拓扑绝缘体与铁磁金属界面磁斯格明子的理论研究

Since the Nobel Prize in physics was awarded to three physicists for theoretical discoveries of topological phase transitions and topological phases of matter, a flurry of research on the topology of both moment space and real space were initialized. Magnetic skyrmion, a particle-like swirling-spin configuration characterized by a topological index called the skyrmion number in a magnetic material is attracting enormous attention. The nontrivial topology of magnetic skyrmion can result in a number of intriguing electromagnetic phenomena, including the topological Hall effect, skyrmion magnetic resonance and thermal induced ratchet motion. These topologically protected properties, together with their nanoscale dimensions (10-100nm), ultralow threshold for current-driven motion, behaviour as particles that can be moved, created and annihilated, all make them suitable for applications as low-energy memory and logic elements for conventional and neuromorphic computing applications that go beyond Moore’s law. However, a prerequisite for the use of skyrmions in devices is the ability to stabilize small individual skyrmions at room temperature and in zero or very small applied fields. Here, in this proposal, we will use the approach we developed for DMI calculations based on ab initio DFT calculations to evaluate the DMI of the bilayer and its distribution in the successive atomic layers of FM and topological insulators(TI). We'll study the main features and microscopic mechanisms of DMI in FM/TI bilayers. Then we will combine the first-principles calculated DMI and magnetic anisotropy to do micromagnetic simulations to study the creation, annihilation, and motion of skyrmions at the FM/TI interfaces. The proposal will pave the way for both topological insulator topic and magnetic skyrmions field.

对材料自旋结构的设计、构筑以及调控一直是自旋电子学中的核心问题。实空间的拓扑磁结构-磁斯格明子，因受到拓扑保护具有非易失、尺寸小、低功耗、在超低电流驱动下即可迅速运动等诸多优点，是下一代磁存储的重要发展方向[Rev.Mod.Phys.89,025006;Nature,539,509(2016)]。然而如何实现室温稳定可控的磁斯格明子还是一大挑战,其物理本质乃是找到适合的Dzyaloshinskii-Moriya(DM)相互作用以及垂直磁各向异性(PMA)的界面材料。本申请项目利用我们发展的第一性原理计算界面DMI的方法[PRL115,267210(2015)]，结合拓扑绝缘体(TI)的诸多优点来研究其与铁磁金属(FM)接触时界面的DMI及PMA的物性，并进一步做基于第一性原理的微磁模拟，探索在TI/FM体系实现磁斯格明子的物理机制、室温稳定性、尺度大小、输运性质以及调控方法。

我们围绕磁性界面或二维磁体中的DM相互作用展开研究，在本自然科学基金面上项目的有力支持下，取得了一下研究成果：（1）首次提出对称性破缺的Janus结构MnXY(X/Y = S, Se, Te, X ≠ Y)中具有DM相互作用。该强度与传统铁磁/重金属异质结中的DM相互作用值可比；利用蒙特卡洛算法对该材料体系进行了微磁模拟，发现在零场下MnSeTe与MnSTe中出现了手性磁畴壁，并通过增加外磁场的方式诱导出了磁斯格明子。（2）利用二维多铁材料CrN内禀的Rashba效应，不仅诱导出大的DM相互作用，还实现了电场调控磁斯格明子，该工作预言了二维材料中通过多铁性实现磁斯格明子的一体化电学调控。（3）发现了一系列本征具有对称性破缺的二维磁体AX2 (A：3d过度金属元素；X：第四主族和第五主族元素)及MCuX2 (A：3d过度金属元素；X：第四主族元素)，并实现了多种各向异性拓扑磁结构，包括：铁磁/反铁磁反斯格明子，反铁磁磁涡旋-反涡旋对等。（4）理论证明了Pt/Co/Ta多层中椭圆型斯格明子存在主要与DM相互作用和PMA有关。（5）基于Fert-Lévy三位点模型证明了CoPt或FePt、CoCu或FeCu、FeGd和FeNi等较厚的薄膜中成分梯度诱导磁体不对称性和自旋-轨道耦合(SOC)对DMI的贡献。

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