The magnetic properties of Nd9Fe84B7 alloys melt spun and subsequently annealed under various conditions were investigated. It was found that the melt-spun alloys were fully amorphous when they were spun at and above 35 m/s. After annealing for 5~15 minutes at 650~750oC, all alloys melt spun at 40 m/s were transformed to nanocrystalline mixtures of mainly a-Fe and Nd2Fe14B. The evolution of the two phases tended to be more complete at higher temperatures, above 700oC. In particular, the alloys annealed for 10~15 minutes at 700oC, 7~12 minutes at 725oC, or 5~7 minutes at 750oC exhibited smooth demagnetization curves just like those of the quasi-single phase Nd-Fe-B alloys, suggesting the exchange interaction took place effectively in these alloys. Mr/Ms was more than 70% in the alloys annealed above 700oC. The magnetic properties obtained from an alloy annealed for 5 minutes at 750oC were Mr/Ms=72%, iHc=3.4 kOe, Br=12.7 kG, and (BH)max=14.2 MGOe. The estimated grain sizes of Nd2Fe14B and a-Fe in the alloy were ~55 nm and ~10 nm, respectively.

Keywords: Exchange interaction, Nanocomposite alloys, Coercivity, Remanence, Melt spinning