近年来，由于在单层极限下具有奇异的性质和潜在的应用前景，二维磁性材料成为备受关注的研究热点，为下一代尺寸更小、速度更快、能耗更低的自旋电子学器件的实现提供了新的可能。早前，研究人员通过在非磁性二维层状材料中引入缺陷或利用近邻效应来诱导磁性。直到最近，本征的二维铁磁性才在CrI3和Cr2Ge2Te6单层中首次被发现，掀起了层状材料磁性研究的热潮。目前为止，大多数的二维铁磁材料都是通过机械剥离法获得，其厚度和尺寸均不易控制。考虑到外延生长的单层薄膜的表面均匀性、厚度可调性，以及与成熟的硅技术集成的重要性，直接在Si衬底上生长出铁磁性单层薄膜是很有必要的，并且在基础研究和器件应用方面都有重大意义。

最近，南京大学李绍春教授课题组与吴镝教授、孙建教授课题组合作，通过引入原子层级缓冲层的方法，成功地在Si衬底上生长出了MnSn单层薄膜。另外，MnSn的厚度可控，且磁性表现出明显的厚度依赖性，为进一步探索和调控二维铁磁性提供了一个新的平台，也为将磁性单层集成到硅基技术中提供了可能性。

单层MnSn薄膜外延生长在Si衬底上的示意图以及不同温度下的M-H曲线。

以上工作已发表在CPL Express Letters栏目

Ferromagnetic MnSn Monolayer Epitaxially Grown on Silicon Substrate

Qian-Qian Yuan (袁茜茜), Zhaopeng Guo (郭照芃), Zhi-Qiang Shi (石志强), Hui Zhao (赵辉), Zhen-Yu Jia (贾振宇), Qianjin Wang (王前进), Jian Sun (孙建), Di Wu (吴镝), and Shao-Chun Li (李绍春)

Chin. Phys. Lett. 2020, 37 (7): 077502

应编辑部邀请，南佛罗里达大学Matthias Batzill教授为本文作了点评！

Matthias Batzill

Department of Physics, University of South Florida, Tampa, FL 33620, USA    

Chin. Phys. Lett. 2020, 37 (8): 080101

Van der Waals materials with ferromagnetic properties have attracted recent interest because of their promise to enable combination of materials with atomically sharp interfaces, facilitating spin transport and spin injection across these interfaces. The discovery of ferromagnetic ordering to persist to the two-dimensional (2D) limit for Cr2Ge2Te6 and CrI3 has sparked interest of 2D ferromagnetism and their potential for fundamentally new magnetic van der Waals heterostructures and spintronics applications. However, these materials have been exfoliated from bulk and are not easily processible for applications. A bottom up synthesis by direct growth is required to integrate different materials for practical applications and to enable detailed characterization of magnetic interfaces. The growth of VSe2 and MnSe2 by molecular beam epitaxy (MBE) have created hope that such materials can exhibit strong magnetism with high Curie temperatures. However, subsequent detailed work on the properties of VSe2 have indicated that it is not an itinerant magnetic material, and the observed magnetism in these materials remains a controversial topic. Other potential ferromagnetic materials that can be reduced to the 2D limit and grown by MBE are currently investigated. Such materials include chromium telluride-based compounds. More generally, direct growth methods, like MBE, allow the development of new 2D materials that are not inherently layered materials and thus would expand the possible materials systems for investigating 2D magnetism.

The paper by Yuan et al. neatly demonstrates the potential of MBE growth for the synthesis of novel ferromagnetic 2D materials. In this work, a single layer MnSn alloy on a Si(111) substrate is demonstrated to exhibit ferromagnetic properties in the 2D limit. MnSn has been predicted to be a half-metallic compound with a zinc-blende structure in the bulk. The zinc-blende structure has a 3D covalent bonding and thus may not be thinned to a monolayer by exfoliation. MnSn was directly grown on a Sn-buffer layer on Si(111), detailed STM characterization combined with crystal structure search allowed them to identify the new material they synthesized and showed that it can be grown in a structure with alternating Sn and Mn layers. Importantly, they show that a single molecular layer that is structurally the same as multilayer films can be obtained. Convincing agreement between the simulated STM images of the proposed structure with the experiments verified their obtained compound. Interestingly, this single monolayer exhibits large magnetic moments of ～2.3μB per Mn-atom and a Tc of ～54 K is measured. The Tc for the single layer is strongly reduced compared to the multilayer films that have a Tc～ 235–245 K. A strong decrease of Tc with the reduction of layers is expected due to thermal fluctuations destroying the magnetic ordering in single layer materials and thus this effect supports the notion of a monolayer ferromagnet. The fact that this material can be grown on silicon also shows potential of integrating it with the mature silicon technology. While the initial characterization of the MnSn single layer material is very promising, additional work will be needed to fully understand the magnetic properties of this material. For instance, the in-plane magnetic anisotropy and loss of anisotropy for the multilayers are surprising. Magnetic anisotropy is required to counteract the destruction of spin ordering by thermal fluctuation and usually an out-of plane easy axis is observed for 2D magnets. This may raise the question of the importance of the interface with the substrate to stabilize the magnetic ordering in this system.

Very few layered materials that can be exfoliated exhibit ferromagnetic ordering. Therefore, the development of new 2D materials that have no layered bulk counterparts are required. Such materials can only be synthesized as 2D materials by direct growth methods like MBE. Thus, these materials science methods are critically important to advance this field. The study by Yuan et al. showed that new exciting materials can be obtained, but for exploring more such systems the challenge remains in reliable theoretical predictions. And while theoretical guidance is necessary to motivate the synthesis of certain materials systems, another lesson from the work by Yuan et al. is that initial expectations for a compound may have to be adjusted for understanding the structure of the real material.

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