New types of stress sensitive and magnetic field tunable microwave composite
materials are discussed where embedded short ferromagnetic microwire inclusions are used as
controllable radiative elements. The dc external magnetic field is applied to the whole composite
structure. And, the local stress is transferred to the individual microwires through the
accommodating composite matrix. The spatial and angular distributions of microwires can be
random, partly ordered, or completely ordered. For a wide frequency range, the free-space
microwave response of a wire-filled composite can be characterized by a complex effective
permittivity with resonance frequency dispersion. The latter depends on the conductive and
magnetic properties of the microwire inclusions that contribute to the ac microwire
magnetoimpedance (MI). In the vicinity of the so-called antenna resonance frequency, which is
defined by the length of microwires and matrix dielectric constant, any variations in the MI of the
microwires will result in large changes of the effective permittivity, and hence the reflection and
transmission coefficients for an incident microwave. The field or stress dependence of the effective
permittivity arises from the corresponding field or stress sensitivity of the MI in the ferromagnetic
microwires with induced circumferential or helical magnetic anisotropy, respectively. The strong
field tunable effect in the proposed composite materials can be utilized to introduce reconfigurable
microwave properties in coatings, absorbers, and randomizers, and also in new media such as
microwave metamaterials and bandgap wire structures. A maximum field tunability of 30 dB was
achieved for free-space transmission measurements when the external magnetic field changed from
zero to ~40 Oe. The stress sensitivity of reflection and transmission coefficients opens up new
possibilities for the distant non-destructive testing and evaluation of composite materials both in the
laboratory environment and large scale applications. The stress tunability of transmission
coefficient may reach up to 5-8 dB within the elastic limit. The reflection coefficient usually
demonstrates less tunability in both cases (field and stress dependent) and may require a multilayer
structure to achieve better results, but it is always strong enough for the stress sensing applications.
Microwave Properties of Ferromagnetic Nanowires and Applications to Tunable Devices