Effect of tensile stress during ultra-rapid annealing on the soft magnetic properties of Fe-B based nanocrystalline alloys

Abstract

The effect of applied stress during ultra-rapid annealing (URA) on the magnetic properties has been investigated for rapidly-solidified (Fe1-xCox)86B13Cu1 (x = 0–0.05) alloys. Nano-meter scale grains with an average size of about 15 nm and a small coercivity value of 5 ± 1 A/m are confirmed for all the alloys after URA at 763 K for 0.5 s. The saturation magnetic polarization (Js) of these nanocrystalline alloys shows a slight increase with Co content from 1.88 T at x = 0 to 1.94 T at x = 0.05. A clear creep-induced anisotropy (Ku) up to 460 ± 20 J/m3 is observed with a hard axis along the direction of applied tensile stress for x = 0–0.04 while an easy axis is confirmed in the same direction for x = 0.05. The saturation magnetostriction (λs) of nanocrystalline (Fe1-xCox)86B13Cu1 increases monotonously from + 13 ± 2 ppm to + 20 ± 2 ppm with an increase of x from 0 to 0.05 and the observed change in the anisotropy axis cannot be attributed to the bulk magnetoelastic effect. The magnetostriction was also measured for polycrystalline Fe1-xCox (x = 0–0.1) binary alloys prepared for comparison and λs of bcc-Fe is confirmed to change its sign from negative to positive by Co addition. A similar trend is confirmed for the local magnetostriction estimated for the bcc-Fe(Co) phase in the nanocrystalline samples by assuming the inverse effect of the local magnetostriction, suggesting that Ku in nanocrystalline (Fe1-xCox)86B13Cu1 (x = 0–0.05) alloys is due to the strain retained within the bcc-Fe(Co) nanocrystallites. Our results demonstrate that the shape of the hysteresis curve in the URA Fe-B based nanocrystalline alloys (HiB-Nanoperm) can be controlled by stress applied to the precursor amorphous ribbons during annealing.