Magnetic field and mechanical stress tunable microwave properties of composites containing Fe-based microwires

We report the magnetic field and stress tunable microwave behaviour of microwire-epoxy composites containing Fe-based microwires of different lengths. Observation of a crossover magnetic field at 600 Oe, when the wire length is less than 35 mm, is reasoned to be related to the competition between giant magnetoimpedance and ferromagnetic resonance in addition to the wire's intrinsic properties. Samples containing 25 mm microwires exhibit a linear increase of the permittivity tunability as the tensile strain is increased up to 2.0%. This can be attributed to the longitudinal anisotropy field and the wire alignment under strain.