  This paper systematically studies the changes in the microstructure and
magnetic properties of CoFeSiBNb$_3$ metallic microfibers before and after AC
annealing. The influence of current intensity on the magneto-impedance (MI)
effect of the microfibers was analyzed and the microstructure changes of the
microfibers before and after annealing were explored by means of
high-resolution transmission electron microscopy. Using this data, the
mechanism of current annealing to improve the MI characteristics was further
revealed. The results show that the surface of the CoFeSiBNb$_3$ metallic
microfibers after AC current annealing is smooth and continuous; its general
magnetic properties and MI ratio increase at first with current intensity and
then decrease at higher intensities. During the current annealing process,
Joule heat eliminates residual stresses in the microfibers while forming
atomically ordered micro-domains, which improves the degree of its
organizational order. Meanwhile, a stable toroidal magnetic field is generated,
which promotes the distribution of the magnetic domain structure of the
microfibers, thereby improving the MI effect.
