Magnetic properties and microstructure of nanocomposite R-2(Fe,Co,Nb)(14)B/(Fe,Co) (R = Nd, Pr) magnets

Nanocomposite R-2(Fe, Co, Nb)(14)B/(Fe, Co) (R=Nd, Pr) magnets prepared by crystallizing the as-made R-8(Fe, Co, Nb)(86)B-6 amorphous melt-spun ribbon s have been studied. The coercivity is found to depend mainly on the grain size of the soft phase which is very sensitive to the sample composition. T he average grain size is about 30 nm in R8Fe86B6, but the microstructure is not homogeneous and there are several large alpha-Fe grains with sizes up to 50-100 nm. The coercivities are 3.3 kOe in Nd8Fe86B6 and 4.9 kOe in Pr8F e86B6 samples. Nb substitution significantly reduces the grain size of alph a-Fe and increases the coercivity. The highest coercivities obtained are 5. 5 kOe in Nd-8(Fe0.97Nb0.03)(86)B-6 and 9.3 kOe in Pr-8(Fe0.92Nb0.08)(86)B-6 samples. Co substitution for Fe increases the grain size of both the 2:14: 1 phase and alpha-Fe and dramatically decreases the coercivity. Increasing the B content in Co substituted samples leads to the formation of a more ho mogeneous and finer microstructure and thus to a partial recovery of the co ercivity from 2.3 kOe in Nd-8((Fe0.5Co0.5)(0.97)Nb-0.03)(86)B-6 to 4.3 kOe in Nd-8((Fe0.5Co0.5)(0.97)Nb-0.03)(82)B-10 and from 2.1 kOe in Pr-8((Fe0.5C o0.5)(0.94)Nb-0.06)(86)B-6 to 6.5 kOe in Pr-8((Fe0.5Co0.5)(0.94)Nb-0.06)(82 )B-10. It is further found that Co substitution improves the temperature de pendence of the saturation magnetization. (C) 1999 American Institute of Ph ysics. [S0021-8979(99)47208-1].