Coercivity limits and mechanism in nanocomposite Nd-Fe-B alloys

The largest coercivity in the Nd-Fe-B system was achieved by systematically tailoring the microstructure from strongly interacting Nd2Fe14B grains to magnetically isolated Nd2Fe14B grains in a Nd-rich matrix. The crystal structure, phase purity and magnetic properties of the Nd-Fe-B samples were extensively measured. In particular energy-filtered images, using spatially resolved measurements of inner-shell ionization edges, was critical in evaluating the particle shapes (platelets with the crystallographic c-axis normal to the plate), size (similar to 100 x 40 x 25 nm) and distribution. For randomly oriented, non-interacting particles, the largest observed coercivity, mu H-0(c) similar to 2.75 T is - 83% of the theoretical limit expected for Stoner-Wohlfarth coherent rotation behavior including demagnetization effects. Initial magnetization curves of thermally demagnetized Nd-Fe-B samples show a systematic increase in susceptibility with an increase in Nd concentration as a result of a competition between contributions from strongly interacting Nd2Fe14B grains with grain sizes smaller or equal to the magnetic domain size and completely isolated multidomain Nd2Fe14B grains. The possible coercivity mechanisms in such Nd-Fe-B samples are discussed. Crown Copyright (C) 2001 Published by Elsevier Science B.V. All rights reserved.