Effect of grain size and distribution on the anisotropy and coercivity of nanocrystalline Nd2Fe14B magnets

The effect of grain size and distribution on the anisotropy and coercivity of nanocrystalline Nd2Fe14B magnets has been studied by adopting a Gaussian distribution model of grain sizes. The results show that the effective anisotropy K-eff and coercivity H-c decrease with reducing average grain size (D) over bar, and the decrease is more rapid when (D) over bar < 15 nm. The nonideal distribution of grain size results in a further decrease of K-eff and H-c. The grain size distribution has greater effect on K-eff when <(D)over bar> > 15 nm. Taking values of the microstructure constant P-c = 0.9 and distribution coefficient sigma = 0.5, the variation of H-c with (D) over bar is consistent with published experiment results. The decrease of H-c is mainly attributed to the decrease of K-eff. In order to obtain high effective anisotropy and coercivity in nanocrystalline Nd2Fe14B magnets, (D) over bar should be larger than 15 nm and the grain size distribution should be as centralized as possible. (C) 2006 Elsevier B.V. All rights reserved.