Evidence for a single-layer van der Waals multiferroic

TL;DR

This paper reports the discovery of a single-layer van der Waals material, NiI2, exhibiting type-II multiferroic order with coupled magnetic and electric polarization, persistent down to monolayer thickness, opening new avenues for 2D multiferroic research.

Contribution

It provides the first evidence of intrinsic multiferroic order in a single atomic layer of NiI2, demonstrating coupled magnetic and electric properties in 2D transition metal dihalides.

Findings

NiI2 exhibits a proper-screw spin helix with handedness.

Magneto-chiral ground state confirmed by circular dichroic Raman.

Polar order persists in monolayer NiI2.

Abstract

Multiferroic materials have garnered wide interest for their exceptional static and dynamical magnetoelectric properties. In particular, type-II multiferroics exhibit an inversion-symmetry-breaking magnetic order which directly induces a ferroelectric polarization through various mechanisms, such as the spin-current or the inverse Dzyaloshinskii-Moriya effect. This intrinsic coupling between the magnetic and dipolar order parameters results in record-strength magnetoelectric effects. Two dimensional materials possessing such intrinsic multiferroic properties have been long sought for harnessing magnetoelectric coupling in nanoelectronic devices. Here, we report the discovery of type-II multiferroic order in a single atomic layer of the transition metal-based van der Waals material NiI2. The multiferroic state of NiI2 is characterized by a proper-screw spin helix with given handedness, which couples to the charge degrees of freedom to produce a chirality-controlled electrical polarization. We use circular dichroic Raman measurements to directly probe the magneto-chiral ground state and its electromagnon modes originating from dynamic magnetoelectric coupling. Using birefringence and second-harmonic generation measurements, we detect a highly anisotropic electronic state simultaneously breaking three-fold rotational and inversion symmetry, and supporting polar order. The evolution of the optical signatures as a function of temperature and layer number surprisingly reveals an ordered magnetic, polar state that persists down to the ultrathin limit of monolayer NiI2. These observations establish NiI2 and transition metal dihalides as a new platform for studying emergent multiferroic phenomena, chiral magnetic textures and ferroelectricity in the two-dimensional limit.