Measurement of field-dependent surface impedance tensor in amorphous wires with circumferential anisotropy

The surface impedance tensor sigma is found for 120 mu m diameter Co-based amorphous wires as a function of a dc axial magnetic field H-ex. Contrary to the usual practice of giant magneto-impedance experiments, the voltage response is measured not only across the wire but also in the external coil (secondary coil), and the wire is subjected to an ac current flowing through it or the coil (primary coil), which allows all the components of the impedance tensor sigma(zz), sigma(Phi Phi) and sigma(z Phi)=sigma(Phi z) to be determined. Under certain conditions, they all are very sensitive to H-ex (especially sigma(Phi Phi): 100-250%/Oe) but exhibit a different field behavior: the diagonal terms sigma(zz) and sigma(Phi Phi) are symmetrical with respect to H-ex whereas the off-diagonal terms sigma(z Phi)=sigma(Phi z) are antisymmetrical, and at high frequencies, sigma(zz) has a minimum at H-ex=0 where sigma(Phi Phi) has a maximum. The effect of a moderate dc bias current I-b is demonstrated to be favored for sigma(z Phi) and sigma(Phi z), which are nearly zero for I-b=0. These new magneto-impedance characteristics may further expand the application opportunities for the GMI effect. <(C)> 2000 American Institute of Physics.