  Amorphous Co-rich microwires with excellent soft magnetic and mechanical
properties produced by melt-extraction technique are emerging as a
multifunctional material for a variety of applications ranging from
ultrasensitive magnetic field sensors to structural health self-monitoring
composites. There is a pressing need for enhancing these properties to make the
microwires practical for integration into new technologies. Conventional heat
treatments at temperature below crystallization may improve the magnetic
softness of an as-quenched amorphous wire, but usually deteriorate the good
mechanical characteristic of the wire due to crystallization. To overcome this,
we propose a new approach that utilizes the advantages of a multi-step Joule
current annealing method to design novel (nanocrystal, amorphous)/amorphous
core/shell structures directly from as-quenched amorphous microwires. These
results show that the density and size of nanocrystals in the core can be
optimized by controlling the Joule current intensity, resulting in the large
enhancement of soft magnetic and giant magneto-impedance properties, while the
amorphous shell preserves the excellent mechanical strength of the microwire.
This study also provides a new pathway for the design of novel core/shell
structures directly from rapidly quenched amorphous magnetic materials that are
currently exploited in high frequency transformers, sensing and cooling
devices.
