## 3 Results and Discussion
### Effects of Stress-Annealing on Magnetic Properties and GMI Effect of Co-Rich Microwires  
In contrast to as-prepared Fe-rich samples, as-prepared Co-rich microwires with low negative magnetostriction values (\(-3\times 10^{-7}\)) [41] displayed linear and almost unhysteretic hysteresis loops (see Figure 6a). However, as we recently reported elsewhere [43, 44], considerable magnetic hardening (coercivity rising by more than one order of magnitude) and transformation of linear hysteresis loop into rectangular is observed even after short-time annealing of Co-rich microwires (Figure 6b). In spite of magnetic hardening, annealed Co-rich samples can show higher GMI effect [44]. Stress-annealing of Co-rich microwires can also prevent magnetic hardening, allowing induction of transverse magnetic anisotropy at high enough annealing temperature and GMI ratio improvement [44]. As can be observed in Figure 6c, stress-annealed Co\({}_{69.2}\)Fe\({}_{4.1}\)B\({}_{11.8}\)Si\({}_{13.8}\)C\({}_{1.1}\) microwires showed lower coercivity, H\({}_{\rm c}\). As expected, as-prepared the Co\({}_{69.2}\)Fe\({}_{4.1}\)B\({}_{11.8}\)Si\({}_{13.8}\)C\({}_{1.1}\) microwire displays an enhanced GMI effect, as shown in Figure 7a (GMI ratio up to 285%), with a double-peak magnetic-field dependence. Such\(\Delta\)Z/Z (H) dependencies are ascribed to a circular magnetic anisotropy with high initial permeability and low coercivity typical for Co-rich microwires with vanishing and negative \(\lambda\)s values [43; 44]. Upon stress-annealing at a fixed T\({}_{\text{ann}}\) = 300 \({}^{\circ}\)C /\(\alpha_{\text{a}}\) = 80 MPa and different annealing time, a noticeable modification of the \(\Delta\)Z/Z (H) dependencies, either for 10 min (Figure 7b) or 30 min (Figure 7c), has been observed. The maximum GMI ratio, \(\Delta\)Z/Z\({}_{\text{m}}\), at f = 100 MHz decreased from 280% to 265% and 250% for samples stress-annealed at 10 and 30 min, respectively. However, stress-annealed samples present higher \(\Delta\)Z/Z\({}_{\text{m}}\) values at high frequency region (Figure 7b-d). Both stress-annealed samples exhibited maximum on \(\Delta\)Z/Z\({}_{\text{m}}\) (f) dependence, at about 200 MHz, as can be appreciated from Figure 7d. For annealing time, t\({}_{\text{ann}}\) = 10 min, \(\Delta\)Z/Z\({}_{\text{m}}\)\(\approx\) 310% was achieved (Figure 7b,d). Furthermore, at a lower frequency (10 MHz) the \(\Delta\)Z/Z (H) dependence showed a single peak character associated with an axial anisotropy induced upon stress-annealing. Observed frequency dependence on \(\Delta\)Z/Z (H) can be attributed to the radial distribution of magnetic anisotropies in the core-shell domain structure [46; 47; 48]. Consequently, stress-annealing of Co-rich microwires resulted in an increase in the GMI ratio, at frequencies above 400 MHz, as can be observed in Figure 7d. Based on these observed dependencies, it can be concluded that stress-annealing is an efficient method for improving the GMI effect in Fe-rich magnetic microwires. Although Co-rich still presents higher GMI ratios, its high cost remains undesirable for some particular applications. Consequently, the advantages of the proposed method allowing GMI-ratio enhancement in Fe-rich microwires is that stress-annealed microwires retain amorphous structure, therefore keeping their excellent plasticity and flexibility, which are typical for amorphous materials. Presented results are of potential interest for the development of cost-effective magnetic sensors and, more specifically, pT resolution sensors (Electronic Compass for Smart Phones and Biomagnetic Field Sensing, Cardiac Magnetic Activity) [17; 49; 50] and metamaterials based on the GMI effect [30; 31; 32].