The influence of short time heat treatment on the structural and magnetic behaviour of Nd2Fe14B/α-Fe nanocomposite obtained by mechanical milling

The structure, microstructure and magnetic properties of Nd2Fe14B/alpha-Fe nanocomposite obtained after 8 h of high energy milling and different heat treatments are investigated. Two types of annealing have been performed: a conventional heat treatment at 550 degrees C for 1.5 h is used for comparison with rapid annealing for maximum 3 min at 700, 750 or 800 degrees C. It is found that these rapid annealing are much more efficient to promote the recrystallisation of the hard phase with a limited grain growth of the soft magnetic phase thus leading to better magnetic properties. Indeed, the best coercive field of 0.55 T was obtained for samples annealed at 700, 750 and 800 degrees C for 2.5, 1.5 and 1 min respectively. This behaviour is explained by both, a good recovery of crystallinity for the hard phase and relatively small size Fe nanocrystallites. The different heat treatments do not result in significant changes for remanent magnetization, which is between 93 and 107 A m(2)/kg. X-ray diffraction patterns were used to follow the evolution of the structure and the microstructure. The magnetic behaviour was checked from hysteresis curves and dM/dH vs. H plots. (C) 2011 Elsevier B.V. All rights reserved.