Upper limit for the simultaneous existence of high B_s and low H_c in nanocrystalline FeCoSiBPCu alloys

Recently, nanocrystalline Fe-Si-B-P-Cu soft magnetic alloys, NANOMET^® with high Fe contents of above 83 at% have attracted much attention as a future core material for high performance magnetic applications because of their high magnetic flux density (B_s ∼ 1.80 T), low coercivity (H_c), significantly low core loss (W) and low materials cost [1]-[3]. Small addition of Co (∼ 4 to 5 at%) is shown to be effective in producing wide ribbons for commercial applications (such as transformers, motors etc.). High concentrations of Fe and Co (very close to the limit for the formation of amorphous state) in the alloys were used to obtain high B_s (∼ 1.84 T) [2]. Simultaneous existence of B_s similar to oriented steel and H_c lower than it are the driving factors to further increase in magnetic elements in the alloy. Generally it is believed that the amorphous structure (a broad halo in X-ray diffraction curve) of as quenched ribbons is very important for achieving a uniform nano-crystalline structure with low coercivity (H_c) after optimum heat treatment. We have noticed that the nanocrystalline Fe_81.2Co₄Si_0.5B_9.5P₄Cu_0.8 alloy shows low H_c < 10 A/m, but a minor increase in Co from 4 to 5 at.% (Fe_81.3Co₅Si_0.5B_9.5P₄Cu_0.7) results in a drastic increase in H_c to ∼ 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). The B of this alloy approaches to ∼ 1.9 T.