Mechanism of the Magneto-Impedance Effect in Negative Magnetostrictive Amorphous Wires

The concept of a magneto-impedance (MI) effect in magnetic amorphous wires is put forward. At certain frequencies, the wire impedance is highly responsive to magnetic properties, owing to the dependency of the skin depth on magnetic permeability. The theoretical analysis of the MI effect is based on the exact expression for the impedance of a magnetic wire in which the dynamic permeability related to domain wall displacements is damped by microscopic eddy currents. The circular domain structure typical of negative magnetostrictive amorphous wires is considered. In this case, the penetration depth depends on the circumferential permeability, which is a sensitive function of the dc longitudinal field. This theoretical approach is found to account satisfactorily for the existing experimental data on the magneto-impedance characteristics of negative magnetostrictive amorphous wires. In the frequency region where the skin depth is comparable to the wire radius, the MI ratio can reach more than 50% in magnetic fields of the order of the anisotropy field.