表层氧化对单晶铁薄膜电子磁手性二向色性谱的影响研究

表层氧化对单晶铁薄膜电子磁手性二向色性谱的影响研究

曾志欣，胡绮雯，李建军，符潇潇^*，黄晓旭^*

（1.重庆大学材料科学与工程学院轻合金材料国际合作联合实验室，重庆400044；

2.重庆大学电子显微镜中心，重庆400044）

摘  要  电子磁手性二向色性技术（EMCD）是一种基于透射电镜电子能量损失谱（EELS）的磁表征技术，可实现亚纳米级高空间分辨、元素分辨的局域轨道自旋磁矩定量测量。本文以单晶铁薄膜为例研究了金属透射样品表面氧化对EMCD信号的影响。在Fe/MgO(001)薄膜的平面样品中，从不同区域中提取一系列低能损失谱、Fe-L_2，3峰常规在轴芯能级损失谱和动量转移矢量分辨的离轴芯能级损失谱（q-EELS）。结合低能损失谱和常规在轴芯能级损失谱，可合理忽略所选区域的复散射，且发现样品靠近孔洞边缘的区域较其他区域而言存在更为严重的氧化情况。表层氧化物相对含量的增加导致所选数据中常规在轴芯能级损失谱以及动量转移矢量分辨离轴芯能级损失谱的白线比值均呈单调递增趋势，这种现象原理上会导致从EMCD谱中提取的轨道-自旋磁矩比值的偏差相对递增。此外，作者还发现样品表层氧化、非晶化、样品弯曲效应和外延质量等因素综合引起了实验中局部衍射条件的轻微不规则变化，这些因素同样对EMCD定量的结果有较大的影响，最终导致所观测区域EMCD谱中实际提取的轨道-自旋磁矩比值呈不规则变化。本工作建议在进行易氧化金属的EMCD实验时，可预先进行会聚束辐照以降低表层氧化物厚度，并注意关注衍射条件的细微变化。

关键词  EMCD；表层氧化；复散射；动量分辨芯能级损失谱

中图分类号：TG115；TM27；TB43 文献标识码：A   doi：10.3969/j.issn.1000-6281.2024.05.008

Influence of surface oxidation on the electron magnetic chiral dichroism of single-crystalline iron thin films

ZENG Zhixin¹, HU Qiwen¹, LI Jianjun ¹, FU Xiaoxiao^1，2*, HUANG Xiaoxu^1，2*

(1. International Joint Laboratory for Light Alloys (MOE), College of Materials Science and Engineering, Chongqing University, Chongqing 400044; 2. Electron Microscopy Center, Chongqing University, Chongqing 400044, China)

Abstract  Electron magnetic chiral dichroism (EMCD) is a magnetic characterization technique based on the electron energy loss spectroscopy (EELS) in transmission electron microscopy. It enables quantitative measurement of local orbital spin magnetic moments with sub-nanometer spatial resolution and elemental resolution. This paper used a single-crystal iron thin film to study the effect of surface oxidation on the EMCD signal in metal transmission samples. In the planar sample of the Fe/MgO (001) thin film, a series of low-energy loss spectra, Fe-L_2,3 edge spectra, and momentum transfer q-resolved off-axis core loss spectra (q-EELS) were extracted from different areas. By combining the low-energy loss spectra and the conventional on-axis core loss spectra, complex scattering in the selected areas was reasonably neglected. Areas near the edges of the holes in the sample exhibited more severe oxidation compared to other areas. The increase in the relative content of surface oxides led to a monotonic increase in the white line ratio in both the conventional on-axis core loss spectra and the momentum transfer vector-resolved off-axis core loss spectra in the selected data. Theoretically, this would cause a relative increase in the deviation of the orbital-spin magnetic moment ratio extracted from the EMCD spectra. In addition, factors such as surface oxidation, amorphization, sample bending effects, and epitaxial quality of the sample collectively contributed to slight irregular changes in local diffraction conditions during the experiment, significantly impacting the quantitative results of EMCD. Ultimately these factors led to irregular changes in the actual orbital-spin magnetic moment ratio extracted from the EMCD spectra of the observed areas. This work suggests that in EMCD experiments involving oxidizable metals, pre-irradiation with a convergent beam can reduce the thickness of surface oxides. Furthermore, attention should be paid to subtle changes in diffraction conditions.

Keywords  EMCD；surface oxidation；plural scattering；q-EELS

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