for spin-up electrons. Finally, the total magnetic moment of the system gradually recovers to the preexcitation level on the order of picoseconds. This work provides new insight into how photoexcitation controls the magnetic properties of 2D materials.