Presentation Information
[O4-1]Three-dimensional multimodal analyses on Nd-Fe-B magnets
*Satoshi Okamoto1, Tomomi Suwa1, Takuya Taniguchi1, Kousuke Kato1, Motohiro Suzuki2, Yujiro Hayashi3, Jaemyung Kim3, Takahiko Iriyama4, Hiroshi Miyawaki4, Tadakatsu Ohkubo5 (1. Tohoku University (Japan), 2. Kwansei Gakuin University (Japan), 3. RIKEN (Japan), 4. Daido Steel Co. (Japan), 5. National Institute for Materials Science (Japan))
Keywords:
Nd-Fe-B,3D-SEM,Magnetic tomography,3D-XRD
Coercivity and hysteresis curve shape are very basic magnetic properties of Nd-Fe-B magnets, but they are still not fully understood. Since they are related to the microstructure of nm-scale grain boundary phase, mm-scale main phase grain, and tens mm-scale grain alignments. Moreover, they are also strongly related to the magnetization processes of nm-scale nucleation, mm-scale domain wall depinning, and tens mm-scale domain structure formation. In addition, crystalline orientation of each main phase grain is also an important factor of coercivity and hysteresis curve shape. From these points of view, numerous studies about multiscale microstructure and magnetic domain observations of Nd-Fe-B magnets have been reported so far. Recently, 3-dimensional (3D) visualization techniques of scanning electron microscopy (SEM) and X–ray magnetic circular dichroism (XMCD) microscopy have been developed for 3D-microstructure and magnetic domain structure observations, respectively. And very recently, we successfully demonstrated the combined 3D-magnetic domain structure and microstructure observations of a Tb-Cu infiltrated Nd-Fe-B sintered magnet along the magnetic hysteresis curve with varying the external magnetic field [1]. This was the first observation of the magnetic domain evolution inside the Nd-Fe-B magnet body, and the strong relationship between the magnetic domain evolution and the microstructures was observed. However, the very low signal to noise ratio (SNR) of XMCD signal hinders us from the detailed analysis. Moreover, the complicated microstructures and magnetic domain structures of the Nd-Fe-B sintered magnet also makes the analysis very difficult. Therefore, as a next step, the combined observations of 3D-SEM and 3D-XMCD were employed for a Nd-Fe-B hot-deformed magnet. Since the Nd-Fe-B hot-deformed magnet has a much more uniform microstructure and simple magnetic domain structure, much easier analysis is highly expected. After the elaborate noise reduction processes, very clear magnetic domain evolution was successfully observed. As a result, it is possible to discuss the major domain wall pinning sites and magnetic domain evolution process of the Nd-Fe-B hot-deformed magnet. The last piece of 3D microscopy, which is important to understand the coercivity and the magnetic hysteresis curve, is the crystalline orientation of constituent grains. For the polycrystalline steel, 3D-crystalline orientation imaging was reported by using a 3D X-ray diffraction (3D-XRD) technique [2]. The steel has the grain size of tens mm in diameter and the simple crystalline structure of bcc. On the other hand, the grain size of Nd-Fe-B sintered magnets is one order of magnitude smaller than that of steel, and their crystal structure has lower symmetry. Solving these difficulties, the 3D-XRD microscopy was successfully demonstrated for a Nd-Fe-B sintered magnet. These recent studies for the 3D multimodal analyses on Nd-Fe-B magnets will be introduced in this talk.
[1] M. Takeuchi et al., NPG Asia Mater. 14, 70 (2022).
[2] Hayashi et al., Science 366, 1492 (2019).
[1] M. Takeuchi et al., NPG Asia Mater. 14, 70 (2022).
[2] Hayashi et al., Science 366, 1492 (2019).