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 Keff and coercivity Hc decrease with reducing average grain size D¯, and the decrease is more rapid when D¯<15nm. The nonideal distribution of grain size results in a further decrease of Keff and Hc. The grain size distribution has greater effect on Keff when D¯>15nm. Taking values of the microstructure constant Pc=0.9 and distribution coefficient σ=0.5, the variation of Hc with D¯ is consistent with published experiment results. The decrease of Hc is mainly attributed to the decrease of Keff. In order to obtain high effective anisotropy and coercivity in nanocrystalline Nd2Fe14B magnets, D¯ should be larger than 15 nm and the grain size distribution should be as centralized as possible.