Fig. 2: Thickness-dependent multiferroic order in NiI₂.

It shows different rotational anisotropy of the second-harmonic generation (SHG) pattern as a function of NiI₂ thickness from 70 layers (70 L) to monolayer (1 L). Red and blue background colours highlight the transition to the polar state upon decreasing temperature. The red colour represents the non-ferroelectric state with a minute SHG response of six petals. In comparison, the blue colour represents the ferroelectric state with a significant SHG response of four petals. The ferroelectric state is maintained by an exfoliated NiI₂ bilayer on the SiO₂/Si substrate. (Fig. 2 is remade from Fig. 3 in Ref. ¹²).