# A Review of Magnetoelectric Composites Based on ZnO Nanostructures
## 3 ZnO Magnetoelectric Composites
### ZnO Micro- and Nano-Rods  
In 2008, [PERSON] et al. [74] demonstrated one of the first magnetoelectric composites utilizing ZnO nanorods for the piezoelectric phase and a Ni-Fe alloy for the magnetostrictive layer, corresponding to the connectivity scheme shown in Figure 1f. The ZnO nanorods were fabricated via the electrochemical deposition method on ITO substrates, depicted in Figure 6a, using an aqueous solution of zinc nitrate, KCL and formamide. A thin-film passivation layer was realized in order to electrically isolate the ITO substrate from the magnetostrictive layer- by spin coating a sol-gel-derived ZnO layer. The magnetostrictive Ni-Fe (75-25% wt.) phase was deposited on top of the as-grown nanorods by DC magnetron sputtering at a \(5\times 10^{-3}\) Torr Ar ambient with a final thickness of 100 nm, shown in Figure 6b,c. The magnetoelectric characterization was performed by the direct ME measurement method, using an Au sputtered top electrode and the ITO substrate, via a lock-in amplifier in both longitudinal (LM) and transverse (TM) magnetization. The ME voltage coefficients (\(\alpha_{\text{H}}\)) for TM and LM (Figure 6d) were found to be 0.34 and 0.48 mV cm\({}^{-1}\) Oe\({}^{-1}\). Due to the shape anisotropy of the ZnO nanorods, the demagnetization factor D for fields perpendicular to the axis of the nanorods is larger than when the field is applied along the longitudinal axis [75], resulting in enhanced ME coupling and a larger ME voltage coefficient along the axis of the nanorods. Compared to the PZT and AlN piezoelectric phases, ZnO is an n-type semiconductor with a wide range of resistivity values [62] depending on the fabrication methods and annealing conditions. The lower ME voltage can be attributed to the leakage current, which is minimized in piezoelectric materials such as PZT with high resistivity values that present larger values of ME voltage coefficients [15]. The interfacing of magnetostrictive materials with nano- and microrods has proven to be a significant technological challenge due to the imperfections that exist on the interfaces, becoming the dominant factor for reduced ME performance. This is contrary to large ME laminates, where mechanical effects such as shear stresses and anchor losses can significantly influence sensitivity and performance. The ZnO microrods were used by [PERSON] et al. [26], who developed a magnetoelectric core-shell composite based on amorphous FeCoBSi, focusing on the investigation of the interface between the two materials. The microrods with a length of up to 1 cm were synthesized via the modified vapor-liquid-solid (MVLS) approach and by the conventional vapor-liquid-solid (VLS) method. The FeCoBSi magnetostrictive thin film was RF sputtered on the VLS-grown rods, and the composite was investigated by TEM. The crystallinity of the ZnO microrods was not affected by the deposited magnetostrictive layer, which was of uniform thickness and amorphous in nature. Investigation of the interface can provide insight about the magnetization-induced stress and strain created through the interaction of the magnetostrictive and piezoelectric phase under the application of an external magnetic field [76]. Additionally, mechanical stress gradients may induce domain wall motion, even in the absence of an applied magnetic field or spin-polarized current. For example, a piezoelectric material generating stress may induce anisotropy in the magnetostrictive layer of an artificial multiferroic to generate domain wall motion [77]. Thus, utilizing the conventional methods of magnetic behavior characterization such as vibrating sample magnetometry (VSM) or the observation of mechanical effects due to strain (cantilever bending method) [46] cannot provide detailed distributions of strain and magnetization at the nanoscale. Magneto-optic Kerr effect (MOKE) microscopy was applied to ZnO microrod and nanorod [78] ME composites in order to investigate the magnetic domain structure, alignment and domain wall motion [79].