# A Review of Magnetoelectric Composites Based on ZnO Nanostructures
## 3 ZnO Magnetoelectric Composites
### Thin Films
#### 3.1.1 Pure ZnO Thin Films
Interestingly, it was also elaborated that the ferromagnetic resonance was detected without the application of an external  
Figure 2: **(a)** SEM cross-sectional image of the grown ZnO thin film grown on Metglas foil via PLD; **(b)** schematic representation of the composite ZnO/Metglas cantilever depicting measurement method and direction of the applied magnetic fields; **(c)** dependence of magnetostriction and piezoelectric coefficient (d\({}_{33}\)) on the applied DC magnetic field; **(d)** direct magnetoelectric coefficient (DME) as a function of the DC magnetic field at an ac magnetic field of 0.9 Oe at 1 kHz. Reproduced with permission from [45]. Copyright [2011], John Wiley and Sons. rf magnetic field due to the electro-acoustical excitation provide by the ZnO piezoelectric transducer. In order to further investigate the characteristics of surface acoustic waves in magnetoelectric heterostructures, a theoretical investigation on the applicability of ZnO piezoelectric layers in magnetoelectric SAW devices was presented by [PERSON] et al. [52], combined with Metglas magnetostrictive layers, shown in Figure 3a. The ZnO/Metglas ME composite showed a proposed 212 MHz/Oe magnetic field sensitivity, as depicted in Figure 3b, under an external bias of 0.9 Oe and a reduced sensitivity of \(-\)154 MHz/Oe at zero bias, suggesting a simpler device topology, eliminating the need of exciting coils. The increased magnetic field sensitivity is due to the giant \(\Delta\)E effect [53; 54] originating from the Metglas substrate, predicting a giant magnetic field sensitivity of 5 \(\times\) 10\({}^{-12}\) T, while also allowing the detection of DC and AC magnetic fields due to the frequency independence for fields below several kHz. The combination of the wave resonators with microelectromechanical systems (MEMS) in the field of magnetoelectrics offer the potential for sensor miniaturization and the integration of tunable and reconfigurable filter elements into microwave transceivers for mobile radios and RF SoCs (system-on-chip). Such a tunable, film bulk acoustic wave resonator was developed by [PERSON] et al. [55]. The FBAR was fabricated on top of a bulk micromachined SiO\({}_{2}\) membrane consisting of a Pt/ZnO/Fe\({}_{65}\)Co\({}_{35}\) multilayer structure, utilizing the magnetostrictive phase as the second electrode of the piezo stack. Resonance peaks were detected at approximately 1.14 GHz (series resonance) and 1.19 GHz (parallel resonance) under unbiased conditions, with the application of a magnetic field resulting in a 7 MHz frequency downshift of both peaks, due to the \(\Delta\)E effect. In a later study, [PERSON] et al. [56] presented another FBAR with a frequency upshift of \(\sim\)106 MHz (S\({}_{11}\)) at a magnetic field of 2 kOe, thus, demonstrating enhanced frequency tuning and a high \(\Delta\)E modulus of 35.06 GPa compared to other reports [57]. Moreover, a MEMS ME magnetic field sensor was fabricated based on FeCo/ZnO thin films [58] utilizing a cantilever array and a film stack of Si/SiO\({}_{2}\)/Pt/ZnO/Pt/FeCo. At resonance (10.45 kHz), the magnetic field sensitivity was evaluated at 57.6 mV/Gauss. In addition, thin-film magnetoelectric composites were developed utilizing ferromagnetic and piezoelectric constituents deposited by a low temperature spin spray process [59]. The Fe\({}_{3}\)O\({}_{4}\)/ZnO heterostructure exhibited a large ferromagnetic resonance frequency shift of 14 Oe, with an ME coupling coefficient of 1.5 \(\times\) 10\({}^{-4}\) Oe cm k/V, demonstrating the viability of low-temperature deposition for static and microwave ME applications. Magnetic-field-induced strain was also studied in ZnO/FeTb [60] and ZnO/FeCoBi [61] thin-film systems via X-ray diffraction (XRD) and grazing incident X-ray diffraction (GIXRD), respectively. These characterization methods allow for the measurement of strain on the atomic scale by directly evaluating the lattice deformation of the ZnO crystal under an applied magnetic field bias.