Approaching the theoretical coercivity of Nd2Fe14B: Microstructural evaluation and interparticle interactions

The microstructure of melt-spun rapidly quenched NddeltaFe13.1B (2.05 less than or equal to delta less than or equal to 147.6) and NddeltaFe14B (delta =40.6,151.7) ribbons was tailored by appropriate annealing from strongly in teracting to magnetically isolated single domain Nd2Fe14B grains embedded i n a nonmagnetic matrix of alpha-Nd and gamma-Nd. This microstructure, as ch aracterized by a variation in the magnetic interaction between Nd2Fe14B gra ins, was found to have a large impact on coercivity, mu(0)H(c), i.e., coerc ivity increases with an increase in the Nd concentration from 1.2 T in Nd2. 05Fe13.1B to 2.75 T in Nd147.6Fe13.1B at 290 K. A detailed study of the mic rostructure of NddeltaFe13.1B (delta=38.1,148.7), carried out by convention al transmission electron microscopy, energy-filtered imaging, and energy di spersive x-ray microanalysis, showed that the majority of the Nd2Fe14B grai ns are completely isolated only in Nd147.6Fe13.1B. The Nd2Fe14B grains, in Nd147.6Fe13.1B, are found to be randomly oriented platelets with the c axis normal to the platelet and an average size of 100x40x25 nm. For these rand omly oriented, noninteracting, single domain Nd2Fe14B grains, the coercivit y was calculated using the Stoner-Wohlfarth model and including the shape a nisotropy of the grains. The observed coercivity of Nd2Fe14B in Nd147.6Fe13 .1B is similar to 83% of this theoretical value and is the largest so far r eported for Nd2Fe14B. (C) 2000 American Institute of Physics. [S0021-8979(0 0)05621-8].