# Review of Magnetoelectric Effects on Coaxial Fibers of Ferrites and Ferroelectrics
## 4 Spinel Ferrite-Ferroelectric Coaxial Fibers
### Cobalt Ferrite-PZT Core-Shell Nanofibers  
CoFe\({}_{2}\)O\({}_{4}\) (CFO) is one of the well-studied spinel ferrites in ME composites due to its large magnetostriction (\(-\)110 ppm) and high Curie temperature (520 \({}^{\circ}\)C) [60]. Ferroelectric lead zirconate titanate, PbZr\({}_{x}\)Ti\({}_{1\text{-}}\)O\({}_{3}\) (PZT), in particular with a morphotropic phase boundary composition of x = 0.52, has the desired large piezoelectric coefficient (d\({}_{33}\) = 200-400 pC/N) and high Curie temperature (386 \({}^{\circ}\)C) [61; 62]. The combination of these two ferroic phases has attracted significant interest in the fabrication of ME materials with a variety of mechanical connectivities [56; 57; 63; 64; 65].  
The first reported studies on CFO-PZT fibers were carried out by [PERSON] et al. [56; 57]. Sols made with metal nitrates or acetates were used to synthesize the fibers by electrospinning and annealed fibers were characterized in terms of ferroic order parameters by PFM and magnetization measurements [56]. In another study on CFO and PbZr\({}_{x}\)Ti\({}_{1\text{-}}\)O\({}_{3}\) with x = 0.2, annealing of the fibers at high temperatures was reported to result in minor deformations in their shape, as seen in Figure 4a [40]. Structural characterization of annealed fibers by X-ray diffraction showed diffraction peaks corresponding to CFO and PZT and the absence of any impurities (Figure 4b).  
Figure 3: Diagram of electrospinning setup for synthesis of coaxial core–shell nanofibers.  
[PERSON] et al. confirmed the ferroelectric nature of the fibers by PFM measurements [40]. The PFM amplitude versus voltage data and the 180\({}^{\circ}\) contrast in the phase angle hysteresis loop in Figure 4c are clear evidence of the piezoelectric nature of the fiber rather than it having originated from electrostatic interactions. The estimated piezoelectric coefficient d\({}_{33}\) from the PFM data was 30 pm/V. Measurements of d\({}_{33}\) by PFM on CFO-PbZr\({}_{0.52}\)Ti\({}_{0.48}\)O\({}_{3}\) fibers yielded values ranging from 125 to 157 pm/V, depending on the molar fraction of CFO and PZT [56]. Ferromagnetic characterization on the fiber was performed by measuring the magnetization M with a vibrating sample magnetometer. Figure 4d shows the expected M vs. H hysteresis loop at room temperature with a saturation magnetization of 12 emu/g, which is a smaller than expected value for pure CFO since the fiber also contains nonmagnetic PZT.  
Several investigators have reported on the measurements of ME coupling in individual fibers by scanning probe microscopy. [PERSON] et al. [40] carried out MFM of CFO-PZT under an applied voltage to determine the strength of a converse ME (CME) effect. The magnetic domains were imaged (Figure 5A) first by the MFM mode. Then, the fiber was poled electrically by applying a DC voltage to the tip, operating under the PFM mode. This was followed by MFM to determine any change in the magnetization in CFO due to the piezoelectric strain in PZT (Figure 5B). The procedure could be used to determine the strength of the static converse ME coupling from data on the shifts in MFM frequency maps. The profiles in Figure 5C show significant changes in the frequency shifts, \(\Delta\)f, before and after applying a poling voltage of 6 V to the fiber, and the shift in turn is proportional to the magnetization of CFO. From these data, the estimated ME susceptibility is 1.2 \(\times\) 10\({}^{-8}\) s/m [40]. [PERSON] et al., on the other hand, reported MEVC = 29.5 V/cm Oe from PFM studies under a static magnetic field [57]. It is one of highest MEVC reported for a composite of CFO and PZT and is two orders of magnitude higher than values reported for bulk or layered composites of CFO and PZT [4].  
Figure 4: (a) SEM image of CFO-PZT core–shell fiber after heat treatment. (b) XRD profile of annealed CFO-PZT nanofiber. (c) PFM amplitude and phase angle as a function of applied voltage for a CFO-PZT fiber. (d) Magnetization M vs. static magnetic field H for the fibers [40].