# A Review of Magnetoelectric Composites Based on ZnO Nanostructures
## 1 Introduction  
Magnetoelectricity, the coupling between electric polarization and magnetization, has been gaining increased scientific interest over the last few years since its first discovery in single-phase antiferromagnetic Cr\({}_{2}\)O\({}_{3}\) at a low temperature [1]. Progress in multiferroic, ferromagnetic, ferroelastic and composite magnetoelectric materials has extended the reach of magnetoelectric composites into functional magnetoelectric devices [2; 3; 4], taking part in significant innovations in the sensor, medical and energy sectors [5; 6; 7; 8]. The recent developments in magnetoelectric sensors based on novel materials and fabrication methods have made the detection of low intensity magnetic fields possible, reaching limits of detection previously achievable only by superconducting quantum interference devices (SQUIDs) [9]. Thin-film technology, in conjunction with the optimal combination of ferromagnetic and piezoelectric materials, has sparked the development of magnetoelectric sensors with high sensitivity at room temperature, compatibility with CMOS microfabrication, reduced fabrication cost and increased spatial resolution [10; 11; 12]. These properties make magnetoelectricsensors a promising candidate for biomagnetic measurements and imaging such as magnetocardiography (MCG), magnetoencephalography (MEG) [13] and magnetoneurography (MNG) [14]. In a recent review [15], the structure, fabrication, operation mechanism and noise analysis of thin-film magnetoelectric sensors for biomagnetic sensing were presented. Apart from the biomagnetic sensing and due to the inherent ability of magnetic fields to penetrate tissue non-invasively, magnetoelectric materials have the potential to facilitate the manipulation of important electrophysical effects at the subcellular level. The control of cell-to-cell interactions can be a significant catalyst in the development of advanced targeted treatments, enabled by magnetoelectric materials at the micro- and nanoscale. Recently, the application of magnetoelectric composites in drug delivery applications and cancer treatment, as well as in tissue engineering, brain stimulation and wireless energy transfer for implantable devices, were discussed in detail by [PERSON] et al. [16], highlighting the prospects of magnetoelectric materials in future biomedical applications. Due to the coupling between ferromagnetic and piezoelectric domains, magnetostrictive materials and magnetoelectric composites were also employed in energy harvesting applications by exploiting both the direct and inverse magnetoelectric effect. Capturing the ambient energy available in the environment presents a viable method for powering the widespread adoption of the Internet of Things (IoT) in a decentralized and grid-independent way. Advancements in piezoelectric, magnetostrictive and magnetoelectric materials for energy harvesting emphasize the increased research interest in this area, which is highlighted in a recent review article [17], presenting the material, fabrication characterization and modelling achievements in the field. Based on the aforementioned applications and research efforts in micro- and nanofabrication, magnetoelectric composite research is mainly focused in material science and engineering, aiming to develop advanced composite structures with vastly different electrical, chemical and mechanical properties that allow the applicability of magnetoelectric materials in areas where biocompatibility, manufacturability, cost and magnetoelectric response are equally important. Currently, piezoelectric materials, including lead zirconate titanate (PZT), AlN, polyvinylidene fluoride (PVDF), AlScN, LiNbO\({}_{3}\), etc., combined with FeGa, FeCoBSi, FeCo, Metglas (commercial alloys containing Fe, Co, Ni, B, Si) and Terfenol D (Tb\({}_{x}\)Dy\({}_{1-x}\)Fe\({}_{2}\)) magnetostrictive materials, amongst others, have found widespread application in magnetoelectric composites and were studied extensively.