Exchange coupled R-2(Fe,Co,Nb)(14)B/(Fe,Co) (R=Nd,Pr) and Sm-2(Fe,Co,Cr)(17)C-2/(Fe,Co) nanocomposite magnets

Fe-rich exchange coupled nanocomposite R-8(Fe,Co,Nb)(86)B-6 (R=Nd, Pr) (con sisting of hard 2:14:1 and soft alpha-Fe(Co) phases) and Sm-2(Fe,Co)(15)Cr2 C2 magnets (consisting of hard 2:17 and soft alpha-Fe(Co) grains) have been studied. In both systems the nanocomposite hard/soft microstructure is obt ained by crystallizing the as-made;amorphous melt-spun ribbons. The coerciv ity is found to depend mainly on the grain size of the soft phase which is very sensitive to the sample composition and annealing conditions. In the R -(Fe,Co,Nb)-B system, it is found that substitution of Nb for Fe significan tly decreases the grain size of the alpha-Fe(Co) and increases the coercivi ty of the magnets. Substitution of Co for Fe increases the grain size and r esults in a dramatic decrease of coercivity. Increasing the B content of Go -substituted samples leads to a partial recovery of the coercivity. Magneti c properties M-r/M-s=0.7 and H-c=6.5 kOe have been obtained for Pr-8((Fe0.5 Co0.5)(0.94)Nb-0.06)(82)B-10. In the Sm-2(Fe,Co)(15)Cr2C2 system, it is fou nd that a higher annealing temperature combined with a shorter annealing ti me leads to a higher coercivity. The optimum annealing condition is at 900 degrees C for 1 min where the highest coercivity of 12.1 kOe is obtained. T he grain size of alpha-Fe in annealed ribbons becomes much smaller with Co substitution, leading to a stronger exchange coupling between the 2:17 and alpha-Fe phase. A significant enhancement in the Curie temperature of the 2 :17 phase is observed in these magnets as compared to their parent alloys d ue to the exchange coupling effect between the 2:17 phase and the Fe(Co) ph ase. (C) 1999 Elsevier Science S.A. All rights reserved.