[论文解读] Layer thickness crossover of type-II multiferroic magnetism in NiI2

Shuang Wu, Xinyu Chen+7 more|arXiv (Cornell University)|Jul 20, 2023

Multiferroics and related materials被引用 7

一句话总结

本研究通过SHG和拉曼光谱揭示了NiI2中类型-II多铁磁性在层厚依赖性上的转变，发现双层及更厚的薄膜因层间交换耦合破坏自旋简并而表现出磁致二次谐波产生（SHG）效应，而单层NiI2则无此响应。极化分辨SHG与拉曼光谱证实层间交换作用在稳定螺旋磁有序和铁电性方面起关键作用，三明治层NiI2表现出体相行为。

ABSTRACT

The discovery of atomically thin van der Waals ferroelectric and magnetic materials encourages the exploration of 2D multiferroics, which holds the promise to understand fascinating magnetoelectric interactions and fabricate advanced spintronic devices. In addition to building a heterostructure consisting of ferroelectric and magnetic ingredients, thinning down layered multiferroics of spin origin such as NiI2 becomes a natural route to realize 2D multiferroicity. However, the layer-dependent behavior, widely known in the community of 2D materials, necessitates a rigorous scrutiny of the multiferroic order in the few-layer limit. Here, we interrogate the layer thickness crossover of helimagnetism in NiI2 that drives the ferroelectricity and thereby type-II multiferroicity. By using wavelength-dependent polarization-resolved optical second harmonic generation (SHG) to probe the ferroic symmetry, we find that the SHG arises from the inversion-symmetry-breaking magnetic order, not previously assumed ferroelectricity. This magnetism-induced SHG is only observed in bilayer or thicker layers, and vanishes in monolayer, suggesting the critical role of interlayer exchange interaction in breaking the degeneracy of geometrically frustrated spin structures in triangular lattice and stabilizing the type-II multiferroic magnetism in few-layers. While the helimagnetic transition temperature is layer dependent, the few-layer NiI2 exhibits another thickness evolution and reaches the bulk-like behavior in trilayer, indicated by the intermediate centrosymmetric antiferromagnetic state as revealed in Raman spectroscopy. Our work therefore highlights the magnetic contribution to SHG and Raman spectroscopy in reduced dimension and guides the optical study of 2D multiferroics.

研究动机与目标

• 研究NiI2中类型-II多铁磁性对层厚的依赖性。
• 确定少层NiI2中二次谐波产生（SHG）的起源，并区分铁电性与磁序作为其来源。
• 阐明层间交换相互作用在解除NiI2三角晶格中几何自旋简并方面的角色。
• 明确少层NiI2中磁序及其转变温度的厚度演化行为。
• 通过建立磁致非线性光学响应对SHG和拉曼信号的贡献，为二维多铁材料的光学表征提供指导。

提出的方法

• 采用波长依赖、极化分辨的二次谐波产生（SHG）技术，探测少层NiI2中铁电对称性破缺。
• 利用拉曼光谱识别少层体系的磁相和对称性。
• 研究聚焦于双层、三层和单层NiI2，以比较其层依赖的磁性和电子性质。
• 理论解释将SHG响应与螺旋磁序引起的反演对称性破缺联系起来，而非铁电性。
• 系统比较不同厚度下SHG强度与拉曼模式，揭示磁序的转变行为。
• SHG极化各向异性分析证实了非线性光学响应的磁性起源。

实验结果

研究问题

• RQ1 少层NiI2中二次谐波产生（SHG）信号的起源是什么——铁电性还是磁序？
• RQ2 层间交换相互作用如何影响少层NiI2中螺旋磁序的稳定性？
• RQ3 NiI2中磁序从单层向双层/三层行为转变的临界厚度是多少？这一转变如何影响多铁性？
• RQ4 NiI2中螺旋磁转变温度是否表现出非单调的厚度依赖性？
• RQ5 SHG和拉曼光谱响应如何随层厚演化？它们揭示了哪些关于磁相的信息？

主要发现

• 极化分辨测量证实，NiI2中的SHG源于螺旋磁序引起的反演对称性破缺，而非铁电性。
• SHG信号仅在双层或更厚的NiI2中观测到，在单层中完全消失，表明层间交换耦合起关键作用。
• 层间交换相互作用通过解除三角晶格中几何自旋简并的简并度，稳定了类型-II多铁磁态。
• 少层NiI2中螺旋磁转变温度表现出非单调的厚度依赖性，三明治层样品表现出体相行为。
• 拉曼光谱揭示三明治层NiI2中存在中间的中心对称反铁磁态，表明其具有独特的磁相演化路径。
• 本研究确立了磁序而非铁电性，是二维多铁材料（如NiI2）中光学非线性响应的主要贡献者。