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

Nanocrystalline Fe81.3Co4Si0.5B9.5P4Cu0.7 alloy shows low coercivity (Hc <10 A/m), but a minor increase in Co from 4 to 5 at.% (Fe81.3Co5Si0.5B8.5P4Cu0.7) results in a drastic increase in Hc (>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 Hc. A large numbers of bcc Fe(-Co) nuclei (∼2-5 nm) in as-quenched state results in a high magnetic flux density (Bs) after optimum annealing. It is possible to obtain a Bs of 1.88-1.90 T in this system but it is at the cost of an increase in Hc.