Development of the 300 kV Magnetic Field-Free Atomic Resolution Electron Microscope

In a conventional high-resolution electron microscope, the sample is placed in a strong magnetic field of approximately 2–3 T because the focusing effect of the magnetic field before and after the sample is utilized as the objective lens. When observing magnetic materials, the sample is affected by the magnetic field of the objective lens, which may change magnetic structure or even cause physical deformation of the sample. As a result, atomic-resolution observation of magnetic materials using electron microscopy has been challenging. Our group has developed a new objective lens that can maintain a magnetic field-free environment for the sample while placing strong magnetic fields for focusing electron beam in its vicinity. This was achieved by positioning two round lenses symmetrically before and after the sample, such that the magnetic fields can cancel out at the sample plane. By combining this objective lens with a higher-order aberration corrector (STEM Delta Corrector), we have successfully realized atomic-resolution observation under a magnetic field-free environment [1]. This microscope has been used for the structural analysis of magnetic materials [2], atomic-scale magnetic field observations [3], magnetic field observations in magnetic devices [4], and so on. We have started a new project for developing the magnetic field-free cryogenic atomic-resolution electron microscope (MACALU), which enables atomic-resolution observations of various materials at liquid helium temperature in a magnetic field-free environment by integrating a cooling function into the magnetic field-free electron microscope. As the column for MACALU, we are developing a new 300 kV magnetic field-free electron microscope. The optical parameters of the magnetic field-free objective lens are designed as follows: a focal length of 4.2 mm, a spherical aberration coefficient of 43 mm, and a chromatic aberration coefficient of 4.0 mm. The pole piece is designed to incorporate a sample cooling stage, resulting in larger optical constants compared with those of the 200 kV magnetic field-free objective lens. However, by operating at 300 kV, the effects of chromatic aberration and diffraction can be reduced. Figure 1 shows the simulated full width at half maximum (FWHM) and d59 (the diameter encompassing 59% of the total intensity) of the electron probe distribution based on these parameters. For comparison, the simulation results for the 200 kV magnetic field-free electron microscope are also plotted. The use of 300 kV allows for a reduction in probe dimension, even when considering the sample cooling system. We are currently building up a magnetic field-free electron microscope equipped with a 300 kV magnetic field-free objective lens and a STEM/TEM delta corrector (Figure 2). This microscope currently uses a room-temperature sample stage and does not have a sample cooling function. However, since the optical configuration is the same as that of MACALU, the optical characteristics of the 300kV magnetic field-free objective lens and the STEM/TEM performance of MACALU should be evaluated. Figure 3(a) shows an aberration-corrected STEM Ronchigram acquired from a thin carbon film. The Ronchigram confirms that aberrations are well corrected over a semi-angle exceeding 20 mrad. Figures 3(b) and 3(c) show an ADF-STEM image of Si observed along the [112] zone axis and the corresponding power spectrum, respectively. As indicated in the inset of Fig. 3(b), the Si–Si spacing along this orientation is 78 pm, which is clearly resolved in the real-space image. Spots corresponding to this 78 pm spacing are also observed in the power spectrum. A newly developed 300 kV magnetic-field-free atomic-resolution electron microscope has achieved higher spatial resolution than our previous 200 kV magnetic-field-free electron microscope. As a next step, this microscope will be equipped with a newly developed STEM corrector capable of correcting aberrations up to the sixth-order three-lobe aberration [5]. In this presentation, we will report detailed evaluation results of the electron-optical performance and recent progress in the development of MACALU [6]. The FWHM and d59 of the simulated probe distributions for the 300 kV and 200kV magnetic field-fee atomic-resolution electron microscopes. A photograph of the 300 kV magnetic field-free atomic-resolution electron microscope. (a) STEM Ronchigram acquired using a carbon thing film. (b) ADF STEM image of Si from [112] direction. (c) The power spectrum of the image (b).