Effect of grain size and magnetocrystalline anisotropy on exchange coupling in nanocomposite two-phase Nd-Fe-B magnets

We have studied the exchange coupling in two-phase nanocrystalline alloy ribbons of the composition Nd-4-xSmxFe77.5B18.5 (x = 0.0, 0.1, 0.2, 0.3, 0.4 and 0.5). The grain size of the Fe3B phase changed from 16 to 23 nm by annealing between 670 degreesC and 760 degreesC for 5 min. From the measurements of the magnetic properties and the reversibility of magnetization, we derived that the mean critical grain size of the Fe3B phase required for full exchange coupling by the Nd2Fe14B phase is about 20 nm. A detailed study of the magnetization-reversal process was made on samples in which the magnetocrystalline anisotropy of the Nd2Fe14B phase was reduced by substitution of Sm for Nd. We showed that the exchange-coupled range can be markedly expanded and that the reduced remanence can be increased with increasing Sm content. (C) 2001 Elsevier Science B.V. All rights reserved.