Asymmetric torsion giant impedance in nearly-zero magnetostrictive amorphous wires with induced helical anisotropy

The torsion giant impedance (TGI) ratio, (DeltaZ/Z)(xi), has been measured in as-cast (Co0.94Fe0.06)(72.5)B15Si12.5 amorphous wire and after current a nnealing under torsion stress up to 33 pi rad min(-1) at 450 mA for up to t (ann) = 105 min, Strong changes have been induced in the TGI ratio by Joule heating under these conditions. The slightly asymmetric character of the ( DeltaZ/Z)(xi) dependence on applied torsion xi with a broad maximum at arou nd xi = 2.5 pi rad m(-1) has been observed in the as-cast state where the m aximum value of (DeltaZ/Z)(xi) reaches about 155%, Samples treated by Joule heating under applied torsion exhibit a (DeltaZ/Z)(xi) dependence showing a tendency to achieve a finally sharp and rather asymmetric shape with a sh arp maximum at a certain torsion, xi (m). Pre-annealing by Joule heating al so introduces some changes in the observed dependences. A maximum (DeltaZ/Z )(xi) ratio of 330% is obtained after optimal conditions of current anneali ng under torsion. The asymmetric character of the (DeltaZ/Z)(xi) in the as- cast state and after torsion annealing could be ascribed to the spontaneous or induced (after torsion annealing) helical magnetic anisotropy, which ca n be compensated by the application of certain torsion stress. The evolutio n of(DeltaZ/Z)(xi) (xi) dependences with annealing under torsion reflects t he kinetics of the induced magnetic anisotropy in this studied amorphous wi re.