[论文解读] Spontaneous topological Hall effect induced by non-coplanar antiferromagnetic order in intercalated van der Waals materials

H. Takagi, R. Takagi+15 more|arXiv (Cornell University)|Mar 8, 2023

Advanced Condensed Matter Physics被引用 5

一句话总结

本研究在插层范德华材料CoM₃S₆（M = Nb, Ta）中发现了一种植入的拓扑霍尔效应，其源于具有有限标量自旋手性的非共面反铁磁序。通过偏振中子散射与第一性原理计算，作者证实了全入全出的非共面自旋纹理，该纹理产生一种赝磁质，即使净磁化强度接近零，也能诱导出巨大的拓扑霍尔响应，从而为反铁磁自旋电子学器件开辟了新平台。

ABSTRACT

In ferromagnets, electric current generally induces transverse Hall voltage in proportion to magnetization (anomalous Hall effect), and it is frequently used for electrical readout of the up and down spin states. While these properties are usually not expected in antiferromagnets, recent theoretical studies predicted that non-coplanar antiferromagnetic order with finite scalar spin chirality (i.e. solid angle spanned by neighboring spins) can often induce large spontaneous Hall effect even without net magnetization or external magnetic field. This phenomenon, i.e. spontaneous topological Hall effect, can potentially be used for the efficient electrical readout of the antiferromagnetic states, but its experimental verification has long been elusive due to the lack of appropriate materials hosting such exotic magnetism. Here, we report the discovery of all-in-all-out type non-coplanar antiferromagnetic order in triangular lattice compounds CoTa3S6 and CoNb3S6, by performing the detailed magnetic structure analysis based on polarized neutron scattering experiments as well as systematic first-principles calculations. These compounds are reported to host unconventionally large spontaneous Hall effect despite their vanishingly small net magnetization, and our analysis revealed that it can be well explained in terms of topological Hall effect, which originates from the fictitious magnetic field associated with scalar spin chirality in non-coplanar antiferromagnetic orders. The present results indicate that the scalar spin chirality mechanism can offer a promising route to realize giant spontaneous Hall response even in compensated antiferromagnets, and highlight intercalated van der Waals magnets as an unique quasi-two-dimensional material platform to enable various nontrivial manner of electrical reading and possible writing of non-coplanar antiferromagnetic domains.

研究动机与目标

• 确定在净磁化强度可忽略的情况下，CoM₃S₆（M = Nb, Ta）中异常大自发霍尔效应的微观起源。
• 通过实验验证非共面反铁磁序，解决CoM₃S₆磁结构长期存在的争议。
• 建立补偿型反铁磁体中无净磁化强度下标量自旋手性与拓扑霍尔效应之间的关联。
• 证明插层范德华材料为实现反铁磁体系中拓扑响应提供了独特的二维平台。
• 验证理论预测：非共面自旋纹理可通过贝里相位产生赝磁质，从而导致拓扑霍尔效应。

提出的方法

• 开展偏振中子散射实验，探测磁结构因子的面内与面外分量，区分自旋翻转与非自旋翻转散射通道。
• 进行飞行时间中子劳厄衍射，测量198个磁性衍射峰，并比较观测值与计算值的磁结构因子。
• 采用第一性原理密度泛函理论（DFT）计算，评估可能的磁结构及其与实验数据的兼容性。
• 计算标量自旋手性χ_ijk = S_i · (S_j × S_k)，以量化几何相位及其对赝磁质的贡献。
• 在零磁场及外加磁场条件下，分析霍尔电阻率ρ_xy与纵向电阻率ρ_xx，以提取自发霍尔响应。
• 基于磁化强度、磁化率与电阻率随温度和磁场的变化测量，构建磁相图。

实验结果

研究问题

• RQ1 CoM₃S₆（M = Nb, Ta）的真实磁结构是什么？该结构如何在净磁化强度可忽略的情况下产生大的自发霍尔效应？
• RQ2 该体系中具有有限标量自旋手性的非共面反铁磁序能否通过实验手段得以证实？
• RQ3 CoM₃S₆中的拓扑霍尔效应在多大程度上源于非共面自旋纹理相关的几何贝里相位？
• RQ4 全入全出自旋构型中的标量自旋手性如何产生赝磁质，从而诱导出可测量的霍尔响应？
• RQ5 插层范德华材料是否能在无净磁化强度或自旋-轨道耦合的条件下，实现巨大的拓扑响应？

主要发现

• 在相I中，CoTa₃S₆的磁结构被确认为调制矢量q = (1/2, 0, 0)的全入全出非共面反铁磁序。
• 偏振中子散射数据明确显示自旋纹理同时具有面内与面外分量，证实了该序的非共面特性。
• 中子劳厄衍射测得的磁结构因子与全入全出构型的计算结果高度一致，验证了所提出的自旋纹理。
• 在2.2 K时，自发霍尔电阻率可达约100 nΩ·cm，表明尽管净磁化强度可忽略，仍存在巨大的拓扑霍尔响应。
• 标量自旋手性χ_ijk为有限值且具有空间调制特性，其产生的赝磁质通过贝里相位积累诱导出拓扑霍尔效应。
• 该体系表现出两种时间反演对称性相反的态，沿c轴方向的有效磁质符号相反，从而可实现反铁磁态的电学读出。