Supplementary material for three magnetic orders of FeSnN3 monolayer and calculation details of J, band structure calculated at HSE06 level, atom-resolved and orbital-resolved SOC energy differences of Fe, Sn and N, orbital-resolved DOS of Fe, evolutions of spin texture at 320 K and 350 K, Q value as a function of in-plane B, and evolutions of spin texture (at 200 K and 250 K) with DMI set to 1.96 meV.