Evidence for a single-layer van der Waals multiferroic

Qian Song, Connor A. Occhialini, Emre Ergeçen, Batyr Ilyas, Danila Amoroso, Paolo Barone, Jesse Kapeghian, Kenji Watanabe, Takashi Taniguchi, Antia S. Botana, Silvia Picozzi, Nuh Gedik and Riccardo Comin ()
Additional contact information
Qian Song: Massachusetts Institute of Technology
Connor A. Occhialini: Massachusetts Institute of Technology
Emre Ergeçen: Massachusetts Institute of Technology
Batyr Ilyas: Massachusetts Institute of Technology
Danila Amoroso: c/o Università degli Studi ‘G. D’Annunzio’
Paolo Barone: Area della Ricerca di Tor Vergata
Jesse Kapeghian: Arizona State University
Kenji Watanabe: National Institute for Materials Science
Takashi Taniguchi: National Institute for Materials Science
Antia S. Botana: Arizona State University
Silvia Picozzi: c/o Università degli Studi ‘G. D’Annunzio’
Nuh Gedik: Massachusetts Institute of Technology
Riccardo Comin: Massachusetts Institute of Technology

Nature, 2022, vol. 602, issue 7898, 601-605

Abstract: Abstract Multiferroic materials have attracted wide interest because of their exceptional static1–3 and dynamical4–6 magnetoelectric properties. In particular, type-II multiferroics exhibit an inversion-symmetry-breaking magnetic order that directly induces ferroelectric polarization through various mechanisms, such as the spin-current or the inverse Dzyaloshinskii–Moriya effect3,7. This intrinsic coupling between the magnetic and dipolar order parameters results in high-strength magnetoelectric effects3,8. Two-dimensional materials possessing such intrinsic multiferroic properties have been long sought for to enable the harnessing of magnetoelectric coupling in nanoelectronic devices1,9,10. 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. Combining birefringence and second-harmonic-generation measurements with theoretical modelling and simulations, we detect a highly anisotropic electronic state that simultaneously breaks three-fold rotational and inversion symmetry, and supports 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.

Date: 2022
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DOI: 10.1038/s41586-021-04337-x

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