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 interacting to magnetically isolated single domain Nd2Fe14B grains embedded in a nonmagnetic matrix of alpha-Nd and gamma-Nd. This microstructure, as characterized by a variation in the magnetic interaction between Nd2Fe14B grains, was found to have a large impact on coercivity, mu(0)H(c), i.e., coercivity 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 microstructure of NddeltaFe13.1B (delta=38.1,148.7), carried out by conventional transmission electron microscopy, energy-filtered imaging, and energy dispersive x-ray microanalysis, showed that the majority of the Nd2Fe14B grains 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 randomly oriented, noninteracting, single domain Nd2Fe14B grains, the coercivity was calculated using the Stoner-Wohlfarth model and including the shape anisotropy 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 reported for Nd2Fe14B. (C) 2000 American Institute of Physics. [S0021-8979(00)05621-8].