Song Qian, Occhialini Connor A, Ergeçen Emre, Ilyas Batyr, Amoroso Danila, Barone Paolo, Kapeghian Jesse, Watanabe Kenji, Taniguchi Takashi, Botana Antia S, Picozzi Silvia, Gedik Nuh, Comin Riccardo
Department of Physics, Massachusetts Institute of Technology, Cambridge, MA, USA.
Department of Material Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA.
Nature. 2022 Feb;602(7898):601-605. doi: 10.1038/s41586-021-04337-x. Epub 2022 Feb 23.
Multiferroic materials have attracted wide interest because of their exceptional static and dynamical 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 effect. This intrinsic coupling between the magnetic and dipolar order parameters results in high-strength magnetoelectric effects. Two-dimensional materials possessing such intrinsic multiferroic properties have been long sought for to enable the harnessing of 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 NiI. The multiferroic state of NiI 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 NiI. These observations establish NiI and transition metal dihalides as a new platform for studying emergent multiferroic phenomena, chiral magnetic textures and ferroelectricity in the two-dimensional limit.
多铁性材料因其独特的静态和动态磁电特性而备受广泛关注。特别是，II型多铁性材料呈现出一种打破反演对称性的磁序，该磁序通过各种机制，如自旋电流或反Dzyaloshinskii-Moriya效应，直接诱导铁电极化。磁序参数与偶极序参数之间的这种内在耦合导致了高强度的磁电效应。长期以来，人们一直在寻找具有这种内在多铁性特性的二维材料，以便在纳米电子器件中利用磁电耦合。在此，我们报告在基于过渡金属的范德华材料NiI的单原子层中发现了II型多铁性序。NiI的多铁性状态由具有特定手性的适当螺旋自旋螺旋表征，它与电荷自由度耦合以产生手性控制的电极化。我们使用圆二色拉曼测量直接探测磁手性基态及其源自动态磁电耦合的电磁子模式。结合双折射和二次谐波产生测量与理论建模和模拟，我们检测到一种高度各向异性的电子态，它同时打破了三重旋转对称性和反演对称性，并支持极性序。光学特征随温度和层数的演变令人惊讶地揭示了一种有序的磁极化状态，这种状态一直持续到单层NiI的超薄极限。这些观察结果将NiI和过渡金属二卤化物确立为研究二维极限下新兴多铁性现象、手性磁纹理和铁电性的新平台。
