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

Fe/sub 82-x/Co/sub x/Nb/sub 3/Ta/sub 1/Mo/sub 1/B/sub 13/(0 /spl les/ x /spl les/ 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/sub x1/ and T/sub x2/. The Curie temperature of the amorphous phase, T/sub C,amorphous/ increased with x and exceeded T/sub x1/ at x = 18, where nanocrystalline /spl alpha/-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 /spl alpha/-FeCo phase and the loss of magnetization due to the /spl alpha//spl rarr//spl gamma/ phase transition at temperatures between 940 and 970/spl deg/C. In the range 18 /spl les/ x /spl les/ 41, except for x = 20.5, an abrupt drop in magnetization was observed at T/sub x2/ corresponding to the secondary crystallization of a (FeCo)/sub 23/B/sub 6/ phase. For the x = 20. 5 composition, the magnetization was observed to increase at T/sub x2/, fall gradually, and then drop abruptly 140/spl deg/C above T/sub x2/. Therefore, a large magnetic moment was retained to about 790/spl deg/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.