## 4 Discussion
The maximum values for the ME coefficient obtained in our work were comparable with previously obtained values in classical layered composites combining piezoelectric (PE) ceramics with metals or alloys [35; 36; 37]. In papers [38; 39], laminate composites were investigated, where a ferromagnetic layer comprised magnetic particles that were embedded into a flexible matrix. In [36] a composite structure consisted of a mechanically bonded nickel layer and an ME composite of (0-3) type. The maximum values of \(\alpha\) reported in the present paper were either comparable or exceed those reported in [38; 39; 40] by an order of magnitude. Furthermore, the presented design concept compared well with an alternative approach [41; 42; 43; 44] for polymer-based ME composite materials, where magnetic nanoparticles were embedded in PVDF, PVDF-TrFE, or polyurethane matrices. The ME voltage coefficient demonstrated in the present paper was about 2 orders of magnitude larger than in [41; 42; 43; 44] while keeping the same level of bio-compatibility. In [45; 46; 47; 48; 4] PVDF was used as the PE phase. We obtained a comparable magnitude of ME coupling to the heterostructures of [47; 48], where the amorphous alloy Metglas was employed as a ferromagnetic phase. In [45; 46; 4] values of the ME coefficient were at least an order of magnitude larger for two-layered structures than in the present work. Nevertheless, it should be noted that in [35; 36; 37; 38; 39; 48] measurements were carried out either at a higher resonance frequency, ranging from units to tens of kHz, or far from resonance. We are aware of only two works [4; 5] in which the magnitude of the ME effect at a similarly low resonance frequency exceeded the values of \(\alpha\) we obtained. We believe that an increase in the ME coefficient in MAE/PEP structures could be achieved by further optimization of the properties of the magnetic and PE layers. In this work, a composite structure consisting of PVDF and polyester layers was used as the PE phase. Since polyester does not possess PE properties, it did not contribute to the ME response of the structure. Since PVDF has a lower Young's modulus and higher density than polyester, according to (1), usage of only PVDF material would also reduce the resonance frequency. If the bending oscillation mode will be used and the neutral axis lay within the PE material the PE material should be realized as a PE bimorph. Furthermore, it was shown that increasing the thickness of the PVDF layer and optimizing the properties of the adhesive layer could significantly enhance the ME response [46]. Another important step towards the optimization of MAE-PEP structures might be the direct deposition of the PE layer(s) on the MAE. Alternatively, the PE layer could be realized as a micro-structured cellular PDMS-based material (known as ferroelectret or piezoelectric-tet). It was shown that micro-structured cellular PDMS materials presented PE coefficients that were ten times larger than that of PVDF [49; 50]. An additional advantage of using similar materials for the PE and magnetostrictive layers would be that they have similar mechanical properties. As PDMS is significantly softer than PVDF, it can be expected that the resonance frequency would be further reduced. ME coupling in the presented heterostructures could be even larger in the T-T configuration [20]. This seems to be contradictory to the current concepts of enhanced ME coupling. However, mechanically soft magnetoactive cantilevers, such as those presented in this paper, can easily morph (bend to a great extent) towards the direction of an applied magnetic field, so that their demagnetizing factor would drastically decrease and the ME coupling might become highly efficient. The characterization of MAE/PEP structures by the method of sinusoidal field excitation in the T-T geometry should be explored next.