Effect of microstructure on the coercivity of HDDR Nd-Fe-B permanent magnetic alloy

28-Jul-2008

The School of Physics, Shandong University, has shown that the coercivity mechanism of HDDR Nd-Fe-B permanent magnetic alloy is greatly related to its microstructure defect at the grain boundary. The investigation can provide a clear understanding of coercivity mechanism, and hence will be reported in Science in China Series G-Physics, Mechanism and Astronomy.

The study found that the coercivity mechanism can contribute to different structure defect thickness at the grain boundary, and also found that the coercivity reaches the maximum when the structure defect thickness is 2r0/lex=1.67.

''Some author had experimentally reported the different coercivity mechanisms of HDDR Nd-Fe-B magnetic powders. Our research aimed to theoretically investigate the effect of microstructure defect on coercivity,'' said Dr. Liu Min, a noted principal investigator with School of Physics, Shandong University, "this research is the first paper to theoretically investigate the relationship between the microstructure defect and the coercivity mechanism".

The coercivity was investigated from the nucleation and pinning mechanisms. When the critical field of the irreversible domain wall displacement is smaller than the demagnetization nucleation field, the irreversible domain wall displacement takes place more easily than the demagnetization nucleation, and the coercivity is controlled by the pinning mechanism. However, if the demagnetization nucleation field is smaller than the critical field of the irreversible domain wall displacement, the demagnetization nucleation takes place more easily than the irreversible domain wall displacement, and the coercivity is controlled by the nucleation mechanism.

The main conclusion reported by the investigator is that the coercivity mechanism of HDDR Nd-Fe-B permanent magnetic alloy is greatly related to its microstructure defect at the grain boundary. For a fixed lex, when 2r0/lex1.67, the coercivity is controlled by the pinning mechanism; when 2r0/lex1.67, it is determined by the nucleation mechanism. And the coercivity reaches the maximum 1198 KA/m when 2r0/lex=1.67, which is well consistent with the experimental result given by Morimoto et al.

Original publication: Yue, M. Liu, X. B. Xiao, Y. F. et al., "Magnetization reversal mechanism of anisotropic HDDR Nd2Fe14B-based magnet powder", J. Magn.Magn.Mater, 2004, 269: 227-230.

Moritmoto, K. Kato, K. Igarashi, K. Nakayama, R.; "Magnetic properties of anisotropic Nd- Fe-B HDDR powders prepared from strip cast alloys", J.Alloys.Comp, 2004, 366: 274-278.

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