Host: The Japan Society of Vacuum and Surface Science
Name : Annual Meeting of the Japan Society of Vacuum and Surface Science 2023
Location : [in Japanese]
Date : October 31, 2023 - November 02, 2023
In 2019, we developed the hollow-cone Foucault imaging method as a new method for observing the magnetization of specimen materials using transmission electron microscopes [1-3]. This method can be regarded as third Lorentz microscopy, which makes it possible to simultaneously visualize both magnetic domains and domain walls as well as acquire contrast-inverted hollow-cone Foucault images under both bright-field and dark-field conditions. Furthermore, the schlieren images are obtained under the special boundary condition, schlieren condition, between the bright-field and dark-field conditions, we observed a contrast caused by the electromagnetic field in the space around the specimen [4]. In this study, we precisely controlled the schlieren condition on the basis of the inclination angle of the illumination electron beams and successfully observed the electromagnetic field around the specimen in the vacuum region.
Under the magnetic field-free condition for the specimen with the lens current of the objective lens turned off, the electron beam transmitted through the specimen was made to crossover at the selected-area (SA) aperture position. The SA aperture functioned as a reciprocal space aperture (diffraction aperture) to shielding out the deflected electron beams at and around the specimen, and this asymmetric imaging led to the contrast caused by the electromagnetic fields. To analyze the vector component of the electromagnetic fields, the image was recorded every 10 degrees of the azimuth angle by the illumination beams and used for image processing instead of the full-azimuthally integrated images as in the previous hollow-cone Foucault imaging method. This recording procedure is similar to that of laminography in X-ray experiments. The transmission electron microscope used was the JEM-2100 Plus, with an acceleration voltage of 200 kV.
We numerically reconstructed the series of azimuthally separated schlieren images. Figure 1(a) shows the distribution of the electric field from latex balls of 2 mm in diameter on a 50 nm thick Si3N4 membrane. The intensity of the electric field is indicated by brightness, and the direction is indicated by color. The electric charges are generated at the surface of the latex balls by the irradiation of the electron beams, and it can be seen that the electric field distribution is similar around the latex balls even with varying numbers of balls. Figure 1(b) shows the magnetic field leaking from the amorphous magnetic film into the vacuum. The orientation of the magnetic field is indicated by the azimuth of arrowheads and colors, and the intensity of the magnetic field is indicated by the size of the arrowheads. The colored area inside the thin film is the region where the electron beam was transmitted and the magnetization distribution inside the material was detected. Although the intensity of the recorded electron beams differed greatly between the vacuum and the inside of the specimen, simultaneous visualization in one micrograph should be feasible by further analysis for data processing.
We have demonstrated that our developed schlieren electron microscopy using thermal electron beams can be used to observe a wide area of electric and magnetic fields, which are difficult to observe even with electron holography using field emission electron beams.
