# A Review of Magnetoelectric Composites Based on ZnO Nanostructures
## 3 ZnO Magnetoelectric Composites
### ZnO Nanocposites  
As discussed above, thin-film- and micro/nanorod-based ZnO magnetoelectric composites rely on strain transfer between the magnetostrictive and piezoelectric phase for magnetoelectric coupling. On the other hand, nanocomposites are essentially a single-phase material where the magnetoelectric coupling arises from the interaction between magnetization and electric polarization in the volume of the material [88]. Magnetostriction is induced in the magnetic domains during magnetization, which involves domain movement that spontaneously induces an electrical polarization due to the piezoelectric effect. Cobalt-doped (Co-doped) ZnO nanoparticles were developed by [PERSON] et al. [89] via a chemical precipitation method using zinc acetate and cobalt acetate tetrahydrate as precursors, oxal acid as the precipitating agent, ammonium hydroxide as a pH buffer and tetraethylammoniumhydroxoxide as an anti-agglomeration agent. The resulting nanopowder exhibited nano-crystalline domains of ZnO and Co\({}_{3}\)O\({}_{4}\) with diffraction peak intensities modulated by Co content for both ZnO and Co\({}_{3}\)O\({}_{4}\), accompanied by a reduction in the crystallite size, indicating the substitution of Co\({}^{+2}\) at the Zn\({}^{+2}\) sites. Ferromagnetic behavior  
Figure 8: (**a**) Schematic representation of a 2-2 ME composite sensor during oscillation via an externally applied magnetic field; (**b**) SEM image of a hexagonal ZnO micro needle; (**c**) ZnO micro needle ME sensor with Ag and Al/Au/Ag contacts on an FeCoSiB cantilever; (**d**) Piezotronic (green) and piezoelectric (blue) measurements of an Ag/ZnO/Al/Au/Ag ME sensor in reverse bias at a resonant frequency of 79.4 Hz and a magnetic field of 0.6 mT; (**e**) I–V characteristics of an Ag/ZnO/Al/Au/Ag ME sensor. The equivalent circuit represents the ideal Schottky contact (D1) and space charge contribution (D2), depicting a large piezotronic current in the forward direction. Reproduced with permission from [36]. Copyright [2016], [PERSON] and Sons. was observed in the Zn-Co-O nanocomposites at 5K and was attributed to the exchange interaction of d-d coupling cobalt ions. In addition, oxygen vacancies heavily contribute to the reported ferromagnetism via a bound magnetic polaron (BMP), formed between trapped electrons by an oxygen vacancy and the trapped electron at an orbital overlapping with the d shell of a cobalt ion. Oxygen vacancies are identified in the doped samples by energy dispersive X-ray spectroscopy (EDS) and UV-visible and fluorescence (FL) spectra. The magnetoelectric coupling was verified by measuring the ME coefficient using the dynamic method, i.e., the measurement of the ME voltage developed across the sample under the simultaneous application of DC and AC magnetic fields. The magnitude of the magnetoelectric coefficient was evaluated at 8.65 mV/cm Oe at a magnetic field of 2 kOe for the Zn\({}_{0.82}\)Co\({}_{0.18}\)O sample, suggesting that the 18% Co-doped ZnO nanocomposites can be applied to functional device applications. Ferromagnetic ordering, due to the inherent oxygen vacancies and BMP formation, was also reported in Mg-doped ZnO nanoparticles (Figure 9a,b) [90] where an ME voltage coefficient of 4.13 mV/cm Oe was identified, depicted in Figure 9d. Another ferromagnetic material used in ZnO nanocomposites stems from the combination of Ni and Co with Fe\({}_{2}\)O\({}_{4}\) (ferrite). Nickel ferrite (NiFe\({}_{2}\)O\({}_{4}\)) composites were developed by [PERSON] et al. [32], presenting room-temperature magnetoelectric voltage coefficients from 3.7 to 8 mV/cm Oe (Figure 9c,e). A similar magnetoelectric response was reported from La\({}_{0.7}\)Sr\({}_{0.3}\)MnO\({}_{3}\)-ZnO nanocomposites, presenting a maximum ME voltage coefficient of 2.6 and 2.4 mV/cm Oe for the transversal and longitudinal modes, respectively [91].