Effects of Metastable Phase Formation on the Magnetic Properties of Pr_4.5Fe₇₇B_18.5 Amorphous Ribbons

Abstract

The effects of isothermal annealing on the magnetic properties of Pr_4.5Fe₇₇B_18.5 amorphous ribbons have been investigated. Two separate reactions increasing coercive force (H_c) were recognized during the progress of annealing. In the first reaction, hard magnetic Pr₂Fe₁₄B particles with nanoscale diameters precipitated in the vicinity of Fe₃B and Pr₂Fe₂₃B₃ particles. The precipitation of fine Pr₂Fe₁₄B particles increased H_c largely but reduced saturation magnetization (M_s) significantly. In the second reaction, the Pr₂Fe₂₃B₃ particles decomposed into Pr₂Fe₁₄B particles containing very fine α-Fe particles within them. This resulted in the second increase in H_c due to both the volume fraction increase of Pr₂Fe₁₄B particles and the exchange-coupling interaction between the Pr₂Fe₁₄B grains and α-Fe particles within them. This phenomenon was observed originally in the present study. In the final stages of annealing, both H_c and M_s were decreased substantially by the decomposition of Pr₂Fe₁₄B into PrFe₄B₄ particle in addition to the coarsening of α-Fe particles.