Approaching the Theoretical Coercivity of Nd2 Fe14 B: Microstructural Evaluation and Interparticle Interactions

The microstructure of melt–spun rapidly quenched Nd d Fe 13.1 B (2.05 £ d £ 147.6) and Nd d Fe 14 B ( d = 40.6,151.7) ribbons was tailored by appropriate annealing from strongly interacting to magnetically isolated single domain Nd 2 Fe 14 B grains embedded in a nonmagnetic matrix of a -Nd and g -Nd. This microstructure, as characterized by a variation in the magnetic interaction between Nd 2 Fe 14 B grains, was found to have a large impact on coercivity, m 0 H c , i.e., coercivity increases with an increase in the Nd concentration from 1.2 T in Nd 2.05 Fe 13.1 B to 2.75 T in Nd 147.6 Fe 13.1 B at 290 K. A detailed study of the microstructure of Nd d Fe 13.1 B ( d = 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 Nd 2 Fe 14 B grains are completely isolated only in Nd 147.6 Fe 13.1 B. The Nd 2 Fe 14 B grains, in Nd 147.6 Fe 13.1 B, are found to be randomly oriented platelets with the c axis normal to the platelet and an average size of 100×40×25 nm. For these randomly oriented, noninteracting, single domain Nd 2 Fe 14 B grains, the coercivity was calculated using the Stoner– Wohlfarth model and including the shape anisotropy of the grains. The observed coercivity of Nd 2 Fe 14 B in Nd 147.6 Fe 13.1 B is ~83% of this theoretical value and is the largest so far reported for Nd 2 Fe 14 B.