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The saturation induction Bs and the relation between Curie temperature Tc and crystallization temperature Tx were systematically studied for Co-rich (FeCo)100−y(SizB1−z)y systems with zero magnetostriction in the range of 20⩽y⩽30 and 0⩽z⩽0.6. With increasing metalloid content y, Bs and Tc decrease and Tx increases. Small addition of Si into FeCo-B alloys greatly increases Tx, while it slightly decreases Bs and Tc . The highest Bs alloy with Tx⩾Tc was determined to be (FeCo)75.5(Si0.15B0.85)24.5 (Bs = 9.8 kG). Magnetic annealing was studied for toroidal shape alloys with different Si/(Si+B) content z but with the same Bs of 10.6 kG (Tx<Tc). It was found that residual induction Br for an alloy containing no Si (z = 0) was anomalously small (5.6 kG), while small additions of Si (z = 0.1) greatly increased Br (10 kG). This is mainly attributed to increases in the magnetically induced anisotropy Ku by a small addition of Si. The change in Ku is unexplainable by the pair model. Another contribution to the change in Br may be the weak random local anisotropy due to the intrinsic amorphous structure of the alloy. Interstitially sited Si plays an important role in increasing the magnetic ordering as well as structural stability.

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The metalloid content dependence of zero magnetostrictive composition of an amorphous (FexCo1−x)100−y(Si0.4B0.6)y alloy in an as-quenched state is experimentally determined. The iron content (x) of the zero magnetostrictive composition was found to decrease with an increase in the metalloid content (y). The Curie temperature and saturation magnetization decrease with an increase in the metalloid content, while crystallization temperature increases. With an increase in the saturation magnetization of the present alloys the coercive force increases as Hc∝M3s.