Strain-induced anisotropy in amorphous alloys and the effect of toroid diameter on magnetic properties

Abstract

The magnetic properties of toroids wound from amorphous Fe-B-Si alloy ribbons has been examined as a function of toroid diameter and for ribbon widths from 0.1 cm to 2.5 cm. For the smallest ribbon widths, as the diameter decreased from 20 cm to 1 cm, the magnetic coercivity before annealing increased by a factor of ten and the remanence-to-saturation ratio decreased from ∼0.7 to less than 0.4. After annealing in a field, the coercivity still increased with decreasing diameter by a factor of ten and the ratio of remanence-to-saturation dropped from ∼0.9 to ∼0.45. The losses after annealing similarly increased by a factor of ten and the permeability decreased. These lowest losses, obtained from large diameter toroids, are below those of the best commercial alloys (Supermalloy, 4-79 Mo-Permalloy, and Deltamax) and are well below any previously reported amorphous alloy. Toroids made from wider tapes, 1 cm and 2.5 cm in width, of similar Fe-B-Si compositions, were found to have properties essentially independent of their diameter. This difference in behavior between narrow and wide tapes can be understood on the basis of their differences in strain distribution. The approach to saturation after annealing was found to improve with increasing nickel in the amorphous alloys Fe x Ni 80-x B 20 . This is attributed to the decrease in magnetostriction which decreased the anisotropy arising from strain induced ordering. This assumption was confirmed when it was noted that the approach to saturation became much easier when the inner half of the thickness of an annealed toroid tape was etched away removing material with annealed in strain induced ordering. The impairment in properties of toroids with decrease in diameter is thus attributed to the increase in the strain induced ordering anisotropy. In order to eliminate the effect of toroid diameter on the magnetic properties we wound the toroids with sufficient tension to overcome the compressive forces developed in the tapes due to their radius of curvature. Rather than improving the properties, winding with tension significantly deteriorated the properties. This is believed to be due to the radial forces introduced by the winding tension. The effects of tensional loads and of compressive loads on the surface of straight ribbons were studied and confirmed the results obtained in the toroids.