Microscopic origin of multiferroic order in monolayer NiI₂

Abstract

The discovery of multiferroic behavior in monolayer NiI2 provides a new symmetry-broken state in van der Waals monolayers, featuring the simultaneous emergence of helimagnetic order and ferroelectric order at a critical temperature of T=21 K. However, the microscopic origin of multiferroic order in NiI2 monolayer has not been established, and in particular, the role of non-collinear magnetism and spin-orbit coupling in this compound remains an open problem. Here we reveal the origin of the two-dimensional multiferroicity in NiI2 using first-principles electronic structure methods. We show that the helimagnetic state appears as a consequence of the long-range magnetic exchange interactions, featuring sizable magnetic moments in the iodine atoms. We demonstrate that the electronic density reconstruction accounting for the ferroelectric order emerges from the interplay of non-collinear magnetism and spin-orbit coupling. We demonstrate that the ferroelectric order is controlled by the iodine spin-orbit coupling, and leads to an associated electronically-driven distortion in the lattice. Our results establish the microscopic origin of the multiferroic behavior in monolayer NiI2, putting forward the coexistence of helical magnetic order and ligand spin-orbit coupling as driving forces for multiferroic behavior in two-dimensional materials.

Multiferroicity in a Two-Dimensional Non-van der Waals Crystal of AgCr2X4 (X = S or Se)

“…The potential type II multiferroicity of monolayer and few-layer NiI 2 could be derived from the electric polarization induced by spin helical order. 26 In addition, multiferroics can also be achieved by designing extrinsic FM−FE multiferroic van der Waals (vdW) heterostructures by integrating these two Over the past five years, researchers have made a significant breakthrough in achieving 2D magnetic ultrathin films from non-vdW materials, 29,30 which overcomes the limitation of both mechanical exfoliation and layered crystals of bulk counterparts. 31−40 To date, 2D non-vdW crystals of CrSe, 31 CrTe, 32 CrTe 2 , 33 Cr 2 Te 3 , 34 MnTe, 35 FeSe, 36 α-MnSe 2 , 37 Fe 7 Se 8 , 38 and VO 2 39 have been successfully synthesized, exhibiting a high Curie or Neél temperature.…”

“…Notably, Song et al and Ju et al have recently found experimental evidence of intrinsic multiferroicity in monolayer and bilayer NiI 2 . The potential type II multiferroicity of monolayer and few-layer NiI 2 could be derived from the electric polarization induced by spin helical order . In addition, multiferroics can also be achieved by designing extrinsic FM–FE multiferroic van der Waals (vdW) heterostructures by integrating these two kinds of components together.…”

Topological multiferroic order in twisted transition metal dichalcogenide bilayers

“…For the particular case of electric and magnetic orders, a magnetoelectric coupling [34] provides a venue for the electric control of magnetic orders, a feature with a huge potential interest. Different multiferroic mechanisms have been studied over the past years, both in bulk compounds [35][36][37] and recently in two-dimensional monolayers [38][39][40]. In the realm of moiré materials, the strongly correlated states emerging in twisted systems provide an additional platform to artificially engineer multiferroics associated with the moiré length scale.…”

Microscopic origin of magnetism in monolayer $3d$ transition metal dihalides

“…For example, type-II multiferroicity was reported in bulk NiBr 2 22 , NiI 2 23 and MnI 2 24,25 , where the onset of helimagnetic phases breaks inversion symmetry leading to spin-induced ferroelectricity. Such a multiferroic phase has been recently reported to survive down to the monolayer limit of NiI 2 26,27 , while monolayer NiBr 2 and NiCl 2 have been proposed as half-excitonic insulators 28 . The electronic structure of the 3d M X 2 compounds has been recently analyzed 29,30 ; however the magnetic exchange couplings and SOC-driven anisotropic contributions have not been fully addressed and require deeper investigation.…”