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

Nanocrystalline Fe-based alloys have softer magnetic properties such as lar ger saturation inductions and permeabilities, smaller magnetostrictions, co ercive 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 ribbon s made of Fe73.5Cu1Nb3Si16.5B6 were annealed at different temperatures unti l crystallization, leading to devitrification of the initial state into a t wo-phase system (amorphous + FeSi grains), to study the correlations betwee n the evolution of the domain widths, in both surfaces of the annealed samp les, as well as other parameters such as saturation magnetostriction and el ectrical resistivity, with the MI effect response. Magnetic domains have be en observed by means of the Bitter technique and we have established that a fter heat treatments, the domain width diminishes from 4.5 +/- 0.1 mum for the as-cast ribbon to 2.7 +/- 0.1 mum for the sample annealed at 475 degree sC, which is in agreement with theoretical predictions. We also show that t he maximum MI ratio is obtained for the ribbons annealed in the temperature range of 550 580 degreesC, when the samples show an optimum nanocrystallin e state and exhibit softer magnetic properties. (C) 2001 Elsevier Science B .V. All rights reserved.