Asymmetric torsion stress giant magnetoimpedance in nearly zero magnetostrictive amorphous wires

The effect of the torsion strain on the electrical impedance in (Co0.95Fe0.05)(72.5)Si12.5B15 amorphous wire has been investigated. The torsion stress impedance ratio, defined as (Delta Z/Z)(xi) = [(Z(xi)-Z(xi(max))]/Z(xi(max)), has an asymmetric character with a broad maximum at around 20 rad/m. After Joule heating without torsion (Delta Z/Z)(xi) has a tendency to achieve finally a sharp and nearly symmetric shape. Maximum changes of (Delta Z/Z)(xi), of 270% is obtained after optimal conditions of current annealing. Besides, GMI ratio, (Delta Z/Z)(H) = [(Z(H)-Z(H-max)]/Z(H-max) can be significantly improved too reaching 350% in the particular case of current annealing at 550 mA for 0.5 min. The asymmetry of the (Delta Z/Z)(xi) in the as-cast state and after torsion annealing could be ascribed to the spontaneous or induced helical anisotropy. (C) 2000 American Institute of Physics. [S0021-8979(00)56708-5].