supplementary material

Top and side views of the structure of M3XSe4 monolayers. The planar framework of X-M and two distinct M-S bonds. Phonon spectra and free energy variation during AIMD simulation after 6 ps at 300 K of M3XSe4. The elastic constants, 2D Young's modulus and exchange coupling parameters. Young's modulus of M3XSe4 monolayers as a function of the angle θ. Four antiferromagnetic configurations considered in M3XSe4 monolayer. Angular dependence of the magnetic anisotropic energy (MAE). The contribution to MAE from the SOC interaction between different M-d, X-p and Se-p orbitals along [100] and [010] directions for V3SSe4 monolayer. The energy difference as a function of strain intensity. Band structures of V3SSe4 under 4% tensile strain, V3TeSe4 under 1% tensile strain, and Cr3SSe4 under -5% compressive strain. The POSCAR files of M3XSe4 monolayers.