# A Review of Magnetoelectric Composites Based on ZnO Nanostructures
## 3 ZnO Magnetoelectric Composites  
### Thin Films  
#### 3.1.1 Pure ZnO Thin Films  
ZnO thin films were extensively used in device applications [39] and, specifically, in chemical, optical and biological sensors [40] and light-emitting-diodes (LEDs) [41], as well as in thin-film transistors, gas sensors [42] and surface acoustic wave (SAW) devices [43]. Since ZnO is a well-established piezoelectric material with favorable piezoelectric and semiconducting properties [44], ZnO thin films have found widespread application in magnetoelectric composites. One of the first reports on ZnO thin-film magnetoelectric composites was presented by [PERSON] et al. [45], in which ZnO thin films were grown on Metglas substrates by pulsed laser deposition (PLD), as shown in Figure 2a, forming an ME cantilever with 2-2 connectivity (Figure 1d), depicted in Figure 2b. A highly c-axis-oriented ZnO film with a thickness of 2 \(\upmu\)m was grown, presenting a columnar structure. Magnetoelectric characterization was performed using the DC and AC magnetic fields, and parameters such as the magnetostriction (\(\lambda\)), piezomagnetic coefficient d\({}_{33,\mathrm{m}}\) and magnetoelectric coefficient \(\alpha_{\mathrm{ME}}\) were evaluated. A value of the saturation magnetostriction of 27 ppm at a magnetic field intensity of 60 Oe was measured with a maximum d\({}_{33,\mathrm{m}}\) value of 2 ppm/Oe at a DC magnetic field of 17 Oe (Figure 2c). In addition, the evaluation of the direct ME (DME) (Figure 2d) and converse ME (CME) coefficient was included, showing a strong magnetic coupling between the ZnO piezoelectric phase and the Metglas substrate. Cantilever-based magnetoelectric devices represent the backbone of magnetoelectric detec  
Figure 1: Composite magnetoelectric materials with different types of connectivity. **(a)** 0-0; **(b)** 0-3; (c) 1-3; **(d)** 2-2; **(e)** core-shell nanocomposite; **(f)** core-shell nanorods/wire. Light blue represents the PE phase, red represents the FM phase, and light green represents a neutral matrix. tion and characterization since the introduction of the cantilever method by [PERSON] [46] and its refinement by [PERSON] et al. [47]. In fact, tuning fork-like structures (two cantilevers) have shown remarkable sensitivity and a detection limit of 500 fT/(Hz)\({}^{1/2}\)[27] utilizing PZT for the piezoelectric material at resonance. Despite the widespread adoption of cantilever structures, ZnO-based ME research has also focused on SAW [48; 49] in addition to bulk (BAW) and film (FBAR) bulk acoustic wave resonator devices [50], integrated with magnetostrictive materials. [PERSON] et al. [51] presented a layered magnetoelectric heterostructure consisting of yttrium iron garnet (YIG) as the magnetostrictive phase grown on gadolinium gallium garnet (GGG) substrates. The ZnO thin-film layer used for the excitation of bulk, transversal and longitudinal acoustic waves was sputtered on top of the multilayer, with Al serving as the electrodes of the piezoelectric transducer. The influence of the external magnetic field on the BAW spectrum was studied at both low-intensity fields and at saturation. A shift in the acoustic resonance frequency was detected by increasing the field intensity from 0 to 100 Oe; however, only one of the shear modes was influenced, which was attributed to the polarization vector of that mode being parallel to the applied magnetic field. At saturating fields (0-2 kOe), an effective frequency tuning of 0.25 MHz at 2 GHz was observed due to the resonance magnetoelastic behavior in the YIG magnetostrictive film.