  We investigate the microwave properties of epoxy-based composite containing
melt-extracted CoFeBSiNb microwires fabricated by a combined current-modulation
annealing (CCMA) technique. We observe a shift of the resonance peak in the
effective permittivity spectra of the composite sample containing annealed 25
mm Nb-doped microwires as an applied magnetic field is increased. This
observation is consistent with the absorption-dominated impedance for thick
microwires and the ferromagnetic resonance phenomenon. It is shown that CCMA is
an appropriate technique to release internal residual stresses. Hence, for
samples containing small amounts of Nb, we observe that CCMA allows us to
suppress the high frequency resonance peak observed in samples containing
as-cast wires. However, for samples containing a high amount of Nb, the high
frequency peak remains despite the CCMA treatment. In this case, the
observation of a two-peak feature in the permittivity spectra is attributed to
the coexistence of the amorphous phase and a small amount of nanocrystallites
distributed at the wire surface. However, due to large magnetostatic energy of
long (35 mm) and short (15 mm) as-cast wires and imperfect wire-epoxy bonding
no shift of the resonance peak and the characteristic double peak of the
permittivity spectrum can be detected. Overall, CCMA emerges as a promising
strategy to control microwave permittivity in composites with melt-extracted
microwires.
