Influence of combinatory effects of STEM Setups on the sensitivity of differential phase contrast imaging

Abstract

Differential phase contrast (DPC) imaging in scanning transmission electron microscopy (STEM) mode has been suggested as a new method to visualize nanoscale electromagnetic features of materials. However, image quality of the DPC image is very sensitive to the electron beam alignment, microscope setups and specimen conditions. Unlike normal STEM imaging, microscope setup variables in DPC mode are not independent, but rather correlated decisive factors for the field sensitivity. Here, we systematically investigated independent and combinatory effects of microscope setups on the sensitivity of the DPC image in a hard magnet Nd2Fe14B alloy. When it comes to the sensitivity, smaller overlaps of the electron beam with annular detectors and large camera length are required. But they cannot be controlled independently in the two-condenser-lens system. In that linked system, the effect of the camera length on the DPC sensitivity was slightly predominant than overlap. And DPC signal was noisy and scattered at small overlap below 11% since the electron beam disk is keen to shift by sample thickness variation and scanning distortion. Electron beam current does not affect evidently to the sensitivity. In addition, DPC sensitivity depending on the sample thickness was also examined, the optimum thickness for the high sensitivity was about 65 nm for the Nd2Fe14B hard magnetic material. This practical approach to the STEM setups and sample thickness may provide experimental guideline for further application of the DPC analysis method.