Rapid-annealing effect on the microstructure and magnetic properties of the Fe-rich nanocomposite magnets

The rapid-annealing effect on the microstructure and magnetic properties of the nanocomposite Fe93-x-yCoxNb2(Nd, Pr)(y)B-5 (x=0-20, y=5-7 at. %) alloys produced by crystallization of an amorphous phase have been investigated. The melt-spun ribbons consist of an amorphous phase in the as-quenched state and the amorphous phase changes to a nanocomposite structure consisting mainly of bcc-(Fe, Co), (Nd, Pr)(2)(Fe, Co)(14)B and residual amorphous phases after annealing at temperatures of 973-1023 K. The nanocomposite alloys exhibit improved values of remanence (J(r)), coercive force (H-cJ), and maximum energy product [(BH)(max)] upon annealing at a higher heating rate (alpha) in the temperature range corresponding to the primary crystallization temperature of bcc-Fe phase. As alpha increases, the grain sizes of each phase decrease, especially for the Nd2Fe14B phase, and the ratio of total surface area of the Nd2Fe14B to bcc-Fe phases (S-hard/S-soft), which are evaluated assuming each grain is a sphere, becomes close to 1 for the Fe88Nb2Nd5B5 alloy. This result indicates that the homogeneity of soft and hard magnetic phases is improved by rapid annealing, and it causes the improvement of hard magnetic properties of the Fe-rich nanocomposite alloys. (C) 2000 American Institute of Physics. [S0021-8979(00)23908-X].