Differential phase contrast (DPC) imaging in scanning transmission electron microscopy is a technique to visualize electromagnetic field distribution inside specimens at high spatial resolution. However, diffraction contrast strongly hampers electromagnetic contrast in DPC images especially in polycrystalline samples. In this paper, we develop an imaging technique to effectively suppress diffraction contrast in DPC images. It is shown that a magnetic structure in a Nd-Fe-B permanent magnet was clearly visualized by averaging 64 DPC images with various specimen-tilt conditions. This is because the diffraction contrast in DPC images sensitively and randomly varies with crystal orientation and thus almost vanishes by averaging specimen-tilt image series. We further investigated two types of residual diffraction contrast in the tilt-series averaged DPC images: weak contrast inside grains and strong contrast at grain boundaries. We found that the former can be suppressed by averaging more DPC images, while the latter can be suppressed by the tilt-series averaging with wider range of specimen tilt. The tilt-series averaging method enables DPC to visualize electromagnetic structures even inside polycrystalline materials.

中文翻译：

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通过样品倾斜系列平均 DPC STEM 对多晶材料的磁结构成像

扫描透射电子显微镜中的微分相衬 (DPC) 成像是一种以高空间分辨率可视化样品内部电磁场分布的技术。然而，衍射对比度强烈阻碍了 DPC 图像中的电磁对比度，尤其是在多晶样品中。在本文中，我们开发了一种成像技术来有效抑制 DPC 图像中的衍射对比度。结果表明，Nd-Fe-B 永磁体中的磁性结构可以通过在各种样品倾斜条件下对 64 个 DPC 图像进行平均来清晰地可视化。这是因为 DPC 图像中的衍射对比度敏感且随机地随晶体取向而变化，因此通过平均样本倾斜图像系列几乎消失。我们进一步研究了倾斜系列平均 DPC 图像中两种类型的残余衍射对比度：晶粒内部的弱对比度和晶界的强对比度。我们发现前者可以通过对更多 DPC 图像进行平均来抑制，而后者可以通过具有更宽样本倾斜范围的倾斜系列平均来抑制。倾斜系列平均方法使 DPC 甚至可以将多晶材料内部的电磁结构可视化。