  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.
