Effects of minor precipitation of large size crystals on magnetic properties of Fe-Co-Si-B-P-Cu alloy

Nanocrystalline Fe_81.3Co₄Si_0.5B_9.5P₄Cu_0.7 alloy shows low coercivity (H_c <10 A/m), but a minor increase in Co from 4 to 5 at.% (Fe_81.3Co₅Si_0.5B_8.5P₄Cu_0.7) results in a drastic increase in H_c (>60 A/m). In terms of structure both the alloys in as-quenched state exhibit similar X-ray diffraction patterns (i.e. X-ray amorphous). Similar to low concentration of Co in the alloy, it is possible to realize a fine nanocrystalline structure made from bcc Fe-Co phase for higher concentration of Co. However, existence of a very few large sized crystals (above 100 nm) in as-quenched state enhances the H_c. A large numbers of bcc Fe(-Co) nuclei (∼2–5 nm) in as-quenched state results in a high magnetic flux density (B_s) after optimum annealing. It is possible to obtain a B_s of 1.88–1.90 T in this system but it is at the cost of an increase in H_c.