The 2D ising model describes the behavior of FePS 3, CrI 3. and Fe 3 GeTe 2, while Cr 2 Ge 2 Te 6 and MnPS 3 behaves like isotropic Heisenberg model. The intrinsic anisotropy in CrI 3 and Fe 3 GeTe 2 is caused by strong spin–orbit coupling, allowing them to remain magnetic down to a monolayer, while Cr 2 Ge 2 Te 6 has only exhibit magnetism as a bilayer or thicker. The XY model describes the case where n = 2 . In this system, there is no transition between the ordered and unordered states, but instead the system undergoes a so-called Kosterlitz–Thouless transition at finite temperature T K T , where at temperatures below T K T the system has quasi-long-range magnetic order. It was reported that the theoretical predictions of the XY model are consistent with those experimental observations of NiPS 3. The Heisenberg model describes the case where n = 3 . In this system, there is no transition between the ordered and unordered states because of the Mermin-Wagner theorem. The experimental realization of the Heisenberg model was reported using MnPS 3.