Effect of annealing on torsion giant impedance of Co-rich amorphous wires with vanishing magnetostriction

Torsion giant impedance ~TGI! ratio, (DZ/Z)j , has been measured in as-cast and annealed ~Co0.94Fe0.06)72.5B15Si12.5 amorphous wire. Two-step current annealing has been employed: pre-annealing ~550 mA, 2 min! plus current annealing under torsion stress ~33 p rad/m, 450 mA, up to tann5105 min!. Pre-annealing results in a drastic increase of (DZ/Z)j ~from 155% to 260%! and decrease of the asymmetry. The consequent annealing under torsion of pre-annealed sample gives rise to the appearance of significant asymmetry of the TGI. In this case (DZ/Z)j has a tendency to achieve rather asymmetric shape with a sharp maximum at certain torsion jm . The value of jm depends on the time of annealing under torsion and on the applied torsion during the annealing. The asymmetric shape of the (DZ/Z)j in as-cast state and after torsion annealing could be ascribed to the spontaneous or induced helical anisotropy, which can be compensated by the application of certain torsion stress. Evolution of (DZ/Z)j(j) and jm ~j! as a function of annealing time reflects the kinetic of the induced magnetic anisotropy. © 2002 American Institute of Physics. @DOI: 10.1063/1.1447519#