Experimental realization of a single-layer multiferroic*

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 inverse Dzyaloshinskii-Moriya effects. 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 magnetoelectriccoupling 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. We use circular dichroic Raman measurements to directly probe the magneto-chiral ground state and its long-lived electromagnon modes which reflect the presence of dynamic magnetoelectric coupling. From birefringence and second-harmonic generation measurements, we detect a highly anisotropic electronic state simultaneously breaking three-fold rotationaland 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.