Amorphous magnetic wires can exhibit unique magnetic properties, such as magnetic bistability [1] and/or Giant Magneto-Impedance, GMI, effect associated with excellent magnetic softness [2]. Additionally, amorphous materials are also characterized by superior mechanical and corrosion properties [3]. Such combination of physical properties makes the amorphous wires attractive for a variety of industrial applications, such as magnetic and magnetoelastic sensors or tunable metamaterials [2,4]. One of the latest trends in the development of amorphous magnetic wires is to reduce their size and expand their functionality through protective coatings. Among the most effective solutions for the production of thin amorphous magnetic wires is the so-called Taylor-Ulitovsky method, allowing the preparation of microwires with rather extended diameters range from 100 nm to 100 µm coated with an insulating, flexible and biocompatible glass coating [4]. The performance of GMI effect based sensors and devices can be significantly improved by using materials with higher GMI effect. Typically, the highest GMI ratio of about 200-300% is observed in Co-rich magnetic wires with vanishing magnetostriction coefficients, λ_s [2]. While, in carefully processed magnetic microwires, GMI ratios of up to 650% have been obtained [4]. However, the reported GMI ratios are still below the theoretically predicted 3000% [2]

Consequently, in this paper we provide our latest attempt on optimization of the magnetic softness and GMI effect in Co-rich glass-coated magnetic microwires.We studied the effect of annealing on the hysteresis loops and the GMI ratio of Co-rich microwires. Surprisingly, after conventional annealing, in most of Co-rich microwires, magnetic hardening and transformation of a linear hysteresis loop into a rectangular one with a higher coercive force are observed. However, stress-annealing allows preventing magnetic hardening and remarkably improve GMI ratio. Properly stress-annealed samples present almost unhysteretic loops with coercivity about 2 A/m and magnetic anisotropy field about 35A/m. A remarkable GMI ratio improvement up to 735% is observed after annealing of Co-rich microwires at appropriate conditions. Observed magnetic softening and GMI ratio improvement have been discussed considering the internal stresses relaxation, induced magnetic anisotropy and a change in the magnetostriction coefficient sign and values with increasing of annealing temperature.