Magnetic properties of Co-substituted Fe-Nb-Ta-Mo-B amorphous alloys

Fe_82-xCo_xNb₃Ta₁Mo₁B₁₃(0 ≤ x ≤ 41) amorphous alloys were produced by rapid solidification. The Co substitutions for Fe on the magnetic and structural properties of as-cast and crystalline alloys were studied. Co substitutions were shown to enhance the saturation magnetization of as-quenched ribbon samples from 108 emu/g at x = 0 to a maximum of 171 emu/g at x = 18. Co substitutions resulted in only slight changes in the primary and secondary crystallization onset temperatures, T_x1 and T_x2. The Curie temperature of the amorphous phase, T_C,amorphous increased with x and exceeded T_x1 at x = 18, where nanocrystalline α-FeCo phase was observed. The T dependence of the magnetization, with decreasing temperature (after secondary crystallization), was used to determine the demarcation between the Curie transition of the α-FeCo phase and the loss of magnetization due to the α→γ phase transition at temperatures between 940 and 970°C. In the range 18 ≤ x ≤ 41, except for x = 20.5, an abrupt drop in magnetization was observed at T_x2 corresponding to the secondary crystallization of a (FeCo)₂₃B₆ phase. For the x = 20. 5 composition, the magnetization was observed to increase at T_x2, fall gradually, and then drop abruptly 140°C above T_x2. Therefore, a large magnetic moment was retained to about 790°C in this alloy. This composition was the best candidate for the soft magnetic application due to their high moments up to high temperature and low measured coercivity.