# Review of Magnetoelectric Effects on Coaxial Fibers of Ferrites and Ferroelectrics
## 6 Discussion  
It is clear from the discussion so far that the somewhat very limited efforts on ME composites with core-shell nano- and microfibers of ferromagnetic and ferroelectric phases have been primarily on oxides of ferrimagnets and ferroelectrics, since it is rather easy to synthesize fibers free of chemical and structural impurities by electrospinning. There were many reports on fibers with spinel ferrites and ferroelectrics, whereas very few studies focused on hexagonal ferrites and PZT/BTO. Early studies were limited to structural, magnetic, and ferroelectric characterization of the fibers and were lacking any measurements on the nature of ME interactions. Initial reports on ME interactions in core-shell fibers were on CFO-PZT and CFO-BTO and utilized SPM techniques for ME measurements. Piezo force microscopy studies under H were employed to estimate the strength of DME in single fibers [56; 57; 64; 65; 66; 67]. An ME susceptibility of 1.2 \(\times\) 10\({}^{-8}\) s/m was measured by a combination of PFM and MFM in CFO-PZT fibers [40]. Reported values of DEVC varied from 29.5 V/cm Oe for CFO-PZT [57] to 35 V/cm Oe for CFO-BTO [67]. Another spinel ferrite-based fiber system studied in the past was NFO-PZT/BTO. Even though XRD data did not reveal any impurity phases and SEM and SPM images showed defect-free fibers, both the magnetization and ferroelectric polarization were reported to be much smaller than for bulk materials [39; 69]. Although DME studies by PFM on single fiber are yet to be reported, converse ME effects by FMR were carried out on NFO-PZT using an SMM, and a voltage-CME coefficient of Av = \(-\)24 Oe/V was reported [68]. Low-frequency DEVC results were reported for 2D films of the fibers of both NFO-PZT and NFO-BTO and were orders of magnitude smaller than for bulk or thick-layered composites [39; 69]. A much higher DEVC, however, was measured in 3D films of the fiber composites. Similar studies on NZFO-PZT coaxial fibers also reported low DEVC values for fiber films [59]. Films of fibers of hexagonal ferrites and PZT/BTO showed a higher DEVC than their spinal ferrite/ferroelectric counterparts [76; 79]. The most significant observation in these hexagonal ferrite-based fiber systems was the large zero-bias DEVC due to a built-in bias field provided by the very high magneto-crystalline anisotropy in the ferrites. Hexaferrite/ferroelectric fibers and fiber assemblies with high zero-bias ME effects have the potential for use in both AC and DC magnetic field sensors and in energy harvesting applications since they eliminate the need for a bias magnetic field. Next, we compared the DEVC for single core-shell fibers in films and discs of fiber samples, with reported values in Table 2 for composites of various mechanical connectivity including bulk (0-3) and layered (2-2) samples [83; 84; 85; 86; 87; 88; 89; 90; 91; 92; 93; 94]. It is clear from Table 2 that films of core-shell fibers had the lowest DEVC, whereas measurements on individual coaxial fiber had values comparable to bulk and layered composites. It is also noteworthy that the DEVC measured on individual fibers was in fact higher than the theoretical estimates in Figures 1 and 2. The relatively small DEVC for films of the fibers compared with theoretical estimates and values for individual fibers could be attributed to the effects of porosity and dipole-dipole interactions in the fiber films. As discussed in the theory section, the model was only valid for isolated fibers. It is essential to consider the effects of (i) both magnetic and electric dipole-dipole interactions between the cores and shells of ferrites and ferroelectrics and (ii) the porosity (or fiber discontinuity) in films. Magnetic dipole-dipole interactions, in particular, between ferrite cores or shells will give rise to demagnetizing effects and reductions in the magnitudes of applied AC and DC magnetic fields and a reduction in DEVC [47].