Recent experiments have discovered a giant magneto-impedance (MI) effect in FeCoSiB amorphous wires. This effect includes a sensitive change (as much as 60%) in a high frequency wire voltage by an applied dc magnetic field and is thus a high frequency analog of giant magnetoresistance. We consider this phenomenon in terms of ac complex resistance or impedance. The giant MI effect is demonstrated to arise from a combination of a skin effect and a strong field dependence of the circumferential magnetic permeability associated with circular domain wall movements. The theoretical results agree satisfactorily with the existing experimental data.

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Recent experiments have discovered giant and sensitive magneto-impedance and magneto-inductive effects in FeCoSiB amorphous wires. These effects include a sensitive change in an ac wire voltage with the application of a small dc longitudinal magnetic field. At low frequencies (1–10 kHz) the inductive voltage drops by 50% for a field of 2 Oe (25%/Oe) reflecting a strong field dependence of the circumferential permeability. At higher frequencies (0.1–10 MHz) when the skin effect is essential, the amplitude of the total wire voltage decreases by 40%–60% for fields of 3–10 Oe (about 10%/Oe). These effects exhibit no hysteresis for the variation of an applied field and can be obtained even in wires of 1 mm length and a few micrometer diameter. These characteristics are very useful to constitute a highly sensitive microsensor head to detect local fields of the order of 10−5 Oe. In this paper, we review recently obtained experimental results on magneto-inductive and magneto-impedance effects and present a detailed discussion for their mechanism, developing a general approach in terms of ac complex impedance in a magnetic conductor. In the case of a strong skin effect the total wire impedance depends on the circumferential permeability through the penetration depth, resulting in the giant magneto-impedance effect.

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