Magnetic domains and magnetoimpedance effect during the nanocrystallization of Fe73.5Cu1Nb3Si16.5B6 ribbons

Nanocrystalline Fe-based alloys have softer magnetic properties such as larger saturation inductions and permeabilities, smaller magnetostrictions, coercive fields and anisotropies compared to the same alloys in the amorphous state, which are suitable properties for using these materials as magnetic devices or sensors based on magnetoimpedance (MI) effect. Melt-spun ribbons made of Fe73.5Cu1Nb3Si16.5B6 were annealed at different temperatures until crystallization, leading to devitrification of the initial state into a two-phase system (amorphous + FeSi grains), to study the correlations between the evolution of the domain widths, in both surfaces of the annealed samples, as well as other parameters such as saturation magnetostriction and electrical resistivity, with the MI effect response. Magnetic domains have been observed by means of the Bitter technique and we have established that after heat treatments, the domain width diminishes from 4.5±0.1 μm for the as-cast ribbon to 2.7±0.1 μm for the sample annealed at 475°C, which is in agreement with theoretical predictions. We also show that the maximum MI ratio is obtained for the ribbons annealed in the temperature range of 550–580°C, when the samples show an optimum nanocrystalline state and exhibit softer magnetic properties.