## 3 Results and Discussion
### Evolution of the Microwave Scattering Properties in Co-Based MWs Linear Arrays
The surface impedance depends on both the dynamic permeability and the magnetization orientation angle \(\theta\), according to Equation (5). Therefore, the electric polarization and current distribution depend on the surface impedance and, hence, on the magnetization direction through \(\tilde{\mu}\)[53]. Applying an axial DC magnetic field, the magnetization changes from circumferential to longitudinal  
Figure 3: (**a**) Top part illustrates the sensing array platform layout from 1 to 15 Co-based MWs. Bottom part shows the \(S_{21}\) transmission coefficients in the L and S microwave spectral GHz regions obtained for two linear arrays composed of Co-based MWs with a length of 4 cm and 8 cm from 1 to 15 MWs. (**b**) \(S_{21}\) minimum value and (**c**) \(f_{\text{min}}\) as a function of the number of 4 cm MWs in the array collected from the results showed in the bottom of (**a**). orientation (\(\theta\) in Equation (5) changes from \(\pi/2\) to 0), and that produces a strong variation on the surface impedance of the Co-based MW at the GHz frequencies. The increase in permeability occurs in the magnetization reorientation, which leads to an increase of the surface impedance and, then, it results in a reduction in the induced current in the MW. The reduction in the power absorbed by the MW produces a smoothing in the \(S_{21}\) parameter [43; 51]. Linear array configurations 1, 2, and 3 are employed to investigate the impact of the above phenomena on the microwave scattering spectrum (Figure 4a1-a3, respectively). Subsequently, microwave scattering experiments are performed as a function of a DC magnetic field varying from 0 to 17 Oe (Figure 4b1-b3). In the condition of \(H_{\text{bias}}=0\) Oe, the configuration 1 (Figure 4a1) displays a minimum intensity of the \(S_{21}\) coefficient of \(-\)\(-\)4.6 dB that is located around \(f\sim\) 2.54 GHz (full width high maximum, FWHM = 0.30 GHz). When the system is extended to three linear arrays (Figure 4a2), the minimum intensity continues to increase up to \(-\)\(-\)16.8 dB. In addition, a shift in the frequency of the minimum scattering is noticed reaching \(\sim\)2.37 GHz and its bandwidth is becoming much narrower (FWHM = 0.17 GHz). Interestingly, the most notable changes are observed when two sensing platforms are arranged in tandem (Figure 4a3), since a huge enhancement of the \(S_{21}\) minimum intensity up to \(-\)\(-\)47.9 dB is recorded (Figure 4b3). These values are highly competitive [37], since an enormous increase of the \(S_{21}\) coefficient of \(\sim\)1041% is achieved by changing from configuration 1 to 3. In turn, a drastic narrowing of the scattering is also registered (FWHM = 0.03 GHz), shifting the minimum frequency to \(\sim\)2.21 GHz. Therefore, configuration 3 also enables accurately tracking shifts in the transmission spectra, one of the required criteria for a sensing device. As the magnetic DC field is increased from O to 17 Oe, the \(S_{21}\) coefficient is strongly flattened for all of the analyzed configurations. The most remarkable change is found in configuration 3, with a progressive increase from \(-\)\(-\)47.9 to \(-\)\(-\)11.6 dB (Figure 4b3). The applied DC magnetic field increases the permeability of the MWs, which, in turn, is closely related to their impedance. In Figure 4b1-b3, these effects are noted gradually up to a maximum applied magnetic field of 17 Oe (a value near the anisot  
Figure 4: Effect of an external DC magnetic field with increasing value up to 17 Oe in the microwave scattering spectrum: (**a1–a3**) schematic illustration of MWs configurations and (**b1–b3**) \(S_{21}\) transmission coefficient obtained for the configurations 1, 2, and 3.  
to Figure 2c). Moreover, DC magnetic fields also affect the bandwidth, which increases by one order of magnitude when the field is present (for example, configuration 3 goes from \(\sim\)0.03 GHz at \(H_{\text{bias}}\) = 0 Oe to \(\sim\)0.34 GHz at \(H_{\text{bias}}\) = 17 Oe). The field-based broadening is also observed in composites that contain short Co-based MWs [29].