Torsional stress impedance and magneto-impedance in (Co0.95Fe0.05)72.5Si12.5B15 amorphous wire with helical induced anisotropy

The magneto-impedance effect Delta Z/Z(H) = [Z(H) - Z(H-max)]/Z(H-max) has been measured in (Fe0.95Co0.05)(72.5)B15Si12.5 wire under torsion stress, xi (torsion angle per unit length) with axial magnetic field (H) as parameter. Without stress (Delta Z/Z)(H)(H) dependence has a non-monotonous shape with first an increase of total impedance Z and then a decrease, i.e. shows a maximum at certain axial magnetic field H-m. It was found that the torsion stress dependence of electrical impedance (torsion impedance), (Delta Z/Z)(xi) = [Z(xi) - Z(xi(max))]/Z(xi(max)), has asymmetric character with a clear maximum at torsion angle, xi around 7 pi rad m(-1) in as-cast wire, while (Delta Z/Z)(xi) reaches a maximum around 170%. Thermal treatments under torsion stress (without and with a previous annealing stage) develop a helical anisotropy on the amorphous wire, which drastically modifies the (Delta Z/Z)(xi) response. Such treatments were carried out by using current annealing which resulted in a drastic increase of the maximum (Delta Z/Z)(xi) up to 225%, and a change of torsion dependence of Delta Z/Z with a tendency to a finally symmetric dependence of (Delta Z/Z)(xi) (xi). The maximum (Delta Z/Z)(xi) ratio, (Delta Z/Z)(xi m), was obtained under torsion stress of xi = 20 pi rad m(-1) (in a torsion annealed sample) and xi = 11 pi rad m(-1) (with pre-annealing and torsion annealing). Observed dependences were explained taking into account the frozen-in magneto-elastic anisotropy owing to the internal stress distribution during the fabrication process, the helical anisotropies induced by the torsion strain and that developed by thermal treatment under torsion stress (torsion annealing). The differences in the shape and intensity of the maximum (Delta Z/Z)(xi m), between the torsion annealed and pre-annealed and torsion annealed samples should be ascribed to the visco-elastic character of the helical anisotropy induced by torsion stress.