The origin of the coercivity reduction of Nd–Fe–B sintered magnet annealed below an optimal temperature

• Published in: Journal of magnetism and magnetic materials Vol. 342; pp. 4 - 10
• Main Authors: Akiya, T., Sasaki, T.T., Ohkubo, T., Une, Y., Sagawa, M., Kato, H., Hono, K.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 01.09.2013; Elsevier
• Subjects: Annealing; Coercive force; Coercivity; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Cracks; Domain effects, magnetization curves, and hysteresis; Exact sciences and technology; Ferrous alloys; Low-temperature annealing; Magnetic materials; Magnetic properties and materials; Magnetization curves, magnetization reversal, hysteresis, barkhausen and related effects; Nd–Fe–B sintered magnet; Optimization; Permanent magnets; Physics; Reduction; Studies of specific magnetic materials; Nd–Fe–B sintered magnet; Low-temperature annealing; Coercivity; Grain boundaries; Scanning electron microscopy; Annealing; Nd-Fe-B sintered magnet; Ternary alloys; Permanent magnets; Sintered materials; Neodymium alloys; Focused ion beam technology; Boron alloys; Cracks; Micromagnetism; Coercive force; Microstructure; Iron base alloys
• ISSN: 0304-8853
• DOI: 10.1016/j.jmmm.2013.04.044

In order to understand the origin of the coercivity reduction in a sintered Nd–Fe–B magnet that is annealed below an optimal annealing temperature, we performed focused ion beam/scanning electron microscopy tomography of post-sinter annealed magnets. A number of grain boundary cracks were observed between Nd2Fe14B grains and Nd-rich phases in the sample annealed below the optimal temperature. We deduced micromagnetic parameters α and Neff by fitting the temperature dependence of the coercivity. While α was constant regardless of the annealing conditions, Neff increased in the sample annealed below the optimal temperature with the reduced coercivity. This indicates that the reduction of the coercivity is due to the local stray field at the cracks. •We performed FIB/SEM tomography of post-sinter annealed magnets.•A number of grain boundary cracks were observed in the low-coercivity sample.•Parameters α and Neff were deduced from the temperature dependence of coercivity.•While α was constant, Neff increased in the low-coercivity sample.•The reduction of the coercivity is due to the local stray field at the cracks.