Effect of grain size and distribution on the anisotropy and coercivity of nanocrystalline Nd 2Fe 14B magnets

Abstract

The effect of grain size and distribution on the anisotropy and coercivity of nanocrystalline Nd 2Fe 14B 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 ¯ , and the decrease is more rapid when D ¯ < 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 ¯ > 15 nm . Taking values of the microstructure constant P c=0.9 and distribution coefficient σ=0.5, the variation of H c with D ¯ 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 Nd 2Fe 14B magnets, D ¯ should be larger than 15 nm and the grain size distribution should be as centralized as possible.