Two-peak feature of the permittivity spectra of ferromagnetic microwire/rubber composites

The effective permittivity of CoFeSiB microwire/rubber composites is investigated in the microwave range of frequencies. As the frequency is varied between 0.3 and 6 GHz with or without magnetic field excitation, we identify a two-peak structure of the permittivity spectra. This spectral feature may be attributed to a core-shell structure of the microwires. That is, a core amorphous phase which is surrounded by a small amount of crystalline nanoshell. High-resolution transmission electron microscopy provides a direct means to detect the crystalline nanoshell. Electromagnetic simulations show that Drude-Lorentzs model of dispersion can describe the effective permittivity quite well in this range of frequencies.