Atomic ordering and magnetic properties in Nd57Fe20B8Co5Al10 solids

Nd57Fe20B8Co5Al10 bulk cylinders and ribbons were quenched from the melt at different cooling rates using copper mold casting and melt spinning, respe ctively. Both the melt-spun ribbons and the cast cylinders display x-ray di ffraction patterns without obvious crystalline peaks. However, the ribbons show soft magnetic properties, the cylinders are hard magnetic, and the cry stallized alloys are not ferromagnetic at all. On the other hand, the cylin ders show different exothermic transformation behavior than the ribbons upo n heating to elevated temperatures. While the ribbons are regarded as amorp hous, the structure of the cylinders prepared at slower cooling rate is con sidered to consist of metastable ordered clusters. This structure forms in the undercooled melt most likely due to intensive generation of ordered nuc lei and slow growth kinetics. The magnetic coherence length, which is equiv alent to the dimension of the ordered clusters, may be larger than the exch ange length. Therefore, the cylinders shows much larger coercivity than the ribbons. (C) 2000 American Institute of Physics. [S0021-8979(00)06818-3].