  This paper concerns the theoretical and experimental investigation of the
magneto-impedance (MI) effect in amorphous wires in terms of the surface
impedance tensor. Physical concepts of MI and problems of significant practical
importance are discussed using the results obtained. The theoretical analysis
is based on employing the asymptotic-series expansion method of solving the
Maxwell equations for a ferromagnetic wire with an ac permeability tensor of a
general form associated with magnetisation rotation. The magnetic
structure-dependent impedance tensor is calculated for any frequency and
external magnetic field, and is not restricted to the case when only strong
skin-effect is present. This approach allows us to develop a rigorous
quantitative analysis of MI characteristics in wires, depending on the type of
magnetic anisotropy, the magnitude of dc bias current, and an excitation
method. The theoretical model has been tested by comparing the obtained results
with experiment. For the sake of an adequate comparison, the full impedance
tensor is measured in CoFeSiB and CoSiB amorphous wires having a
circumferential and helical anisotropy, respectively, by determining the S21
parameter. In cases, when the rotational dynamics is responsible for the
impedance behaviour, there is a reasonable agreement between the experimental
and theoretical results. Such effects as the ac biased asymmetrical MI in wires
with a circumferential anisotropy, the transformation in MI behaviour caused by
a dc current (from that having a symmetric hysteresis to an asymmetric
anhysteretic one) in wires with a helical anisotropy are discussed.
