# Review of Magnetoelectric Effects on Coaxial Fibers of Ferrites and Ferroelectrics
## 4 Spinel Ferrite-Ferroelectric Coaxial Fibers
### NiFe\({}_{2}\)O\({}_{4}\)-PZT Core-Shell Nanofibers  
Nickel ferrite (NFO)-based ME composites have been studied extensively in the past [3; 4; 5; 6; 7]. Although the saturation magnetostriction \(\lambda_{\rm s}\) for NFO is in the order of \(-\)30 ppm, which is much smaller than for CFO, the maximum value of the piezomagnetic coupling coefficient q = d\(\lambda\)/dH for NFO is higher than for CFO [4]. Since the low-frequency MEVC is directly proportional to q, one expects a much higher MEVC for NFO- than for CFO-based composites [3; 4; 5; 6; 7]. However, there have been very few studies on ME effects on NFO-PZT nanowires or fibers. [PERSON] et al. synthesized core-shell NFO-PZT nanowires by a three-step process involving a sol-gel process to form PZT tubes, followed by electrodeposition of the NiFe alloy to fill the cores of the tubes, and high-temperature annealing to form the wires with an NFO core and a PZT shell [29]. Arrays of the nanowires were characterized in terms of ferroic order parameters. The first studies on the synthesis and ME characterization of NFO-PZT core-shell fibers were reported by [PERSON] et al. [39]. Fibers with a PZT core-ferrite shell (sample A) or a ferrite core-PZT shell (sample B) were prepared by electrospinning using sols of NFO and PbZr\({}_{0.52}\)Ti\({}_{0.48}\)O\({}_{3}\). Fibers, upon annealing at 650 \({}^{\circ}\)C, had a core diameter in the range 200-450 nm and a shell thickness of 125-175 nm. XRD data, shown in Figure 6a, were indicative of absence of any impurity phases. The compositions of both NFO and PZT were confirmed by energy dispersive X-ray spectroscopy (EDS). SEM images in Figure 6b,c show fibers with uniform core diameters and shell thicknesses and interfaces free of any defects [39]. Further confirmation of the core and shell structure was obtained from  
Figure 5: MFM images of CFO-PZT fiber poled under (**A**) 0 V and (**B**) 6 V. (**C**) Profiles of MFM frequency signals before and after poling of the fiber. The frequency shifts \(\Delta\)f\({}_{1}\) and \(\Delta\)f\({}_{2}\) are proportional to the magnetization of CFO in the fiber [40]. scanning microwave microscope (SMM) images at 2-20 GHz [68]. Figure 6d shows an SMM image of capacitance at 5.7 GHz. The core-shell structure for the fiber is well resolved in the SMM image. Magnetization and ferromagnetic resonance (FMR) measurements of the fibers confirmed the ferromagnetic ordering of NFO in the fibers. Ferroelectric polarization P vs. E for the fibers were indicative of P-values that were one to two orders of magnitude smaller than for bulk or thin films of PZT [39]. Direct ME effects on NFO-PZT fibers were investigated by (i) P vs. E under H in a disk of the fibers pressed in a static magnetic field and (ii) change in the permittivity measured at 20-22 GHz under H in films of the fibers assembled under a magnetic field [39]. For P vs. E under H, films of the fibers were pressed into a disc in a uniform magnetic field of 3 kOe. Partial alignment of the fibers in the H direction was evident from SEM images, as shown in Figure 7a, for fibers with an NFO core and a PZT shell. Measured P vs. E are shown in Figure 7b for H = 0-7 kOe [39]. The fractional change in the remnant polarization \(\Delta\)P\({}_{\rm r}\)/P\({}_{\rm r}\) = [P\({}_{\rm r}\) (H)--P\({}_{\rm r}\) (0)]/P\({}_{\rm r}\) (0) versus H, a measure of DME effects on the fibers, is shown in Figure 7c. A hysteresis showing an increase in P\({}_{\rm r}\) with increasing H was followed by a further increase in P\({}_{\rm r}\) when H was decreased back to zero. A similar behavior, but with a decrease in P\({}_{\rm r}\) under H, was reported for a disc containing fibers of a PZT core and an NFO shell [39]. The overall fractional increase in P\({}_{\rm r}\) for the NFO-PZT fibers was higher by an order of magnitude or more than reported values for bulk or layered NFO-PZT composites [3; 4; 5; 6; 7; 11; 17].