Soft magnetic properties, magnetoimpedance and torsion-impedance effects in amorphous and nanocrystalline FINEMET alloys: Comparison between ribbons and wires

A comparative study on the magnetoimpedance (MI) and torsion-impedance (TI) effects between Fe_73.5Si_13.5B₉Nb₃Cu₁ FINEMET ribbons and wires has been performed. The samples were annealed for 1 h at temperatures below and above their primary crystallization temperature, around 520°C. The magnetic softening of the samples occurs during the nanocrystallization process, which enhances both the MI and TI effects up to 82% and 75%, respectively, in the wire sample, with respect to its as-quenched state. A maximum MI ratio of 50% is obtained in the nanocrystalline ribbon. The modified values of saturation magnetization, coercive field and magnetostriction, and changes in the domain structure improve the impedance response of the nanocrystalline wire for its use as a sensor element for detecting small magnetic fields, with a sensitivity of 17%/Oe and torsional stresses with a sensitivity of 700% rad/cm.