The variation of initial permeability of amorphous ribbons with frequency and annealing conditions

The published values of the initial permeability of amorphous ribbons have been obtained at a flux density of 50 G (5 mT), but it is shown in this study that these values are not the true initial permeability due to the curvature of the B/H characteristic below 20 G (2 mT). The true initial permeability of Metglas 2605SC type material (Fe81B13.5Si3.5C2) and Vitrovac E6010 type material (Co58Fe5Ni10Si11B16amorphous ribbons has been measured, over the frequency range 20 Hz to 20 KHz for various annealing conditions, i.e. temperature from 325°C to 425°C, field and non-field conditions and atmospheres of air, nitrogen and hydrogen. For Metglas 2605SC type material at flux densities below 5 mT (50 G) lower magnetizing fields are necessary for materials annealed at the higher temperatures. Above 5 mT the optimum annealing temperatures depends upon the operating frequency of the material. The initial permeability of Metglas 2605SC type material is more sensitive to annealing temperature than to field conditions. For Vitrovac E6010 type material field annealing dominates the effect of annealing temperature. The highest values of true initial permeability are obtained for field anneals in hydrogen for both materials. These variations in initial permeability with frequency and annealing conditions are explained by considering the domain structure, stress conditions in the ribbon, domain wall anisotropy and the surface effects of the gas atmospheres.