Competing multiferroic phases in monolayer and few-layer NiI2

A recent experiment reported type-II multiferroicity in monolayer (ML) NiI₂ based on a presumed spiral magnetic configuration (spiral-B), which is, as we found here, under debate in the ML limit. Freestanding ML NiI₂ breaks its C₃ symmetry, as it prefers a striped antiferromagnetic order (AABB-AFM) along with an intralayer antiferroelectric (AFE) order. However, substrate confinement may preserve the C₃ symmetry and/or apply tensile strain to the ML. This leads to another spiral magnetic order (spiral-IVX), while bilayer shows a different order (spiral-VX) and spiral-B dominates in thicker layers. Thus, three multiferroic phases, namely, spiral-B+FE, spiral-IVX+FE, spiral-VX+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 NiI₂ and shed light on the role of layer stacking induced changes in noncollinear spin-exchange interactions and magnetic anisotropy in thickness-dependent magnetism.