Discovery of intrinsic ferromagnetism in two-dimensional van der Waals crystals
Discovery of intrinsic ferromagnetism in two-dimensional van der Waals crystals
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DOI：
10.1038/nature22060
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发表时间：
2017-06-08
期刊：
影响因子：
64.8
通讯作者：
Zhang, Xiang
中科院分区：
文献类型：
--
作者：
Gong, Cheng;Li, Lin;Zhang, Xiang
二维货车德瓦尔斯晶体中长程铁磁有序的实现，结合其丰富的电子和光学性质，可能会带来新的磁性、磁电和磁光应用（1-4）。根据Mermin-Wagner定理（5），在二维系统中，长程磁序受到热涨落的强烈抑制;然而，这些热涨落可以被磁各向异性抵消。以前的努力，基于缺陷和成分工程（6-10），或邻近效应，引入磁响应仅局部或外部。在这里，我们报告内在的长程铁磁秩序在原始的Cr2 Ge 2 Te 6原子层，所揭示的扫描磁光克尔显微镜。在这种磁性柔软的二维货车德瓦尔斯铁磁体中，我们使用非常小的磁场（小于0.3特斯拉）实现了前所未有的转变温度（铁磁和顺磁状态之间）控制。这一结果是在对比的不敏感性的转变温度的磁场在三维制度。我们发现，小的外加磁场导致的有效各向异性，是远远大于近零的磁晶各向异性，打开了一个大的自旋波激发间隙。我们解释所观察到的现象，使用重整化自旋波理论，并得出结论，不寻常的领域的转变温度的依赖性是一个标志性的软，二维铁磁货车范德华晶体。Cr2 Ge 2 Te 6是一种近乎理想的二维海森堡铁磁体，因此将有助于研究基本的自旋行为，为探索超紧凑自旋电子学等新应用打开大门。
The realization of long-range ferromagnetic order in two-dimensional van der Waals crystals, combined with their rich electronic and optical properties, could lead to new magnetic, magnetoelectric and magneto-optic applications(1-4). In two-dimensional systems, the long-range magnetic order is strongly suppressed by thermal fluctuations, according to the Mermin-Wagner theorem(5); however, these thermal fluctuations can be counteracted by magnetic anisotropy. Previous efforts, based on defect and composition engineering(6-10), or the proximity effect, introduced magnetic responses only locally or extrinsically. Here we report intrinsic long-range ferromagnetic order in pristine Cr2Ge2Te6 atomic layers, as revealed by scanning magneto-optic Kerr microscopy. In this magnetically soft, two-dimensional van der Waals ferromagnet, we achieve unprecedented control of the transition temperature (between ferromagnetic and paramagnetic states) using very small fields (smaller than 0.3 tesla). This result is in contrast to the insensitivity of the transition temperature to magnetic fields in the three-dimensional regime. We found that the small applied field leads to an effective anisotropy that is much greater than the near-zero magnetocrystalline anisotropy, opening up a large spin-wave excitation gap. We explain the observed phenomenon using renormalized spin-wave theory and conclude that the unusual field dependence of the transition temperature is a hallmark of soft, two-dimensional ferromagnetic van der Waals crystals. Cr2Ge2Te6 is a nearly ideal two-dimensional Heisenberg ferromagnet and so will be useful for studying fundamental spin behaviours, opening the door to exploring new applications such as ultra-compact spintronics.