Because of the prediction of Landau, people are always uncertain of the existence of two-dimensional (2D) materials before graphene was exfoliated successfully. Also for 2D ferromagnetic (FM) materials, the famous Mermin-Wagner theorem points out: Long-range magnetic order of isotropic spin-S Heisenberg model does not exist at nonzero temperature in dimensions d ≤ 2, which discourages many people who want to study 2D ferromagnets. However, people have not completely given up looking for 2D ferromagnets. After unremitting research, it has been found that magnetism can be indued in 2D systems by some external conditions, such as doping[3–5] and defects[6, 7]. Until 2017, 2D FM monolayer CrI3 and Cr2Ge2Te6 were successfully fabricated in experiments, which has attracted extensive attention. Massive researches on monolayer CrI3 and Cr2Ge2Te6 are beginning to emerge[10–17]. People have regained confidence and enthusiasm for the study of 2D ferromagnet. Recently, new 2D FM materials have been constantly predicted theoretically and synthesized experimentally, such as Fe3GeTe2, VSe2, MnSe2, Fe3P, VI3, and so on. The appearance of 2D FM materials provides a good platform for people to study physical properties of magnetic materials under 2D limit.