Transient Laser-Induced Magnetic Order and Relaxation Pathways in TiCrCT2 (T = O, F) MXenes.

Ultrafast spin manipulation in low-dimensional magnetic materials is essential for next-generation spintronic devices, yet the microscopic mechanisms governing spin dynamics, especially the role of surface termination, remain unclear. Here, TiCrCT2 (T = O, F) MXenes are systematically investigated using real-time time-dependent density functional theory and nonadiabatic molecular dynamics. The results reveal that surface termination governs distinct magnetic responses, including a ferrimagnetic-to-ferromagnetic transition in TiCrCF2 driven by Ti spin reversal, while TiCrCO2 preserves its ferrimagnetic order. This behavior originates from spin-conserving, spin-selective charge transfer between Ti and Cr sublattices, followed by asymmetric relaxation of spin carriers determined by the spin-resolved band-edge structure. Electron-phonon coupling dominates over spin-orbit coupling, enabling rapid same-spin transitions, whereas spin-flip processes occur on longer time scales. These findings establish a unified mechanism linking electronic structure, carrier relaxation, and magnetization dynamics, and highlight surface termination as an effective route to tailor ultrafast spin responses in MXenes.