Multiferroicity in NiI2
Fig.4 Layer-dependent magnetic groundstate of NiI2
Arxiv
Varied competition among three multiferroic phases of NiI2 from the bulk to the monolayer limit
Bulk NiI2 undergoes two successive magnetic phase transitions from a para-magnetic (PM) phase to an interlayer antiferromagnetic (AFM) phase at TN1 = 76 K and then to a spiral magnetic phase below TN2 = 59.5 K [12]. The AFM-to-spiral transition is accompanied by both rotational symmetry and inversion symmetry breaks, resulting in electric polarization through inverse DM interaction, as reflected in second harmonic generation (SHG) [13] and birefringence signals [14]
The monolayer (ML) of NiI2 was recently suggested to be a type-II multiferroic material, based on a presumed magnetic configuration and a supposed origin of the enhanced SHG signal. We found that such an assumption is flawed at the monolayer limit where a freestanding ML NiI2 showing broken C3 symmetry prefers to a striped antiferromagnetic order (AABB-AFM) along with an intralayer antiferroelectric (AFE) order[15]. However, the C3 symmetry of the monolayer may preserve under a substrate confinement, which leads to a spiral magnetic order (Spiral-V), a different spiral order from that of the bulk counterpart (Spiral-B). The Spiral-V order persists up to 2L thickness with the C3 symmetry and shows ferroelectricity (FE) ascribed to the inversed DM interaction. Thus, those three type-II multiferroic phases, namely Spiral-B+FE, Spiral-V+FE and AABB-AFM+AFE, emerge for NiI2 with different layer numbers and structural symmetries.
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