Presentation Information
[P1-57]Three-dimensional magnetic domain propagation of a Nd-Fe-B hot-deformed magnet
*Tomomi Suwa1, Motohiro Suzuki2, Tadakatsu Ohkubo3, Iriyama Takahiko4, Hiroshi Miyawaki4, Kaiki Takemura2, Yusuke Akiyama2, Takuya Taniguchi1, Satoshi Okamoto1,3 (1. Tohoku University (Japan), 2. Kwansei Gakuin University (Japan), 3. National Institute for Materials Science (Japan), 4. Daido Steel Company, Limited (Japan))
Keywords:
Three-dimensional analysis,Magnetic reversal process,Magnetic domain,Microstrucutre,Nd-Fe-B magnet,Hot-deformed Nd-Fe-B magnet
The magnetization reversal mechanism of permanent magnets, particularly Nd-Fe-B magnets, involves interactions across multiple scales, including atomic spins, crystal lattices, grains, grain boundaries, and microstructures. This hierarchical complexity, coupled with the intricate nature of microstructures, has posed significant challenges in fully understanding the magnetization reversal process. A magnetization nucleation process has been elucidated gradually through the recent stochastic atomistic calculations of Nd2Fe14B grain1,2,3). However, the magnetization reversal in a bulk magnet takes place as nucleation of a reversed domain followed by domain propagation, and these processes have a strong relationship with the microstructure. To understand the magnetization reversal mechanism in bulk magnets, it is essential to observe the correlation between the microstructure and the magnetic domain evolution. Conventionally, two-dimensional(2D) observations of magnetic domain structure have been widely used, such as magnetic Kerr microscopy or a combination of scanning electron microscopy (SEM) and X-ray magnetic circular dichroism (XMCD) microscopy4,5). However, since the magnetic domain structure is inherently three-dimensional (3D), and unavoidable surface processing damage and demagnetizing fields make magnetic structures distorted, 2D observations are insufficient. Recently, our group developed a method combining 3D SEM and hard X-ray magnetic tomography, enabling 3D observation of microstructure and magnetic domain structure with the same spatial region6). In this study, we report the magnetization reversal process inside a hot-deformed Nd-Fe-B bulk magnet. The sample was a hot-deformed Nd30.9Febal.Co3.5B0.92Ga0.55 magnet. It was mechanically polished and then processed into a shape of 18 µm × 18 µm (c-axis) × 45 µm using a focused ion beam (FIB). The 3D magnetic domain structure and X-ray absorption spectra (XAS) were acquired at the Nd L2 edge using X-ray magnetic tomography with an algebraic reconstruction technique at BL39XU of SPring-8. Following this, the secondary and backscattered electrons images as 3D microstructure were obtained through FIB-SEM imaging. Alignment between the microstructure and magnetic domain structure was achieved using each Nd distribution obtained from backscattered electron images and XAS data. The magnetic domain structure was measured focusing on the thermally demagnetized state and the state around the nucleation region of the hysteresis curve respectively. The minimum magnetic field interval was 10 kA/m, enabling the capture of gradual evolution in magnetic domains. However, due to the technical limits of the reconstruction algorithm, the magnetic domain images contained artifacts such as X-shaped and streak-shaped noises, which hindered precise analysis of magnetic domain evolution. To address this issue, flat-field correction, total variation regularization based on sparse modeling, and phase field calculation were applied for denoising. This combination of processing techniques effectively reduced noise, enabling the high-accuracy identification of magnetic domain positions and their evolution, as well as a precise analysis of magnetic domain wall dynamics inside the magnet. The figure shows the microscopic evolution of magnetic domains in the hot-deformed magnet, measured at intervals of 10 kA/m from 550 kA/m to 590 kA/m. At 550 kA/m, no magnetic domains were observed within the measurement area. As the magnetic field increased, magnetic domains propagated with slight changes. One of the key findings is that magnetic domains are strongly pinned in the large Nd-rich phase at the ribbon boundary. The Nd-rich phase at ribbon boundaries has been conventionally regarded as a nucleation site for magnetization reversal. However, our findings reveal that the large Nd-rich phase instead acts as a pinning site, inhibiting the propagation of the magnetic domain wall along the c-plane direction.
[1]Y. Toga, et al., npj Computational Materials, 6, 67 (2020) [2]S. Miyashita, et al., STAM, 22,1(2021) [3]I. E. Uysal, et al., Phys. Rev. B, 101, 1, 094421(2020) [4] M. Takeuchi, et al., J. Japan Inst. Metals, 86, 1-7 (2022) [5] M. Takeuchi, et al., NPG Asia Materials, 14, 70 (2022)
[1]Y. Toga, et al., npj Computational Materials, 6, 67 (2020) [2]S. Miyashita, et al., STAM, 22,1(2021) [3]I. E. Uysal, et al., Phys. Rev. B, 101, 1, 094421(2020) [4] M. Takeuchi, et al., J. Japan Inst. Metals, 86, 1-7 (2022) [5] M. Takeuchi, et al., NPG Asia Materials, 14, 70 (2022)
