# Review of Magnetoelectric Effects on Coaxial Fibers of Ferrites and Ferroelectrics
## 5 Hexagonal Ferrite and Ferroelectric Core-Shell Fibers
### Y-Type and W-Type Hexaferrite-Based Core-Shell Nanofibers  
Y- and W-type hexaferrite have more complex structures relative to M-type ferrites, but the hexagonal symmetry remains [46]. [PERSON] et al. [79] studied coaxial fibers of Y- and W-type ferrites and PZT or BTO. Y-types with the composition Ba\({}_{2}\)Ni\({}_{2}\)Fe\({}_{12}\)O\({}_{22}\) (Ni\({}_{2}\)Y) and Ba\({}_{2}\)Zn\({}_{2}\)Fe\({}_{12}\)O\({}_{22}\) (Zn\({}_{2}\)Y) with in-plane anisotropy fields H\({}_{\text{a}}\) = 14 kOe and 9 kOe, respectively, were chosen. Both had much lower Curie temperatures compared with SrM [46]. Their choices for W-type ferrites were BaCo\({}_{2}\)Fe\({}_{16}\)O\({}_{27}\) (Co\({}_{2}\)W) and BaZn\({}_{2}\)Fe\({}_{16}\)O\({}_{27}\) (Zn\({}_{2}\)W) with uniaxial anisotropy fields H\({}_{\text{a}}\) = 21 kOe and 12.5 kOe, respectively. Individual fibers of Y  
Figure 13: (**a**) SEM image of annealed fiber of SrM core–BTO shell. (**b**) XRD patterns of PZT-SrM and BTO-SrM fibers. Fractional change \(\Delta\)P\({}_{\text{r}}\)/P\({}_{\text{r}}\) in discs of fibers of (**c**) PZT core–SrM shell and (**d**) SrM core–BTO shell. MEVC for 3D films consisting of fibers of (**e**) SrM core–PZT shell and (**f**) SrM core–BTO shell [76]. and W-type hexaferrites and coaxial fibers with PZT or BTO were prepared, and their direct ME effects were systematically studied. Individual fibers of Y-type and W-type ferrites made by electrospinning were characterized in terms of crystal structure and magnetic parameters, and the magnetostriction of the fibers was smaller than the vales of the bulk samples [47; 79]. The saturation magnetostriction \(\lambda_{\mathrm{s}}\) measured on the annealed disks of the fibers was \(-10\) ppm for both Zn\({}_{2}\)W and Co\({}_{2}\)W, whereas it was \(-13\) ppm for Ni\({}_{2}\)Y and \(-2.5\) ppm for Zn\({}_{2}\)Y. Several groups also previously reported on the synthesis and magnetic characterization of pure hexaferrite fibers [80; 81; 82]. Core-shell fibers of Y- and W-type ferrites and PZT or BTO were annealed at 900-1000 \({}^{\circ}\)C and SMM images showed well-formed and defect-free core-shell structures in all of the fiber systems [79]. The saturation magnetization at room temperature for the fibers ranged from a minimum of 2.5 emu/g for Zn\({}_{2}\)W-BTO to a maximum of 8 emu/g for Co\({}_{2}\)W-PZT. The maximum polarization obtained from P vs. E measurements on the discs of the fibers were in the range 1-2.5 \(\upmu\)C/cm\({}^{2}\). Both 2D and 3D films of the annealed fibers were assembled in a static magnetic field for measurements of MEVC vs. H. Representative data from [79] for 3D films for both DC and AC magnetic fields parallel or perpendicular to the film plane are shown in Figure 14 for Ni\({}_{2}\)Y-BTO, Zn\({}_{2}\)Y-BTO, Zn\({}_{2}\)W-PZT, and Co\({}_{2}\)W-PZT. The data in Figure 14a,b for films with Y-type ferrite and BTO show a large zero-bias MEVC potentially due to a built-in field arising from the magneto-crystalline anisotropy field. For films with Ni\({}_{2}\)Y, MEVC is higher for in-plane fields than for out-of-plane fields and it increases under H for both field orientations and there is no hysteresis in MEVC vs. H. For the films with Zn\({}_{2}\)Y, the MEVC is higher than for Ni\({}_{2}\)Y, although the magnetostriction for Zn\({}_{2}\)Y was measured to be much smaller than for Ni\({}_{2}\)Y [79]. Since MEVC is directly proportional to the piezomagnetic coefficient \(\mathrm{q}=\mathrm{d}\lambda/\mathrm{d}H\), one may infer from the data in Figure 14 that \(\mathrm{q}\) for fibers with Zn\({}_{2}\)Y is higher than for Ni\({}_{2}\)Y. MEVC for films with fibers of Zn\({}_{2}\)W and Co\({}_{2}\)W with PZT are shown in Figure 14c,d.