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

Abstract

The microstructure of melt–spun rapidly quenched NdδFe13.1B (2.05⩽δ⩽147.6) and NdδFe14B (δ=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 α-Nd and γ-Nd. This microstructure, as characterized by a variation in the magnetic interaction between Nd2Fe14B grains, was found to have a large impact on coercivity, μ0Hc, 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 NdδFe13.1B (δ=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 100×40×25 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 ∼83% of this theoretical value and is the largest so far reported for Nd2Fe14B.