Fig. 1. (Color online) (a–c) Schematic diagrams of different mechanisms of ferroelectric phase transition without applied external magnetic field in (a) Heisenberg model, (b) 2D-XY model, and (c) 2D-Ising model. (d) Theoretical comparison of M–T curves around T_C under different mechanisms indicated by corresponding critical exponent β.

Fig. 2. (Color online) Lattice structure of the CrCl₃ monolayer in (a) side view and (b) top view. The sublattice constructed by Cr atoms can be clearly seen.