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.

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.

A new type of metal-dielectric composites has been proposed that is characterised by a resonance-like behaviour of the effective permeability eff µ in the infrared and visible spectral ranges. This material can be referred to as optomagnetic medium. It consists of conducting inclusions in the shape of non-closed contours or pairs of parallel sticks with length of 50-100 nm embedded in dielectric matrix. The analytical formalism developed is based on solving the scattering problem for considered inclusions with impedance boundary condition, which yields the current and charge distributions within the inclusions. The magnetic properties originated by induced currents are enhanced by localised plasmon modes, which make an inclusion resonate at a much lower frequency than that of the half-wavelength requirement at microwaves. It implies that microstructure can be made on a scale much less than the wavelength and the effective permeability is a valid concept. The presence of the effective magnetic permeability and its resonant properties lead to novel optical effects and open new possible applications. In particular, the condition for Brewster's angle becomes different resulting in reflectionless normal incidence from air (vacuum) if the effective permeability and permittivity are the same. The resonant behaviour of the effective permeability of the proposed optomagnetic medium could be used for creation of optical polarizes, filters, phase shifters, selective lenses.

We demonstrate composite media with ferromagnetic wires that exhibit a frequency region at the microwave regime with scattering spectra strongly dependent on an external magnetic field or stress. These tunable composite materials have recently been proposed theoretically; however, no direct experimental verification has been reported. We used composite materials with predominantly oriented CoFeCrSiB glass-coated amorphous wires having large magnetoimpedance at GHz frequencies. The free space measurements of reflection and transmission coefficients were conducted in the frequency range 18 GHz in the presence of an external static magnetic field or stress applied to the whole sample. In general, the transmission spectra show greater changes in the range of 10dB for a relatively small magnetic field of few Oe or stress of 0.1 MPa. The obtained results are quantitatively consistent with the analytical expressions predicted by the effective medium arguments. The incident electromagnetic wave induces an electrical dipole moment in each wire, the aggregate of which forms the effective dipole response of the whole composite structure in the radiative near or far field region. The field and stress dependences of the effective response arise from a field or tensile stress sensitivity of the ac surface impedance of a ferromagnetic wire. In the vicinity of the antenna resonance the variations in the magnetoimpedance of the wire inclusions result in large changes of the total effective response. A number of applications of proposed materials is discussed including the field tunable microwave surfaces and the self-sensing media for the remote non-destructive evaluation of structural materials.

 The effect of tensile stress on magnetoimpedance (MI) in CoMnSiB amorphous wires at microwave frequencies (0.53 GHz) is investigated both experimentally and theoretically. In the presence of the dc bias magnetic field of the order of the anisotropy field, the impedance shows very large and sensitive change when the wire is subjected to a tensile stress: 100% and 60% per 180 MPa for frequencies 500 MHz and 2.5 GHz, respectively. It is demonstrated that this behavior owes mainly to the directional change in the equilibrium magnetization caused by the applied stress and field, which agrees well with the theoretical results for the surface impedance. This stress effect on MI is proposed to use for creating microwave stress-tunable composite materials containing short magnetic wires. The analysis of the dielectric response from such materials shows that depending on the stress level in the material, the dispersion of the effective permittivity can be of a resonant or relaxation type with a considerable change in its values (up to 100% at 600 MPa). This media can be used for structural stress monitoring by microwave contrast imaging. a)

Two approaches for obtaining asymmetrical magnetoimpedance (MI) characteristics in Co-based negative magnetostrictive amorphous wires are analyzed in terms of the surface impedance tensor ζ̂, which is expressed in the form of orthogonal expansions in Bessel functions in a general case of a helical magnetization. The asymmetry in MI behavior with respect to an axial dc magnetic field can be related to either a certain asymmetric arrangement of the dc magnetic configuration or a contribution to the wire voltage due to the ac cross-magnetization process (represented by the off-diagonal component of ζ̂). The first case is realized in a wire having a helical anisotropy and subjected to an ac current superposed with a dc current. In the other approach, the asymmetric voltage response can be obtained by applying the ac current in series through the wire and the coil mounted on it. No helical anisotropy is required in this case. These kinds of asymmetrical MI are especially important for developing autobiased linear MI sensors.

Asymmetrical giant magnetoimpedance (AGMI), which utilizes a high frequency bias field hb, is realized in a Co-based amorphous wire having a circumferential anisotropy in the outer region. No asymmetry in the dc magnetic configuration is needed in this case. AGMI is discussed in terms of the surface impedance tensor, demonstrating that the effect of hb is related to the role of the off-diagonal component of the impedance in the voltage response measured across the wire. This effect is important for developing autobiased linear magnetic sensors. Using two oppositely biased wires, a near-linear voltage output (±4 mV) is obtained in the range of ±5 Oe for the sensed dc field at a frequency of 8 MHz.