Development of highly-sensitive magnetic imaging technique using a scanning electron microscope

抄録

Introduction

The performance of magnetic materials can be improved by understanding the local magnetic domain structure. Magnetic domain contrasts can be observed with a scanning electron microscope (SEM) by detecting secondary electrons (SEs) deflected by the Lorentz force derived from leakage magnetic fields of magnetic specimens. This type of magnetic contrast is generally known as type-I magnetic contrast ^[1]. To obtain SEM images with a clear magnetic contrast using the type-I method, SEs need to be detected under directional-selective detection conditions ^[2]. Areas where SEs are deflected toward a detector are observed as bright, while areas where the SEs are deflected away from the detector are observed as dark.

Experimental Procedures

To establish a convenient observation method for obtaining magnetic domain images using a conventional SEM, we observed a surface-polished Nd-Fe-B alloy specimen using FE-SEM SU5000 (Hitachi High-Tech Corp.). In this study, we sought an optimal condition for observing SEM images with clear magnetic contrast. In the conventional method, a lower detector (mounted beneath an objective lens) is used as shown in figure 1 (a). On the other hand, in this study, we developed a novel observation method using a top detector (mounted above the objective lens) in deceleration mode while tilting the specimen as shown in figure 1 (b). To verify the angular range of detected SEs with our method, we analyzed the initial emission angles of the detected SEs by their trajectory calculations. We calculated the SE trajectories with a 3D electro-optical simulator from the specimen to the top detector considering the electrostatic and magnetic fields distributed in the calculation area.

Results and Discussion

By using the tilting-deceleration method developed in this study, SEM images with a clearer magnetic contrast were obtained, and fine magnetic domains were clearly visualized. On the basis of a calculation analysis of SE trajectories, it was clarified that in the tilting-deceleration condition, SEs were directional-selectively detected, and the tilting-deceleration method much more effectively obtained SEM images with clear magnetic contrast than the conventional method. Furthermore, we found that the anisotropy caused by directional-selective detection can be reduced by combining magnetic domain contrast images of two orthogonal orientations ^[3]. Figure 2 shows comparison results of an SEM image with the tilting-deceleration method (Fig. 2 (a)), a Kerr microscope image (Fig. 2 (b)), and a spin-SEM ^[4] image (Fig. 2 (c)). These images were acquired in the same field of view of the same specimen. The observation technique developed in this study has great advantages because various analysis methods conventionally used in SEMs, such as elemental analysis using energy dispersive X-ray spectroscopy (EDX) and crystal orientation analysis using an electron backscattered diffraction (EBSD), can be easily applied in the same region of interest.

References

[1] L. Reimer, Image Formation in Low-Voltage Scanning Electron Microscopy, Society of Photo Optical (1993).

[2] D. E. Newbury, Magnetic Contrast in the SEM, Advanced Scanning Electron Microscopy and X-Ray Microanalysis, Springer (1986).

[3] H. Morishita, et al., Improvement of type-I method for observing magnetic contrast using scanning electron microscope under tilting-deceleration condition, J. Magn. Magn. Mater., Vol. 546, 168733 (2022).

[4] T. Kohashi, et al., Magnetism in grain-boundary phase of a NdFeB sintered magnet studied by spin-polarized scanning electron microscopy, Appl. Phys. Lett. 104, 232408 (2014).