Terahertz-driven nonlinear phononics induces transient ferromagnetism in antiferromagnetic MnF2

Ultrafast optical manipulation of magnetism provides a promising pathway for next-generation spintronic technologies. Here, we theoretically demonstrate that terahertz-driven nonlinear phononics can induce transient ferromagnetic polarization in antiferromagnetic MnF2. Using first-principles calculations and nonlinear lattice-dynamics modeling, we show that the simultaneous excitation of two degenerate Eu infrared-active phonons drives a rectified displacement of the intrinsic B2g Raman mode through trilinear phonon coupling. The resulting lattice distortion modifies magnetic exchange interactions and produces a finite magnetization in an otherwise collinear antiferromagnetic state. Furthermore, a tailored two-pulse terahertz excitation scheme with distinct pulse widths and controlled delay enhances the rectified Raman displacement and the induced magnetization. Magnetization-dynamics simulations reveal picosecond-scale oscillations and a sizable light-induced magnetic moment approaching 1 μB per unit cell under strong excitation. These results establish a phonon-mediated pathway for ultrafast optical manipulation of antiferromagnetic order and suggest a strategy for controlling magnetism in antiferromagnetic materials using engineered terahertz fields.