## 3 Results and Discussion  
The results of U\({}_{\text{OUT}}\)(f) measurements for sample S1 together with the reference sample R are presented in Figure 4. Measurements were conducted in the frequency range of 0 kHz to 84 kHz. Samples were exposed to DC magnetic field of different intensities: 0, 0.011, 0.058, 0.133, and 0.4 T.  
One can readily observe a few general features. Firstly, all characteristics are inclined relative to the horizontal axis (unlike in, e.g., [12, 13, 22, 23, 24, 25, 26], where the characteristics are rather flat). This slope, or base, of the obtained characteristics is associated with Faraday electromagnetic induction as proven by the measurement of the reference sample. The value of recorded voltage on the R sample increases steadily with increasing frequency and has an approximately linear character. Since the sample S1 contains more than one piezoelectric sheet, the Faraday induction contribution is larger. Secondly, the low-frequency (kHz range) resonance state occurs at different frequencies which is observed as resonant peaks in the characteristics. It may be stated that the reference sample does not undergo resonance  
Figure 4: Magnetoelectric voltage versus AC magnetic field frequency characteristics for sample S1 and reference R: **(a)** for DC field in the range of 0–0.011 T; **(b)** for DC field between 0.058 and 0.4 T.  
(the small anomaly around 82 kHz may be connected with the specific structure of the piezoelectric patch). However, numerous resonant peaks are observed for the sample S1. For each value of the DC field, between five and seven resonant peaks may be seen (e.g., for 0.133 T resonance takes place at \(t_{\text{res}}\) = 32, 58.8, 65.5, 69.2, 73.5, and 77 kHz). All peaks for various DC fields occur at approximately the same frequencies. In comparison to the data published for composites, in which the authors observe one or two separated peaks [12; 13; 22; 23; 27; 28; 29; 30], we detect multiple, overlapping resonant peaks (60-80 kHz range). The existence of resonant peaks shown in Figure 4 may be explained by the structure of the S1 composite. The sample consists of mechanically coupled layers and, like all mechanical systems, has its own resonance modes. In general, the resonance state in composite materials is associated with bending, longitudinal, radial, and thickness acoustic modes [12]. Based on criteria such as frequency range, material constituents, and theoretical models found in the literature, it can be inferred that the resonance state observed in our study is due to bending and longitudinal modes. It should be pointed out that the number, nature, and frequency of resonant peaks also depend on mechanical boundary conditions (i.e., whether the sample ends are free or fixed) [22; 26; 30]. As may be observed from Figure 4, the values of magnetoelectric voltage of sample S1 depend on the external DC field. At 0 and 0.011 T, the recorded U\({}_{\text{OUT}}\) values are similar and reach about 130 mV (Figure 4a). Above 0.011 T, voltage across the sample significantly increases along with the DC field (Figure 4b), especially at resonant frequencies. The maximum value of U\({}_{\text{OUT}}\) (\(\sim\)450 mV) was noted for 0.133 T at \(t_{\text{res}}\) = 69.2 kHz. Interestingly, at 0.4 T the heights of all peaks above 60 kHz are smaller than those measured at 0.133 T. One possible explanation for such behavior is the competition between mechanical stress and material vibrations. It is known that, as the magnetic field increases in value, the magnetostrictive coefficient of Terfenol-D saturates along with the ME signal [22; 29; 31]. At the same time, increasing stiffness of the material has the damping effect on oscillations. Moreover, it can be noted that the peak around \(t_{\text{res}}\)\(\sim\)32 kHz is larger at 0.4 T than at 0.133 T, which suggests its different origin.