1) JASRI, 2) ESICMM / NIMS
Abstract:
Direct observations of the generation and evolution of reversed magnetic domains is one of the key techniques for elucidating the coercivity mechanism in Nd-Fe-B permanent magnets. In a previous study [1], we showed that the coercivity of the fractured surface closely resembles that of the bulk, in stark contrast to the polished surface in which the coercivity is significantly decreased. The higher coercivity of the fractured surface is attributed to the particular way in which Nd-Fe-B sintered magnets fracture, where the majority of the fractured surface remains covered with a thin layer of the grain boundary phase. Although the similarity between the fractured surface and bulk coercivities cry out for magnetic domain observations of the fractured surface, conventional magnetic domain observations using Kerr microscopy, magnetic force microscopy, and photoelectron emission microscopy, have only been conducted on polished surfaces or thin films. In order to observe the magnetic domain structure in the fractured surface under various magnetic fields, we have developed a new scanning soft X-ray spectromicroscope equipped with a superconducting magnet with a maximum magnetic field of ±8 T. When used in combination with X-ray photons of opposite helicity, and total-electron-yield detection, magnetic domain observations of the fractured surface become possible.
In our magnetic domain observations of the fractured surface of a Nd14.0Fe79.7Cu0.1B6.2 sintered magnet, both the microstructure and the magnetic domain contrast are clearly observed. The magnetic field dependence of the magnetic domains has shown that the precise location in which reversed domains are initially generated is always identical, and independent of whether the magnetic field is increased or decreased. Further analysis has allowed us to characterize local magnetic hysteresis (MH) loops for areas ~ 100 nm2. The observed local MH loops show a wide variety of magnetization reversal characteristics depending on the particular grain. This suggests that the magnetization reversal is very sensitive to the local magnetic fields, which are determined by the stray magnetic field from the surrounding magnetic grains, together with the intrinsic coercivity of each grain, and the exchange coupling with the grains in the sub-surface layers.
In our magnetic domain observations of the fractured surface of a Nd14.0Fe79.7Cu0.1B6.2 sintered magnet, both the microstructure and the magnetic domain contrast are clearly observed. The magnetic field dependence of the magnetic domains has shown that the precise location in which reversed domains are initially generated is always identical, and independent of whether the magnetic field is increased or decreased. Further analysis has allowed us to characterize local magnetic hysteresis (MH) loops for areas ~ 100 nm2. The observed local MH loops show a wide variety of magnetization reversal characteristics depending on the particular grain. This suggests that the magnetization reversal is very sensitive to the local magnetic fields, which are determined by the stray magnetic field from the surrounding magnetic grains, together with the intrinsic coercivity of each grain, and the exchange coupling with the grains in the sub-surface layers.
Acknowledgements
Part of this work is supported by the Elements Strategy Initiative Center for Magnetic Materials under the outsourcing project of MEXT.
Part of this work is supported by the Elements Strategy Initiative Center for Magnetic Materials under the outsourcing project of MEXT.
References
[1] T. Nakamura, A. Yasui, Y. Kotani, T. Fukagawa, T. Nishiuchi, H. Iwai, T. Akiya, T. Ohkubo, Y. Gohda, K. Hono, and S. Hirosawa, Appl. Phys. Lett. 105, 202404 (2014).
[1] T. Nakamura, A. Yasui, Y. Kotani, T. Fukagawa, T. Nishiuchi, H. Iwai, T. Akiya, T. Ohkubo, Y. Gohda, K. Hono, and S. Hirosawa, Appl. Phys. Lett. 105, 202404 (2014).
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