# A Review of Magnetoelectric Composites Based on ZnO Nanostructures
## 3 ZnO Magnetoelectric Composites
### ZnO Micro- and Nano-Rods
Domain wall rotation was observed on single microrod composites, shown in Figure 7a, with the application of a magnetic field perpendicular to the rod axis. Initially, the FeCoBSi MS layer is largely in a demagnetized state and, by the application of a transversal field, 180\({}^{\circ}\) domain wall motion occurs, reaching saturation above 15 mT. The complex magnetic domain structure can be identified from the demagnetized to the saturated state, suggesting the existence of magnetic domains with antiparallel alignment terminated by 90\({}^{\circ}\) domains along the [001] direction, which are gradually annihilated through the magnetization process, reaching a complete alignment along the direction of the applied field at saturation (Figure 7b). Although MOKE microscopy provides a detailed analysis of the magnetic domain structure,  
Figure 6: (**a**) SEM image of the as-grown ZnO nanorods on ITO/glass substrate; (**b**) cross-sectional SEM image of the ZnO nanorods, coated with Ni-Fe; (**c**) magnified SEM view of the coated nanorods; (**d**) magnetoelectric voltage coefficient under the application of an external magnetic field, measured in the transversal and longitudinal modes. Reproduced with permission from [74]. Copyright [2008], IOP Publishing, Ltd.  
it does not provide any information regarding the magnetization-induced strain on the interface. For that purpose, X-ray diffraction (XRD) was utilized in order to observe the ZnO lattice deformation that occurred due to the external magnetic field. Employing XRD as a means to evaluate magnetostriction allows us to probe microrords at a fine enough volume to allow for the spatial mapping of the lattice parameters with high resolution, both for the uncoated and coated samples, providing growth and magnetically induced strain information. In a study by [PERSON] et al. [79], the combined MOKE and nano-diffraction studies revealed a magnetic easy axis perpendicular to the rod axis and magnetic-field-induced strains at the edges of the ZnO crystal exceeding the expected theoretical value of \(4\times 10^{-5}\). This enhancement is unique to these microscopic systems due to geometrical considerations, as well as due to the contribution from the deposition induced compressive strain. In a future study by [PERSON] et al. [80], a systematic study of FeCoSiB/ZnO microrod composites is presented (Figure 7d), utilizing nano-XRD (nXRD), shown in Figure 7c.f. Flame transport synthesis was used in order to produce mm-length ZnO microrods [81] that are covered entirely by a 500 nm thick (Fe\({}_{90}\)Co\({}_{10}\))\({}_{78}\)Si\({}_{12}\)B\({}_{10}\) film, deposited by RF magnetron sputtering. The analysis revealed a direct correlation between the magnetically induced strain and the diameter of the rods, which also occurs when studying the intrinsic strain induced by the deposition (Figure 7e). In both cases, the strain increases by reducing the diameter of the rod to a maximum of \(-5\times 10^{-5}\), which presents a 25% increase in comparison to the bulk ZnO ME composites [61]. This investigation of strain distribution directly on the interface between ZnO and an MS layer highlights the impact of miniaturization on ME composites and drives the research to pursue further improvement on magnetoelectric performance in the nanoscale. Recent studies aiming to discover ZnO magnetoelectric applications have also utilized the aforementioned nXRD method for investigating local strain distributions at the interface [82, 83].