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. %) alloy s produced by crystallization of an amorphous phase have been investigated. The melt-spun ribbons consist of an amorphous phase in the as-quenched sta te 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 pha ses after annealing at temperatures of 973-1023 K. The nanocomposite alloys exhibit improved values of remanence (J(r)), coercive force (H-cJ), and ma ximum energy product [(BH)(max)] upon annealing at a higher heating rate (a lpha) in the temperature range corresponding to the primary crystallization temperature of bcc-Fe phase. As alpha increases, the grain sizes of each p hase decrease, especially for the Nd2Fe14B phase, and the ratio of total su rface area of the Nd2Fe14B to bcc-Fe phases (S-hard/S-soft), which are eval uated assuming each grain is a sphere, becomes close to 1 for the Fe88Nb2Nd 5B5 alloy. This result indicates that the homogeneity of soft and hard magn etic phases is improved by rapid annealing, and it causes the improvement o f hard magnetic properties of the Fe-rich nanocomposite alloys. (C) 2000 Am erican Institute of Physics. [S0021-8979(00)23908-X].