# Review of Magnetoelectric Effects on Coaxial Fibers of Ferrites and Ferroelectrics
###### Abstract
Nanocomposites studied in recent years include ordered arrays [17; 18; 19; 20; 21; 22], thin films [23; 24], core-shell particles [25; 26; 27], coaxial wires [28; 29; 30; 31; 32], nanopillars [33; 34; 35; 36], and core-shell fibers [37; 38; 39]. Coaxial fibers, in particular, are of interest due to structural features suitable for realizing strong ME effects. In the case of nanobilayer or nanopillar composites on a substrate, the substrate clamping could result in significant reduction in the strengths of ME interactions [17]. Ordered arrays and nanopillars could be fabricated such that the clamping effect is minimized but not completely eliminated, whereas core-shell fibers are free of any such clamping effects [37; 38; 39; 40; 41; 42; 43; 44]. Coaxial nanofibers could be fabricated by low-cost techniques such as electrospinning, which have been shown to be ideal for the synthesis of composites with uniform cores and shells and free of interface defects. Nano- and microfiber composites, therefore, are one of the most promising candidates for achieving strong ME coupling and for application in sensors, energy harvesting, etc. Although there have only been a few reports on core-shell multiferroic composites in recent years, they do provide clear evidence for the potential to miniaturize ME composite structures and also achieve strong cross-coupling in them. This review article on core-shell micro- and nanofiber composites is aimed at highlighting key advances in this category of multiferroic materials and to promote activities in this important topic of interest, in particular, for applications in magnetic sensors for medical imaging and dual electric and magnetic field tunable high-frequency microwave and millimeter wave devices. The focus here is on structures with spinel ferrites or hexagonal ferrites for the ferromagnetic phase and ferroelectric PZT or BTO. Several ferrimagnetic spinel ferrites, including pure and Zn-substituted nickel and cobalt ferrites, have desirable order parameters such as large magnetostriction \(\lambda\) and piezomagnetic coupling coefficient q, and high Curie temperature T\({}_{\rm c}\) for use in the composites [45]. Similarly, hexaferrites with large uniaxial or planar magneto-crystalline anisotropy fields are of interest for composites with self-magnetic bias and are preferred for applications in magnetic sensors and energy harvesting [46; 47]. Fibers with cobalt ferrite (CFO), nickel ferrite (NFO), Zn-substituted NFO, and M-, Y-, and W-type hexagonal ferrites for the magnetic phase and PZT or BTO for the ferroelectric phase are considered here. Synthesis by electrospinning, structural characterization by X-ray diffraction, electron microscopy, and scanning probe microscopy are described. Characterization of the fibers in terms of ferroic order parameters was carried out in most of the studies covered in this review. Results on the measurements of strengths of DME by low-frequency MEVC in fibers assembled into 2D and 3D films in a magnetic field, static magnetic field H-induced ferroelectric polarization, and piezo force microscopy (PFM) under applied H are provided. Converse ME effects by voltage tuning of ferromagnetic resonance (FMR) and magnetic force microscopy (MFM) under an electric field are discussed. Finally, we conclude with a brief discussion on follow-up studies and avenues for potential device applications for the core-shell fiber composites.