# A Review of Magnetoelectric Composites Based on ZnO Nanostructures
## 3 ZnO Magnetoelectric Composites
### Discussion  
In recent years, there has been substantial research interest in magnetoelectric composites destined for sensor applications, energy harvesting and energy transfer with a heightened interest in applications in biology, medicine and medical diagnostics. The  
Figure 9: **(a,b)** TEM images of Zn\({}_{0.94}\)Mg\({}_{0.06}\)O (**a**) and Zn\({}_{0.88}\)Mg\({}_{0.12}\)O (**b**) nanoparticles; **(c)** ME voltage coefficient \(\alpha_{31}\) as a function of the DC magnetic field at room temperature and a frequency of 1 kHz. Inset shows room-temperature P-E loops for different nanocomposite compositions; **(d)** ME voltage coefficient as a function of an external magnetic field for the Zn\({}_{0.88}\)Mg\({}_{0.12}\)O nanocomposite; **(e)** ME voltage coefficient \(\alpha_{33}\) as a function of the DC magnetic field at room temperature and a frequency of 1 kHz. Inset shows the linear relationship between the ME voltage and the applied AC magnetic field in the transverse and longitudinal directions. **(a,b,d)** reproduced with permission from [90]. Copyright [2019], Elsevier. **(c,e)** reproduced with permission from [32]. Copyright [2019], Elsevier. bulk of this effort focuses on developing magnetoelectric composites with enhanced piezoelectric coefficient, i.e., increased magnetostriction and piezoelectric constants, coupled with low coercivity magnetostrictive materials offering a narrow ferromagnetic resonance response. Advanced thin-film ME composites based on PZT, AlN and AlScN are undoubtedly some of the best performing composite structures, having achieved detections limits as low as a few pT Hz\({}^{-1/2}\) at low frequencies. Furthermore, several reports have focused on improving the noise performance using magnetic and electric frequency conversion techniques [92; 93], allowing for miniaturization, high frequency operation and increased immunity from environmental noise (mechanical vibrations, acoustic noise, etc.). However, thin-film technology relies predominately on advanced preparation methods such as physical and chemical vapor deposition (PVD and CVD) and requires access to semiconductor grade equipment, presenting an increased barrier of entry. In addition, the aforementioned piezoelectric thin films are not biocompatible, thus, in vivo testing and integration in implantable devices requires extensive encapsulation for reliable and safe operation [16]. On the other hand, ZnO-based magnetoelectric composites have the potential to extend the applicability of ME composites in demanding applications while also providing alternative routes for ME material fabrication and are able to fulfill requirements such as bio-compatibility, bioactivity and controllable dissolution in transient electronics [94]. On that front, one of the most important and interesting properties of ZnO is its low toxicity and biodegradability, owning to the important role that Zn\({}^{+2}\) plays in human health [95]. In addition, the chemically active surface of ZnO can be functionalized and decorated by a number of chemical compounds due to the presence of OH\({}^{-}\) radicals. Due to these properties, ZnO has gained ground in antimicrobial coatings for biomedical applications [96; 97] and in biosensors [98]. More specifically, ZnO and its nanostructures were utilized in electrochemical biosensors [99] and optical and field effect transistor-based biosensors, as well as in biosensors leveraging its piezoelectric properties [100]. ZnO and its nanostructures [101] (nanowires/rods, nanoparticles, nanoribbons, etc.) were widely studied, owing to their exceptional piezoelectric, semiconducting and optical properties. Research efforts have also focused on the fabrication methods of ZnO nanostructures, showing that in addition to the common thin-film form factor, ZnO can be engineered into many different forms, utilizing either high-cost (CVD, ALD, etc.) or low-cost chemical routes.