# A Review of Magnetoelectric Composites Based on ZnO Nanostructures
## 3 ZnO Magnetoelectric Composites
### Discussion
This versatility was leveraged into creating ME composites based on ZnO thin films, either doped or pure, micro/nanorods and nanocomposites combined with magnetostrictive materials, which were presented in this review. A summary of the available technologies used in ZnO-based ME composites is presented in Table 1. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Technology Process** & **Structure** & **Magnetostaticity Phase** & **Sensitivity\({}^{*}\)** & **Ref.** \\ \hline \multirow{8}{*}{ZnO thin film} & PLD & Cantilever & Fe\({}_{\text{Na}}\)Co\({}_{\text{Na}}\)Si\({}_{\text{Na}}\)Si\({}_{\text{Na}}\)Si\({}_{\text{Ca}}\) & 47 mV/cm Oe & [45] \\  & ZnO thin film & BAW & Yttrium iron garnet & Al\(\cdot\)0.25 MHz & [51] \\  & \(\text{Sputtering}\) & FBAR & Fe\({}_{\text{Ca}}\)Co\({}_{\text{Si}}\) & Al\(\cdot\)7 MHz & [50] \\  & \(\text{Sputtering}\) & FBAR & Fe\({}_{\text{Ca}}\)Co\({}_{\text{Si}}\) & Al\(\cdot\)106 MHz & [56] \\  & \(\text{Sputtering}\) & MEMS & FeCo & 57.6 mV/Gauss & [58] \\  & \(\text{Sputtering}\) & Cantilevers & FeCo & (10.45 kHz) & [59] \\  & \(\text{Spin-spray}\) & Thin film & \(\text{Fe}_{\text{Di}}\)O\({}_{\text{4}}\) & \(1.5\times 10^{-4}\) Oe \(\text{cm k/V}\) & [59] \\ \hline \multirow{8}{*}{ZnO} & Electrochemical & Nanorod & Ni\({}_{\text{Na}}\)Fe\({}_{\text{Si}}\) & 0.34-0.8 mV/cm Oe & [74] \\  & deposition & Composite & (DC sputtering) & 0.34-0.8 mV/cm Oe & [74] \\ \cline{1-1}  & MVLS & Core-shell & FeCoBi & - & [26] \\ \cline{1-1}  & micro/nano rods & FTS & Microrod & (Fe\({}_{\text{Ca}}\)Co\({}_{\text{Na}}\)Si\({}_{\text{Ba}}\) & - & [80] \\ \cline{1-1}  & \(\text{FTS}\) & Microrod & FeCoBi & 380 pT Hz\({}^{1/2}\) & [36] \\ \hline \multirow{2}{*}{ZnO nanocomposite} & ZnO NPs & Nanocomposite & Co doped & 8.65 mV/cm Oe & [59] \\ \cline{1-1}  & ZnO NPs & Nanocomposite & Mg doped & 4.13 mV/cm Oe & [59] \\ \cline{1-1}  & ZnO NPs & Nanocomposite & NiFe\({}_{\text{Di}}\)O\({}_{\text{N}}\) NPs & 3.7–8 mV/cm Oe & [32] \\ \hline \hline \end{tabular}
* \({}^{*}\) Sensitivity is used as a term to summarize the magnitude of the magnetoelectric effect due to the fact that different coefficients and figures of merit are used for each composite category in the literature.  
\end{table} 
Table 1: Summary of ZnO based magnetoelectric composites. Another advantage for using ZnO compared to other materials is its ability to dissolve in weak acids, bases and water, which is a highly desirable property for the emerging transient and dissolvable electronics. In fact, biocompatible transient electronics and energy harvesters based on ZnO were demonstrated [102; 103; 104], showing the potential of ZnO in advanced transient applications [105]. Essentially, by combining the favorable transient and biodegradable properties of ZnO with biocompatible ferromagnetic elements such as Mg [102], biocompatible magnetoelectric composites can be realized, with potential applications in biomagnetic imaging, energy and data transfer for implantable devices, drug delivery and body function monitoring and sensing. Last, but not least, the exploitation of ZnO, as a piezoelectric phase, opens the possibility of developing a new generation of devices based on the piezotronics and piezophotronic phenomena. As was demonstrated by [PERSON] et al. [106], it is possible to control carrier transport across a metal-semiconductor interface through the piezoelectric charges that are generated upon an external mechanical stress. This phenomenon is complementary to the change in resistance that appears in piezoresistive materials, such as ZnO, although it is not easy to identify the contribution of each mechanism. [PERSON] et al. [107] demonstrated the significance of the piezotronics effect over the change of piezoresistance; however, further research is required to better clarify this point. Despite the successful integration of ZnO nanostructures with magnetostrictive materials, a gap still exists between state-of-the-art ME materials and ZnO-based ME composites in terms of piezomagnetic and ME coefficients.