Theory of giant magneto-impedance effect in amorphous wires with different types of magnetic anisotropy

The effect of various types of magnetic anisotropy on the giant magneto-impedance spectra of amorphous wires with low magnetostriction is studied theoretically. For the first of the model considered the easy anisotropy axis is supposed to be parallel to the wire axis, whereas for the second model it has circumferential direction. In case of a wire with axial type of magnetic anisotropy the transverse magnetic permeability is obtained as a function of external magnetic field amplitude H-0 and other magnetic parameters of the wire. A strong dependence of this quantity on H-0 is shown to explain the giant magneto-impedance (GMI) effect in this type of amorphous wire. Besides, the amplitude of the GMI effect depends substantially on the value of a phenomenological damping parameter. In case of a wire with circumferential anisotropy the classical expression [Landau and Lifshitz, Electrodynamics of Continuous Media, 2nd ed, Pergamon, New York, 1984] for the wire impedance is not valid in the range of external magnetic field 0 less than or equal to H-0 less than or equal to H-a, where H-a is the anisotropy field. In this range of external magnetic field the wire impedance becomes a tenser. It has both longitudinal and transverse components that can be measured experimentally [Antonov et al., IEEE Trans. Magn. 33 (1997) 3367]. In case of a wire with circumferential anisotropy the peak of the longitudinal component of wire impedance as the function of H-0 corresponds to the anisotropy field, whereas in case of axial anisotropy it is found at zero magnetic field. The results obtained show that the good magnetic softness of amorphous wire is one of the most important conditions to observe the GMI effect; neither any domain structure nor circumferential anisotropy are strictly necessary. (C) 1998 Elsevier Science B.V. All rights reserved.