Domain wall permeability limit for the giant magnetoimpedance effect

Abstract

The magnetoimpedance (MI) effect is based on the change of inductance and resistance under the effect of an external magnetic field. In bulk homogeneous wires or ribbons these two components of the impedance are related to the penetration depth of the electromagnetic field in the material. From simple considerations it is shown that the maximum MI ratio is then proportional to the square root of the relative permeability of the material. That limit is reached for frequencies at which the penetration depth is of the order of the transverse dimensions of the sample. At low operating frequencies, typical of thick geometries, the permeability is limited by the microeddy currents associated with domain wall displacements. The permeability relaxation equivalent to these local eddy currents can be calculated and used in the classical expressions for MI. The real part of the permeability is highly reduced at the relaxation frequency and gives rise to a decrease of the inductance, while the imaginary part contributes to the resistance, with a maximum at the same frequency.