2017年，人们首次在二维层状材料中发现了在单原子层厚度具有稳定的铁磁和反铁磁性，为人们研究低维磁性的新奇物理效应提供了新的研究平台，也为发展新一代磁学器件提供了新的机遇。在二维磁性材料中，层间磁耦合作用的能量可与层间范德瓦尔斯作用接近甚至达到同一量级，层间堆叠结构的改变对二维材料的磁性会产生显著影响，而磁结构的变化在理论上也能反向作用于堆叠结构，形成一种新的自旋和晶格耦合效应。为研究二维磁性材料的堆叠结构和磁结构及其相互耦合，项目提出利用非线性二次谐波效应对材料结构的对称性破缺极端敏感的特性，采用这一原位无损的光学技术表征晶格结构、堆叠结构和自旋结构。而且，这一非线性光学技术可以有效结合各种外场调控方式如电场和磁场等。项目拟进一步探究二维磁性的物理规律，获得明确的实验结论，帮助人们进一步厘清在二维磁性体系中尚未明晰的物理现象，为发展新一代电、光、磁学器件提供基础原理的支撑。

Since the discovery of two-dimensional (2D) van der Waals (vdW) materials, intensive research efforts have been paid on them. In 2017, a number of 2D magnets have been found and their magnetism could be stable down to the monolayer limit. The discovery creates new research playground for studying novel magnetic behavior in 2D system, and provides great opportunities for the possible application in spintronics. Because the energy of vdW interaction and interlayer magnetic coupling is on the same order of magnitude, 2D stacking order can affect the interlayer magnetic coupling, and stacking order may also be affected by the 2D magnetism which induces new spin-lattice coupling. In order to study the spin-lattice coupling in 2D vdW magnetic materials, this project proposes to use nonlinear second harmonic generation which is extremely sensitive to the structure symmetry breaking of the material, and can noninvasively measure the structure of lattice, layered stacking and spin order. This method can also combine with the control of multiple external stimuli such as external magnetic field and electric field. This project plans to obtain the first-hand experimental results, understand the physical mechanism for novel phenomena in 2D magnets, and provide the guiding principles for the development of next generation spintronic devices.