[论文解读] 2D correlations in the van der Waals ferromagnet CrBr$_{3}$ using high frequency electron spin resonance spectroscopy

C. L. Saiz, J. A. Delgado+4 more|arXiv (Cornell University)|Oct 29, 2020

2D Materials and Applications参考文献 46被引用 12

一句话总结

本研究采用高频（240 GHz）电子自旋共振（ESR）光谱法研究体相CrBr3中的磁关联，通过ESR线宽和共振场的温度与角度依赖性揭示其二维（2D）行为。数据同时符合贝列津斯基-科斯特里茨-索利斯（Berezinskii-Kosterlitz-Thouless, BKT）和金兹堡-朗道临界模型，表明在三维（3D）铁磁体中存在二维关联及类似BKT的相变；而共振场（3cos²θ −1）和线宽（3cos²θ −1）²的角度依赖性证实了二维磁各向异性，可能源于外加磁场介导的铁磁（FM）与反铁磁（AFM）相共存。

ABSTRACT

Broadening the knowledge and understanding on the magnetic correlations in van der Waals layered magnets is critical in realizing their potential next-generation applications. In this study, we employ high frequency (240 GHz) electron spin resonance (ESR) spectroscopy on plate-like CrBr$_{3}$ to gain insight into the magnetic interactions as a function of temperature (200 - 4 K) and the angle of rotation ${ heta}$. We find that the temperature dependence of the ESR linewidth is well described by the Ginzberg-Landau critical model as well as Berezinskii-Kosterlitz-Thouless (BKT) transition model, indicative of the presence of two-dimensional (2D) correlations. This suggests that the three-dimensional ferromagnet CrBr$_{3}$, which has been described as an Ising or Heisenberg ferromagnet, could present 2D magnetic correlations and BKT-like behavior even in its bulk form; an observation that, to the best of our knowledge, has not been reported in the literature. Furthermore, our findings show that the resonance field follows a $(3cos^2{ heta} - 1)$-like angular dependence, while the linewidth follows a $(3cos^2{ heta} - 1)^2$-like angular dependence. This observed angular dependence of the resonance field and linewidth further confirm an unanticipated 2D magnetic behavior in CrBr$_{3}$. This behavior is likely due to the interaction of the external magnetic field applied during the ESR experiment that allows for the mediation of long-range vortex-like correlations between the spin clusters that may have formed due to magnetic phase separation. This study demonstrates the significance of employing spin sensitive techniques such as ESR to better understand the magnetic correlations in similar van der Waals magnets.

研究动机与目标

• 利用高频ESR光谱法研究体相CrBr3中磁关联的性质。
• 确定二维磁行为（如贝列津斯基-科斯特里茨-索利斯（BKT）相变）是否可在三维范德瓦尔斯铁磁体中出现。
• 通过分析ESR线宽和共振场的温度与角度依赖性，探测自旋动力学与磁各向异性。
• 识别CrBr3中类似二维行为的起源，特别是与磁相分离及层间关联的关系。

提出的方法

• 在200–4 K温度范围内，对单晶CrBr3进行了高频（240 GHz）电子自旋共振（ESR）光谱测量。
• 通过改变角度（θ = 0° 至 162°）进行ESR测量，以研究共振场与线宽的角度依赖性。
• 将ESR线宽的温度依赖性分别拟合至金兹堡-朗道临界模型与贝列津斯基-科斯特里茨-索利斯（BKT）相变模型，以评估临界指数与二维特征。
• 共振场的角度依赖性采用H_res(θ) = F(3cos²θ −1) + G建模，线宽采用∆H(θ) = A(3cos²θ −1)² + B建模，两者均引入相位偏移以校正实验偏移。
• 分析重点在于识别二维磁行为的特征，包括非立方偶极场效应与q ~ 0自旋涨落。

实验结果

研究问题

• RQ1 CrBr3中ESR线宽的温度依赖性是否符合指示二维（2D）磁关联的模型，如BKT或金兹堡-朗道临界模型？
• RQ2 CrBr3中ESR共振场与线宽的角度依赖性如何？其是否反映二维磁各向异性？
• RQ3 是否可在体相三维（3D）铁磁体（如CrBr3）中观测到二维磁行为（如类似BKT的相变）？
• RQ4 磁相分离（FM与AFM共存）在介导CrBr3中长程涡旋型关联方面起何种作用？
• RQ5 ESR实验中施加的外磁场如何影响CrBr3中二维关联的观测？

主要发现

• CrBr3中ESR线宽的温度依赖性由贝列津斯基-科斯特里茨-索利斯（BKT）相变模型良好拟合，临界温度TBKT = 13.74 K，决定系数R² = 0.9906，表明存在类似BKT的行为。
• 金兹堡-朗道临界模型对线宽数据的拟合得到临界指数b = π/2，决定系数R² = 0.9903，证实磁关联具有二维特征。
• 共振场表现出类似（3cos²θ −1）的角度依赖性，与二维磁系统一致，表明存在各向异性偶极相互作用。
• 线宽表现出类似（3cos²θ −1）²的角度依赖性，形成W形曲线，且在θ ≈ 55°附近呈现浅最小值，这是低维磁体的典型特征。
• 共振场与线宽的角度依赖性均通过引入相位偏移的模型得到良好拟合，证实了二维磁各向异性的存在及长程涡旋型关联。
• 所观测到的二维行为归因于体相CrBr3中存在铁磁（FM）与反铁磁（AFM）相的共存，且该共存由ESR测量中施加的外磁场所介导。