Nanocrystalline Nd2Fe17 synthesized by high-energy ball milling: Crystal structure, microstructure and magnetic properties

Nanocrystalline Nd2Fe17 powders have been obtained by means of high-energy ball milling from nearly single-phase bulk alloys produced by arc melting and high temperature homogenization annealing. The rhombohedral Th2Zn17-type crystal structure of the bulk alloy remains unaltered after the milling process, with almost unchanged values for the cell parameters. However, the severe mechanical processing induces drastic microstructural changes. A decrease of the mean crystalline size down to around 10nm is observed, giving rise to a considerable augmentation of the disordered inter-grain boundaries. This modification of the microstructure affects the magnetic behaviour of the milled powders, although the magnetic structure remains collinear ferromagnetic. While a unique ferro-to-paramagnetic transition temperature, TC = 339 2K, is observed in the bulk alloy, the nanocrystalline samples exhibit a more likely distribution of TC values. The latter seems to be responsible for the significant broadening of the temperature range in which magneto-caloric effect is observed, and the lowering of the maximum value of the magnetic entropy change. © 2010 IOP Publishing Ltd.Financial support from FEDER and Spanish MICINN (Projects Nos MAT2008-06542-C04-03, MAT2007-65227 and NAN2004-09203-C04-03) and from the PAI of the Regional Government of Andalucia (Project No. P06-FQM-01823) is acknowledged.Peer Reviewe