Torsional stress impedance and magneto-impedance in (Co0.95Fe0.05)(72.5)Si12.5B15 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, x i (torsion angle per unit length) with axial magnetic held (H) as parameter . Without stress (Delta Z/Z)(H)(H) dependence has a non-monotonous shape wi th 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 cle ar maximum at torsion angle, xi around 7 pi rad m(-1) in as-cast wire, whil e (Delta Z/Z)(xi) reaches a maximum around 170%. Thermal treatments under t orsion stress (without and with a previous annealing stage) develop a helic al anisotropy on the amorphous wire, which drastically modifies the (Delta Z/Z)(xi) response. Such treatments were carried out by using current anneal ing 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 expla ined taking into account the frozen-in magneto-elastic anisotropy owing to the internal stress distribution during the fabrication process, the helica l 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 a nnealed and pre-annealed and torsion annealed samples should be ascribed to the visco-elastic character of the helical anisotropy induced by torsion s tress.