Magnetic properties and microstructure of nanocomposite R2(Fe,Co,Nb)14B/(Fe,Co) (R=Nd, Pr) magnets

Abstract

Nanocomposite R2(Fe,Co,Nb)14B/(Fe,Co) (R=Nd, Pr) magnets prepared by crystallizing the as-made R8(Fe,Co,Nb)86B6 amorphous melt-spun ribbons 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. The average grain size is about 30 nm in R8Fe86B6, but the microstructure is not homogeneous and there are several large α-Fe grains with sizes up to 50–100 nm. The coercivities are 3.3 kOe in Nd8Fe86B6 and 4.9 kOe in Pr8Fe86B6 samples. Nb substitution significantly reduces the grain size of α-Fe and increases the coercivity. The highest coercivities obtained are 5.5 kOe in Nd8(Fe0.97Nb0.03)86B6 and 9.3 kOe in Pr8(Fe0.92Nb0.08)86B6 samples. Co substitution for Fe increases the grain size of both the 2:14:1 phase and α-Fe and dramatically decreases the coercivity. Increasing the B content in Co substituted samples leads to the formation of a more homogeneous and finer microstructure and thus to a partial recovery of the coercivity from 2.3 kOe in Nd8((Fe0.5Co0.5)0.97Nb0.03)86B6 to 4.3 kOe in Nd8((Fe0.5Co0.5)0.97Nb0.03)82B10 and from 2.1 kOe in Pr8((Fe0.5Co0.5)0.94Nb0.06)86B6 to 6.5 kOe in Pr8((Fe0.5Co0.5)0.94Nb0.06)82B10. It is further found that Co substitution improves the temperature dependence of the saturation magnetization.