# A Review of Magnetoelectric Composites Based on ZnO Nanostructures
## 3 ZnO Magnetoelectric Composites
### Thin Films
#### 3.1.2 Doped ZnO Thin Films  
ZnO is an intrinsic n-type semiconductor with a substantial research effort focused on doping with appropriate impurities, such as lithium, copper and nitrogen as acceptors and aluminum, hydrogen and gallium as donors [62]. Doping with those impurities aims to either improve the n-type conductivity or switch the conductivity to p-type, which has proven quite challenging. For the case of magnetoelectric composites, the introduction  
Figure 4: (a) Magnetization loop of the Fe\({}_{\text{5S}}\)Tb\({}_{\text{4Z}}\)-ZnO magnetoelectric composite measured by VSM and magnetostriction curve obtained by applying an in-plane magnetic field along the long axis of a cantilever; (b) (Up) dependence of ZnO (3–31) forbidden Bragg reflection intensity on the external magnetic field, (Down) field-induced strain along [1, 2, 3, 4, 5, 6, 7, 8, 9, 10], overlaid with the magnetostrictive strain; (c) magnetization loops for the (Fe\({}_{\text{90}}\)Co\({}_{\text{010}}\))78 Si\({}_{\text{12}}\)B\({}_{\text{10}}\)-ZnO ME composite measured by VSM and magnetoelastic coupling coefficient b as a function of the magnetic field measured by the cantilever method; (d) magnetic-field-induced strain along [110] and [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] compared with magnetostriction \(\lambda\). (a,b) reproduced with permission from [60]. Copyright [2013], AIP Publishing. (e,d) reproduced with permission from [61]. Copyright [2013], AIP Publishing. of sub bands using shallow donors and acceptors or taking advantage of defects and vacancies were shown to induce ferromagnetism in ZnO films [63; 64]. In addition, doping and the incorporation of non-ferrous metals such as Zn, Al, Pt and C into ZnO revealed ferromagnetic behavior due to p-p interactions between the 2p orbitals of O and C [65] and metal cluster formation into the ZnO matrix [66]. Magnetoelectric, W-doped, ZnO thin films were developed by physical vapor deposition (sputtering) using a ZnO target containing 1-2% tungsten in an Ar/O\({}_{2}\) gas mixture (70/30%) at 420 \({}^{\circ}\)C [67]. The X-ray photoelectron spectroscopy studies revealed that the added W atoms were bonded with oxygen in the ZnO lattice, effectively substituting Zn\({}^{+2}\) with W\({}^{+6}\). This resulted in an increased carrier concentration due to the excess electrons provided by the bond between W\({}^{+6}\) and O\({}^{-2}\); however, an increase in room-temperature sheet resistivity and a reduction in mobility were observed. Magnetoresistivity was observed for both doped and pure thin films at low temperatures (5 K); however, no appreciable change in the magnetic properties of the W-doped thin films was observed. Doping with W does not appear to favor the enhancement of either magnetic or magnetoelectric properties; although metals such as Li, Ni and Mg were reported as potential dopants for enhancing the ferromagnetic and/or ferroelectric properties of ZnO thin films. In fact, [PERSON] et al. [68] presented the effect of Li, Ni and Mg doping on the magnetoelectric coupling and multiferroic properties of ZnO thin films. The pure and doped (5 wt.%) ZnO thin-films were synthesized by the sol-gel route using zinc acetate, lithium acetate, magnesium acetate and nickel nitrate as dopant sources. A non-linear ferroelectric behavior was identified in Ni-, Li- and Mg-doped samples, shown in Figure 5a,b, in contrast to pure ZnO, and the decrease in remnant polarization was attributed to the ionic radius of the dopants creating an electric dipole moment due to the substitution of Zn\({}^{+2}\) in the crystal lattice. The magnetoelectric characterization, under a combination of AC and DC magnetic fields, resulted in magnetoelectric coupling in the Li-doped samples (Figure 5c) and increased magnetoelectric coupling in the Ni- and Mg-doped samples (Figure 5d,e) in contrast to pure ZnO. An enhanced ME voltage was observed in the Ni-doped samples due to the ferromagnetic nature of Ni.