## 3 Results
The maximum \(\chi_{\rm ac}\) at the optimal resonance frequency has the following physical significance. At low frequencies, the alternating-current dipole moment is caused by the oscillation of the volume dipole moment in each domain. The external alternating current magnetic field plays a driving role. The recovery of magnetic interaction is between domains. There is a resonance frequency as the \begin{table}
\begin{tabular}{c c c c} \hline
**CoFeVB (nm)** & **Co (at\%)** & **Fe (at\%)** & **V (at\%)** \\ \hline
10 & 32.02 & 33.66 & 34.32 \\
20 & 32.44 & 32.92 & 34.64 \\
30 & 30.51 & 31.75 & 37.74 \\
40 & 20.52 & 29.61 & 49.87 \\ \hline \end{tabular}
\end{table} 
Table 1: EDX analysis data for the alloy films. Figure 2: EDX patterns of the Co\({}_{40}\)Fe\({}_{40}\)V\({}_{10}\)B\({}_{10}\) thin films with various thicknesses. (**a**) 10 nm, (**b**) 20 nm, (**c**) 30 nm, and (**d**) 40 nm. driving force acting on the system. Therefore, the peak frequency of the low frequency susceptibility corresponds to the resonance frequency of the magnetic dipole moment oscillation in the domain. At frequency, the \(\chi_{\text{ac}}\) peak represents the spin exchange coupling interaction and domain dipole moment [38]. As shown in Figure 4, the best magnetic properties were achieved on the CoFeVB film with a thickness of 20 nm. Figure 4: Maximum alternate-current magnetic susceptibility for the CoFeVB films. Figure 3: For samples with a thickness of 10 nm to 40 nm, the low-frequency alternate-current magnetic susceptibility (\(\chi_{\text{ac}}\)) as a function of the frequency from 10 to 25,000 Hz. The optimal resonance frequency of CoFeVB films is shown in Figure 5. According to the f\({}_{\text{res}}\) value, the maximum \(\chi_{\text{ac}}\) value is measured with the strongest spin sensitivity at this frequency [38; 39]. The f\({}_{\text{res}}\) presented an oscillating trend with a minimum critical frequency of 50 Hz. For all CoFeVB thicknesses, the f\({}_{\text{res}}\) value of all CoFeVB thicknesses was less than 200 Hz, which indicates that the films are beneficial to the application of sensors, transformers, and magnetic recording media at low frequencies. Figure 6 shows the hysteresis loop of the films with an in-plane external field (H\({}_{\text{ext}}\)) for various CoFeVB thicknesses, as determined through AGM. The external field (H\({}_{\text{ext}}\)) was 1000 Oe in the AGM measurement, which was enough to observe the saturated magnetic spin status of the Co\({}_{40}\)Fe\({}_{40}\)V\({}_{10}\)B\({}_{10}\) films. While the magnetization for the 10 nm and 20 nm films was very close to each other, the magnetization for the 30 nm and 40 nm was lower. It can be reasonably concluded that the 40 nm film was not homogeneous and was a discontinuous film, inducing weak magnetization. Table 1, which presents the EDX analysis, also indicates that the 40 nm film was not a homogeneous composition. The enlarged illustration of Figure 6 shows that the largest H\({}_{\text{c}}\) was achieved at 30 nm. The external field (H\({}_{\text{ext}}\)) used had difficulty rotating the spin state, consequently reducing the magnetization and decreasing the \(\chi_{\text{ac}}\) value. From Figure 6, the results indicate that the in-plane hysteresis loop of the CoFeVB films was an easy axis. Moreover, the hysteresis loops also indicate that the CoFeVB films are magnetically soft. The surface roughness and morphology of the CoFeVB films were investigated by AFM, as shown in Figure 7a-d. When the thickness was increased from 10 nm to 40 nm, the root-mean-square values (R\({}_{\text{q}}\)) of surface roughness were 0.57 nm, 0.39 nm, 1.07 nm, and 0.69 nm, respectively. The increased surface roughness can lead to the pinning effect of the domain wall, which is not easy to move, resulting in an increase in coercivity and a decrease in the \(\chi_{\text{ac}}\) value [40; 41].