# Review of Magnetoelectric Effects on Coaxial Fibers of Ferrites and Ferroelectrics
## 4 Spinel Ferrite-Ferroelectric Coaxial Fibers
### NiFe\({}_{2}\)O\({}_{4}\)-BTO Core-Shell Nanofibers  
Barium titanate is one of the preferred ferroelectrics for the replacement of lead-based ferroelectrics in ME nanocomposites [69,70] and was used extensively in the early works on bulk ME composites [71,72]. [PERSON] et al. reported on the synthesis of BTO-NFO coaxial fibers by electrospinning, and magneto-electric characterization by H-induced ferroelectric polarization, low-frequency MEVC, and high-frequency magneto-dielectric  
Figure 8: (**a**) SEM image of a film of NFO-PZT fibers of sample B assembled on a glass slide in a non-uniform magnetic field gradient. The arrow represents the H direction. (**b**) Real part of the relative permittivity \(\epsilon_{\mathrm{r}}^{{}^{\prime}}\) vs. \(t\) for a series of H for the film of sample B. (**c**) Fractional change in \(\epsilon_{\mathrm{r}}^{{}^{\prime}}\) vs. H at 21 and 22 GHz for the assembled film of sample B. [39].  
Figure 9: (**a**) SMM S\({}_{11}\) image of a single fiber of an NFO core– PZT shell. The yellow dot represents the position of the SMM tip during the measurements. (**b**) S\({}_{11}\) amplitude and phase vs. applied magnetic field H at 5.4 GHz showing FMR as a dip in the amplitude and 180\({}^{\mathrm{o}}\) phase shift. (**c**) The resonance field H\({}_{\mathrm{r}}\) as a function of applied DC voltage at 5.4 GHz. [68].  
effects [69]. Fibers with a BTO core-ferrite shell and an NFO core-BTO shell were prepared and annealed at 600-700 \({}^{\circ}\)C. The core--shell structure was confirmed with MFM images. Measurements of P vs. E in the discs of the fibers showed \(p\)-values as high as 3 \(\mu\)C/cm\({}^{2}\) for E = 40 kV/cm. FMR measurements indicated ferromagnetism with a gyromagnetic ratio \(\gamma\) = 3.1 GHz/kOe, and effective magnetization 4\(\pi\)M\({}_{\text{eff}}\) = 480 G.  
Results of ME coupling measurements in H-assembled discs and thin films of the fibers are shown in Figure 10. Magnetic field-induced variation in P\({}_{\text{r}}\) was measured on the discs of the fibers pressed in a magnetic field. Figure 10a,b show the fractional change in P\({}_{\text{r}}\) as a function of H. A hysteresis in (\(\Delta\)P\({}_{\text{r}}\)/P\({}_{\text{r}}\)) vs. H and a sign reversal are seen for the disc with fibers of the BTO core-NFO shell and a maximum increase in P\({}_{\text{r}}\) of 3%. For the disc with fibers of the NFO core-BTO shell, \(\Delta\)P\({}_{\text{r}}\)/P\({}_{\text{r}}\) is positive and increases under H to a maximum of 1.5%. Upon decreasing H back to zero, P\({}_{\text{r}}\) continues to increase to a maximum of 6.5% at H = 0. The results of MEVC versus H measured at 100 Hz in H-assembled films of BTO-NFO fibers are shown in Figure 10c,d. A rapid increase in MEVC to a maximum as H is increased from zero is followed by a decrease in its value for H > 2 kOe. An 180\({}^{\circ}\) phase shift in ME voltage upon reversal of H direction and a hysteresis in MEVC vs. H are evident, as shown in Figure 10. The film with fibers of the NFO core-BTO shell shows a large zero-bias MEVC. Although the H-induced polarization and results of the magneto-dielectric effects [69] provide clear evidence for strong ME coupling in the fibers, the overall magnitude of MEVC in Figure 10 are orders of magnitude smaller than for bulk or layered composites [3; 4; 5; 6; 7; 71; 72], and the possible causes are discussed in Section 6.