A recent experiment reported type-II multiferroicity in monolayer (ML) $\mathrm{Ni}{\mathrm{I}}_2$ based on a presumed spiral magnetic configuration (spiral-B), which is, as we found here, under debate in the ML limit. Freestanding ML $\mathrm{Ni}{\mathrm{I}}_2$ breaks its $C_3$ symmetry, as it prefers a striped antiferromagnetic order (AABB-AFM) along with an intralayer antiferroelectric (AFE) order. However, substrate confinement may preserve the $C_3$ symmetry and/or apply tensile strain to the ML. This leads to another spiral magnetic order ($\mathrm{spiral}-\mathrm{I}{\mathrm{V}}^X$), while bilayer shows a different order ($\mathrm{spiral}-{\mathrm{V}}^X$) and spiral-B dominates in thicker layers. Thus, three multiferroic phases, namely, spiral-B+FE, $\mathrm{spiral}-\mathrm{I}{\mathrm{V}}^X+\mathrm{FE}, \mathrm{spiral}-{\mathrm{V}}^X+\mathrm{FE}$, and an antimultiferroic AABB-AFM+AFE one, show layer thickness dependence and geometry-dependent dominance, ascribed to competition among thickness-dependent Kitaev, biquadratic, and Heisenberg spin-exchange interactions and single-ion magnetic anisotropy. Our theoretical results clarify the debate on the multiferroicity of ML $\mathrm{Ni}{\mathrm{I}}_2$ and shed light on the role of layer stacking induced changes in noncollinear spin-exchange interactions and magnetic anisotropy in thickness-dependent magnetism.

• Received 6 June 2023
• Revised 30 April 2024
• Accepted 2 May 2024

DOI:https://doi.org/10.1103/PhysRevB.109.195422