Amorphous magnetic microwires can be suitable for a variety of technological applications due to their excellent magnetic softness and giant magnetoimpedance (GMI) effect. Several approaches for optimization of soft magnetic properties and GMI effect of magnetic microwires covered with an insulating, flexible, and biocompatible glass coating with tunable magnetic properties are overviewed. The high GMI effect and soft magnetic properties, achieved even in as-prepared Co-rich microwires with a vanishing magnetostriction coefficient, can be further improved by appropriate heat treatment (including stress-annealing and Joule heating). Although as-prepared Fe-rich amorphous microwires exhibit low GMI ratio and rectangular hysteresis loops, stress-annealing, Joule heating, and combined stress-annealed followed by conventional furnace annealing can substantially improve the GMI effect (by more than an order of magnitude).

The influence of Joule heating on magnetic properties, giant magnetoimpedance (GMI) effect and domain wall (DW) dynamics of Fe₇₅B₉Si₁₂C₄ glass-coated microwires was studied. A remarkable (up to an order of magnitude) increase in GMI ratio is observed in Joule heated samples in the frequency range from 10 MHz to 1 GHz. In particular, an increase in GMI ratio, from 10% up to 140% at 200 MHz is observed in Joule heated samples. Hysteresis loops of annealed samples maintain a rectangular shape, while a slight decrease in coercivity from 93 A/m to 77 A/m, after treatment, is observed. On the other hand, a modification of MOKE hysteresis loops is observed upon Joule heating. Additionally, an improvement in DW dynamics after Joule heating is documented, achieving DW propagation velocities of up to 700 m/s. GMI ratio improvement along with the change in MOKE loops and DW dynamics improvement have been discussed considering magnetic anisotropy induced by Oersted magnetic fields in the surface layer during Joule heating and internal stress relaxation. A substantial GMI ratio improvement observed in Fe-rich Joule-heated microwires with a rectangular hysteresis loop and fast DW propagation, together with the fact that Fe is a more common and less expensive metal than Co, make them suitable for use in magnetic sensors.

We provide new experimental results on studies of composites with glass-coated ferromagnetic microwires aligned with the requirements of carbon composites. This work focuses on the free space microwave measurements of composites made from carbon fibers and ferromagnetic microwires inclusion focusing on the electromagnetic properties. We prepared several glass-coated microwires and selected Co-based microwires with optimum soft magnetic properties and high magnetoimpedance effect for composite fabrication. We observed that by using a low-frequency modulating ac magnetic field parallelly oriented to the ferromagnetic microwires allows us to distinguish the microwave signals originated from the ferromagnetic microwires inclusions from the response generated by the carbon fibers. The location of carbon fibers near magnetic microwires has a critical effect on the response signals (parameters S amplitude) obtained from such composites.

We studied the magnetostriction coefficients, λ_s, Curie temperature, T_c, and their dependence on annealing conditions in Fe₄₇Ni₂₇Si₁₁B₁₃C₂ and Co₆₇Fe_3.9Ni₁.₅B₁₁.₅Si_14.5Mo_1.6 amorphous glass-coated microwires with rather different character of hysteresis loops. A positive λ_s ≈ 20 × 10⁻⁶ is observed in as-prepared Fe₄₇Ni₂₇Si₁₁B₁₃C_2, while low and negative λ_s ≈ −0.3 × 10⁻⁶ is obtained for Co₆₇Fe_3.9Ni₁.₅B₁₁.₅Si_14.5Mo_1.6 microwire. Annealing affects the magnetostriction coefficients and Curie temperatures, T_c, of both Fe₄₇Ni₂₇Si₁₁B₁₃C₂ and Co₆₇Fe_3.9Ni₁.₅B₁₁.₅Si_14.5Mo_1.6 glass-coated microwires in a similar way. Observed dependencies of hysteresis loops, λ_s and T_c on annealing conditions are discussed in terms of superposition of internal stresses relaxation and structural relaxation of studied microwires. We observed linear λ_s dependence on applied stress, σ, in both studied microwires. A decrease in the magnetostriction coefficient upon applied stress is observed for Co-rich microwires with low and negative magnetostriction coefficient. On the contrary, for Fe-Ni-rich microwires with a positive magnetostriction coefficient, an increase in the magnetostriction coefficient with applied stress is observed. The observed results are discussed considering the internal stresses relaxation and short range atomic rearrangements induced by annealing on hysteresis loops, magnetostriction coefficients and Curie temperatures of studied microwires.

In this paper, a gradual change in the hysteresis loop of Co-rich glass-coated microwire stress-annealed at variable temperature is observed. Such microwires annealed with a temperature gradient also present a variable squareness ratio and magnetic anisotropy field along the microwire’s length. The obtained graded anisotropy has been attributed to a gradual modification of the domain structure along the microwire originated by a counterbalance between shape, magnetoelastic, and induced magnetic anisotropies. Accordingly, we propose a rather simple route to design graded magnetic anisotropy in a magnetic microwire.