npj: 单层磁性—原来是Kitaev相互作用搞的鬼！

Original: npj 知社学术圈 2019-06-30

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二维（2D）磁性在自然界中很少见，但却是自旋电子学必不可少的内容，也是前沿基础研究的重要方面，要维持2D磁性的稳定，其关键要素是磁各向异性。

最近对CrI₃和CrGeTe₃单层膜的研究，不仅实现了人们一直在寻求的2D铁磁性，还揭示了两个系统中不同的磁各向异性，即CrI₃的Ising行为与CrGeTe₃的Heisenberg行为。这两种材料的结构和电子性质非常相似，这与他们迥异的磁各向异性形成了强烈对比。若能在微观尺度上理解其机理，将对相关研究大有裨益。来自中国复旦大学的向红军教授和美国阿肯色大学的Laurent Bellaiche教授等，通过第一性原理计算，阐明了两种相似的二维铁磁材料表现出不同磁各向异性的机理。他们提出了描述这两种体系磁性的统一哈密顿量，并结合紧束缚模型从微观上研究单层CrI₃和CrGeTe₃磁各向异性。他们惊奇地发现，CrI₃和CrGeTe₃两种单层中二维磁性的起源，是Kitaev各向异性与单离子各向异性之间不同的共同作用方式引起的。他们进一步发现，Kitaev相互作用和单离子各向异性，都是通过重配体（CrI₃的I或CrGeTe₃的Te）p轨道的自旋-轨道耦合而诱导的，而并非是传统的磁性Cr离子3d轨道的自旋-轨道耦合。看来，在非传统体系中研究Kitaev相互作用有助于更好地发现和理解有趣的物理现象。

该文近期发表于npj Computational Materials 4: 57 (2018)，英文标题与摘要如下，点击左下角“阅读原文”可以自由获取论文PDF。

Interplay between Kitaev interaction and single ionanisotropy in ferromagnetic CrI₃ and CrGeTe₃ monolayers

Changsong Xu, Junsheng Feng, Hongjun Xiang & Laurent Bellaiche

Magnetic anisotropy is crucially important for thestabilization of two-dimensional (2D) magnetism, which is rare in nature buthighly desirable in spintronics and for advancing fundamental knowledge.Recent works on CrI₃ and CrGeTe₃ monolayersnot only led to observations of the long-time-sought 2D ferromagnetism, butalso revealed distinct magnetic anisotropy in the two systems, namely Isingbehavior for CrI₃ versus Heisenberg behavior for CrGeTe₃.Such magnetic difference strongly contrasts with structural and electronicsimilarities of these two materials, and understanding it at a microscopicscale should be of large benefits. Here, first-principles calculations areperformed and analyzed to develop a simple Hamiltonian, to investigate magneticanisotropy of CrI₃ and CrGeTe₃monolayers. The anisotropicexchange coupling in both systems is surprisingly determined to be ofKitaev-type. Moreover, the interplay between this Kitaev interaction and singleion anisotropy (SIA) is found to naturally explain the different magneticbehaviors of CrI₃ and CrGeTe₃. Finally, both theKitaev interaction and SIA are further found to be induced by spin–orbitcoupling of the heavy ligands (I of CrI₃ or Te of CrGeTe₃)rather than the commonly believed 3d magnetic Cr ions.