We have presented herein the results of microstructure, surface magnetic domains (SMDs), and giant magneto-impedance (GMI) effect of melt-extracted Co68.15Fe4.35Si12.25B11.25Nb2Cu2 amorphous wires for the first time employed by using a cryogenic Joule annealing (CJA) technique with large DC current amplitude. Compared with the conventional JA method, experimental results indicate that the maximum GMI ratio [ΔZ/Z0]max achieves up to 425% at 8.1 MHz with monotonic increase of the axial magnetic field Hex up to 6.5 Oe for 300 mA (equal to around 1.06 × 106 A/dm−2) CJA-ed wire, which is about 75% larger than the [ΔZ/Z0]max for the 100 mA (nearly 3.53 × 105 A/dm−2) JA-ed microwires. The remarkable features of large and linearly sensitive response field (2.5 ∼ 6.5 Oe) and the sensitivity of 99.4%/Oe with higher GMI ratio simultaneously make the CJA tailored melt-extracted microwires promising candidate materials for miniaturized GMI sensors. Another interesting result of GMI profiles of 200 mA (appropriately equal to 7.07 × 105 A/dm−2) CJA-ed wire show a linear response to Hex (ranging from 10 to 80 Oe or more), this behavior of GMI curves can be explored to fabricate bi-sensor. Large response field proves to originate from the intensive coupling between the radial stress field and the circumferential magnetic field during CJA process. The effect of outer-shell microstructure and complex SMD for 300 mA CJA-ed microwire is attributed to the fact that liquid nitrogen hinders the evolution of circumferential domain structure to some extent and protects the amorphous structure in the shell region.

We present an effective approach to improve the sensitivity of inductance coil sensors by designing a sensor core that consists of multiple soft ferromagnetic microwires. A systematic study of the longitudinally excited magneto-inductive (LEMI) effect has been performed in a non-magnetic copper wire coil with a filler composed of multiple Co-rich amorphous microwires. Melt-extracted microwires of Co68.2Fe4.3B15Si12.5 and glass-coated microwires of Co68B15Si10Mn7 with excellent soft magnetic properties were used for this study. We have shown that the LEMI ratio and field sensitivity of an inductive coil depend strongly upon the filler-to-air ratio inside the coil, the magnetic softness, and the anisotropy axis distribution of the microwire. Relative to a single-microwire based sensor, the LEMI ratio and field sensitivity of a multi-microwire based sensor are enhanced by three to four times, when varying the number of microwires inside the inductive coil. The sensitivity of the sensor using four glass-coated Co68B15Si10Mn7 microwires in the core reaches a maximum value of 1957%/Oe. Our study paves a pathway for the development of novel room-temperature electric contact free magnetic sensors for use in industry, biomagnetism, space science, and geoscience.

The surface roughness, surface magnetic domain structure (SMDS), and high frequency magneto-impedance (MI) response of melt-extracted Co69.25Fe4.25Si13B13.5 microwires with 1 at.% Nb substitution for B have been studied by atomic force microscopy (AFM), magnetic force microscopy (MFM), and impedance analyzer, respectively. We show that the Nb doping significantly increases the domain width from 729 to 1028 nm, while preserving the low surface roughness (∼2 nm) of the base composition. As a result, a greater improvement of the high frequency MI response (∼300%/Oe at 20 MHz) is achieved in the Nb-doped microwire. A well-defined circumferential anisotropy formed with Nb-substitution is key to a highly sensitive MI field sensor.

We study the influence of a direct bias current on the magnetoimpedance (MI) in melt-extracted amorphous CoFeSiB microwires and the effective electromagnetic properties of epoxy composites filled with these microwires. Our analysis reveals two remarkable features of the current dependence of MI in the range of gigahertz frequencies: a redshift of the dielectric resonance frequency and a decrease of the peak resonance of the effective permittivity as the bias current increases. Both effects are intrinsically linked to the influence of the polymer matrix on the magnetic structure and properties of the microwires. A discussion of these results is proposed in terms of two competing effects of the bias current, i.e., the induced additional effective field in the plane normal to the wire axis and the stress relief from Joule heating.