Ferromagnetic Wires Composite Media with Tunable Scattering Spectra at Microwaves

TL;DR

This study experimentally demonstrates ferromagnetic wire composites with microwave spectra that can be tuned by magnetic field or stress, confirming theoretical predictions and enabling applications like tunable microwave surfaces and structural sensing.

Contribution

First experimental verification of theoretically proposed ferromagnetic wire composites with tunable microwave scattering spectra under external stimuli.

Findings

Transmission spectra change by up to 10dB with small magnetic fields or stress.

Results align quantitatively with effective medium theory.

Large impedance variations near antenna resonance cause significant response changes.

Abstract

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 magneto-impedance 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.