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The development of ultra-broadband composite absorbers of electromagnetic waves depends largely on optimal combination of the medium characteristics that determine their absorption capacity and conditions for impedance matching and destructive interference. It is possible to achieve a higher absorption level in a wider frequency band by combining a variety of mechanisms enhancing the loss of electromagnetic field energy, for example, by combining specific constituents in a composite matrix. The analysis of various mixing models for constituent parameters is carried out in the effective medium approximation for ferritedielectric composites. It appears that the standard mixing rules do not explain the increase in the effective permittivity of ferrite composites in comparison with that of bulk ferrites. The proposed mechanism of such increase is based on the conductive properties of the ferrite granules and the equivalent capacitance effect. The developed model of permittivity calculation is based on the equivalent capacitor circuits and gives a satisfactory agreement with the experimental data. [ABSTRACT FROM AUTHOR]

Magnetoimpedance (MI) in Co-based microwires with an amorphous and partially crystalline state was investigated at elevated frequencies (up to several GHz), with particular attention paid to the influence of tensile stress on the MI behavior, which is called stress-MI. Two mechanisms of MI sensitivity related to the DC magnetization re-orientation and AC permeability dispersion were discussed. Remarkable sensitivity of impedance changes with respect to applied tensile stress at GHz frequencies was obtained in partially crystalline wires subjected to current annealing. Increasing the annealing current enhanced the axial easy anisotropy of a magnetoelastic origin, which made it possible to increase the frequency of large stress-MI: for 90mA-annealed wire, the impedance at 2 GHz increased by about 300% when a stress of 450 MPa was applied. Potential applications included sensing elements in stretchable substrates for flexible electronics, wireless sensors, and tunable smart materials. For reliable microwave measurements, an improved SOLT (short-open-load-thru) calibration technique was developed that required specially designed strip cells as wire holders. The method made it possible to precisely measure the impedance characteristics of individual wires, which can be further employed to characterize the microwave scattering at wire inclusions used as composites fillers.