  New types of tunable composite materials are considered, the effective
microwave permittivity of which may depend on an external dc magnetic field or
tensile stress. The composites consist of short pieces of conductive
ferromagnetic microwires embedded into a dielectric matrix. The short wire
inclusions play a role of the elementary scatterers, when the electromagnetic
wave irradiates the composite and induces a longitudinal current distribution
and electrical dipole moment in each inclusion. These induced dipole moments
form the dipole response, which can be characterized by some complex effective
permittivity. The later may have a resonance or relaxation dispersion caused by
the strong current distribution along a wire, which depends on the wire high
frequency surface impedance. In the vicinity of the resonance frequency any
variations in the wire surface impedance result in a large change of the
current distribution, and hence in the dipole moment of each inclusion and the
effective permittivity on the whole. For a ferromagnetic conductive microwire,
the surface impedance may depend not only on its conductivity but also on the
dc external magnetic field and tension through the so-called magneto-impedance
effect (MI). Therefore, the dispersion of the effective permittivity can be
tuned from a resonance type to a relaxation type, when a sufficient magnetic
field or tensile stress is applied to the composite sample. A number of
applications can be proposed, including the stress-sensitive media for remote
non-destructive health monitoring of different structures, and selective
microwave coatings with field-dependent reflection/transmission coefficients.
