[论文解读] Hall micromagnetometry of individual two-dimensional ferromagnets

Minsoo Kim, Piranavan Kumaravadivel+15 more|arXiv (Cornell University)|Feb 19, 2019

2D Materials and Applications参考文献 28被引用 4

一句话总结

本研究提出了一种基于可重复使用石墨烯霍尔棒的弹道霍尔微磁测量技术，用于探测单个二维铁磁体，结果表明单层CrBr3在具有高垂直各向异性的条件下仍保持铁磁性，且居里温度随厚度变化较弱，这与二维伊辛模型的预测相悖，并揭示了原子级范德华异质结构中独特的磁行为。

ABSTRACT

The recent advent of atomically-thin ferromagnetic crystals has allowed experimental studies of two-dimensional (2D) magnetism that not only exhibits novel behavior due to the reduced dimensionality but also often serves as a starting point for understanding of the magnetic properties of bulk materials. Here we employ ballistic Hall micromagnetometry to study magnetization of individual 2D ferromagnets. Our devices are multilayer van der Waals (vdW) heterostructures comprising of an atomically-thin ferromagnetic crystal placed on top of a Hall bar made from encapsulated graphene. 2D ferromagnets can be replaced repeatedly, making the graphene-based Hall magnetometers reusable and expanding a range of their possible applications. The technique is applied for the quantitative analysis of magnetization and its behavior in atomically thin CrBr3. The compound is found to remain ferromagnetic down to a monolayer thickness and exhibit high out-of-plane anisotropy. We report how the critical temperature changes with the number of layers and how domain walls propagate through the ultimately thin ferromagnets. The temperature dependence of magnetization varies little with thickness, in agreement with the strongly layered nature of CrBr3. The observed behavior is markedly different from that given by the simple 2D Ising model normally expected to describe 2D easy-axis ferromagnetism. Due to the increasingly common usage of vdW assembly, the reported approach offers vast possibilities for investigation of 2D magnetism and related phenomena.

研究动机与目标

• 开发一种可在原子尺度上探测单个二维铁磁体的可重复使用、高灵敏度技术。
• 研究不同层数下原子级薄CrBr3的磁化行为。
• 理解临界温度和磁各向异性在二维铁磁体中的演化规律。
• 研究超薄铁磁层中畴壁的动力学行为。
• 检验二维伊辛模型在描述真实二维易轴铁磁体（如CrBr3）时的有效性。

提出的方法

• 制备多层范德华异质结构，将原子级薄的铁磁晶体（如CrBr3）置于石墨烯霍尔棒上。
• 利用封装石墨烯中的弹道霍尔效应，探测单个二维铁磁体产生的局部磁场。
• 采用可重复使用的器件结构，实现二维铁磁层的多次更换。
• 通过测量霍尔电压随外加磁场和温度的变化，实现磁化的定量分析。
• 分析温度依赖的磁化曲线及超薄薄膜中畴壁的传播行为。
• 将实验结果与二维伊辛模型的预测进行对比，以评估该模型的有效性。

实验结果

研究问题

• RQ1 尽管极度减薄，单层CrBr3是否仍保持铁磁有序？
• RQ2 CrBr3的临界温度如何随层数变化？
• RQ3 单层CrBr3的磁矩大小及其各向异性如何？
• RQ4 在原子级薄铁磁体中，畴壁如何移动和演化？
• RQ5 CrBr3的观测行为是否与二维伊辛模型的预测一致？

主要发现

• 单层CrBr3在低温下仍保持铁磁性，表明其在单原子层下具有稳定的磁序。
• CrBr3表现出强烈的垂直磁各向异性，与易轴铁磁性一致。
• CrBr3的临界温度随层数变化极小，表明其具有强烈的层状磁性特征。
• 磁化率的温度依赖性几乎与厚度无关，与简单二维伊辛模型的预期相反。
• 超薄CrBr3中的畴壁传播动力学与二维伊辛模型的预测显著不同。
• 实验结果与二维伊辛模型存在显著偏差，表明真实二维铁磁体中可能存在额外的磁相互作用或各向异性机制。