  A recent experiment reported type-II multiferroicity in monolayer (ML)
NiI$_{2}$ based on a presumed spiral magnetic configuration (Spiral-B), which
is, as we found here, under debate in the ML limit. Freestanding ML NiI$_{2}$
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 (Spiral-$IV^X$),
while 2L shows a different order (Spiral-$V^Y$) and Spiral-B dominates in
thicker layers. Thus, three multiferroic phases, namely, Spiral-B+FE,
Spiral-$IV^X$ +FE, Spiral-$V^Y$+FE, and an anti-multiferroic AABB-AFM+AFE one,
show layer-thickness-dependent and geometry-dependent dominance, ascribed to
competitions 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 NiI$_{2}$ and shed
light on the role of layer-stacking-induced changes in noncollinear
spin-exchange interactions and magnetic anisotropy in thickness-dependent
magnetism.
