Magnetic-Order-Mediated Second Harmonic Generation in a Room-Temperature van der Waals Ferromagnet Fe₃GaTe₂.

Magnetic order coupled second harmonic generation (SHG) provides a unique nonlinear probe of symmetry breaking in low dimensional magnets. However, its capability to resolve nonuniform magnetic domain structures and identify domain boundaries remains largely unexplored. Here, we demonstrate optical SHG imaging of labyrinthine magnetic domains in room temperature Fe₃GaTe₂, showing high sensitivity to magnetic configurations consistent with magnetic force microscopy. Magnetic field dependent SHG measurements reveal coherent interference between structural and magnetization coupled contributions, leading to SHG hysteresis and field tunable polarization rotation. These effects originate from interference between structural and magnetization induced second order susceptibilities, resulting in a magnetically tunable nonlinear optical phase. First-principles calculations support a magnetization coupled χ⁽²⁾ contribution associated with Fe-orbital spin asymmetry and interlayer hybridization. Our results establish SHG as a domain-resolved and fully optical probe of magnetic symmetry in ferromagnets at room temperature.