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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This paper concerns the theoretical and experimental investigation of the magneto-impedance (MI) effect in amorphous wires in terms of the surface impedance tensor ςˆ. 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 ςˆ 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 tensor ςî s measured in CoFeSiB and CoSiB amorphous wires having a circumferential and helical anisotropy, respectively, by determining the 21 S 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.

Recognizing the importance of magnetic thin wires for science and technology, the 6th International Workshop on Magnetic Wires was held on 6–7 July 2010, in Bodrum, Turkey. A wide range of topics were addressed from technological problems of micro‐ and nanowire fabrication, to advanced physical effects and novel applications. Furthermore, critical discussions on the present state of knowledge and future trends arose during the Panel Session on microwires. In the present report, the main aspects addressed at the Discussion Panel and at workshop presentations are summarized with the aim to state the present and future perspectives on these wire systems. This report is focused upon four topics chosen for broad discussions at the Panel: Fabrication and processing, Magnetization reversal and Dynamics, High‐frequency properties, and Technological applications.

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A new type of a composite material is proposed, the microwave permittivity of which changes under the effect of a dc magnetic field applied to the whole composite sample.The composite consists of short ferromagnetic wires embedded into a dielectric matrix. A strong field dependence of the permittivity is seen in the vicinity of the antenna resonance, where the dispersion behaviour can experience a transformation from a resonant spectrum to a relaxation one under the effect of the field. This permittivity behaviour owes to a high sensitivity of the ac surface impedance of a ferromagnetic wire to a magnetic field, known as the magneto-impedance (MI) effect. If the resonance-like dispersion behaviour is realised, the real part of the effective permittivity can be made negative past the resonance for wire inclusion concentrations well below the percolation threshold. Applying a magnetic field, the negative peak continuously decreases as the dispersion tends to become of a relaxation type. The effective permittivity is analysed within a one-particle approximation, by considering a wire piece as an independent scatterer and solving the scattering problem with the impedance boundary condition. A magnetic field is assumed to be applied in parallel to the wire. A new integro-differential equation for the current distribution in a wire is obtained, which is valid for the surface impedance matrix of a general form. This work demonstrates a possibility of using the MI effect to design fieldcontrolled composites and band-gap structures.

-Index Terms -ferromagnetic wire, magneto-impedance sensor, magneto-impedance tensor, diagonal impedance, off-diagonal impedance, antisymmetrical impedance.

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