Evidence For a Giant Magneto-Electric Coupling in Composites of Coaxial Nanofibers of Nickel Zinc Ferrite and PZT

Ferromagnetic-ferroelectric composites are of interests for studies on the nature of magneto-electric (ME) interactions between the two phases and for applications in useful technologies. The interactions are aided by mechanical forces, i.e., magnetostriction in the ferromagnet in a magnetic field and piezoelectric deformation in the ferroelectric in an electric field and the strength of ME coupling depends on efficient transfer of strain fromone phase to the other. A significant enhancement in the coupling strength can be accomplished in a nanocomposite in which the ratio of surface area-to-volume is orders of magnitude higher than in bulk or thick film composites [1,2]. <br/>This work is on the synthesis of nanofibers of nickel zinc ferrite, Ni(1-x) Znx Fe2O4 (x = 0.5-0.9) (NZFO) and lead zirconate titanate (PZT) by electrospinning and measurements of strengths of ME interactions in discs made of the nanofibers. Two types of fibers were made: (i) Core-shell nanofibers with either ferrite or PZT core and (ii) Composites fibers of ferrite and PZT. In electrospinning of core-shell fibers sol-gels of the ferrite and ferroelectrics were dispensed through a dual chamber needle. For composite fibers sols of equal volumes of ferrite and PZT were mixed and dispensed through a single needle. Electric fields, 1.5 – 2 kV/cm, was applied between the needle and a collector made of a rotating aluminum drum. Fibers are formed when the electrostatic forces overcome the surface tension. Core-shell fibers and composite fibers with an average diameter of 200-500 nm were prepared. Fibers were annealed at 700-900 C and characterized in terms of structure by electron microscopy, X-ray diffraction, and scanning probe microscopy (SPM). Fibers were free of impurities, ferromagnetic parameters for the fibers compared favorably with parameters for bulk NZFO, but the saturation ferroelectric polarization was two orders of magnitude smaller than for bulk samples. <br/>The fibers were pressed into discs for measurements of strength of ME coupling by static magnetic field H induced polarization and low-frequency ME voltage coefficients (MEVC). The fractional change in the remnant polarization dPr/Pr measured as a function of H= 0-7 kOe and a giant ME coupling was evident in discs made of core-shell fibers. The maximum change in dPr/Pr increased with increase in Zn concentration, from -3% for x=0.1 to 82% for x=0.3 and then decreased with increase in Zn concentration to -1% for x=0.5. A hysteresis in dPr/Pr vs H was measured for all of the discs. Discs of composite fibers, however, had much smaller dPr/Pr-values of -1 to -3%. Data on MEVC vs bias field <i>H</i> obtained by applying a static field H and an ac magnetic field h at 30 Hz, both fields parallel to each other and parallel to the plane of a disk of fibers. Discs of core-shell fibers showed a large ME response at H = 0 and was indicative of a built-in magnetic field likely due to anisotropy in the fibers. With increase in H, MEVC increased to values as high as 17 mV/cm Oe and then with increase in H decreased to near-zero value. The maximum MEVC for dics of NZFO-PZT composite fibers was small, on the order of 2-3 mV/cm Oe. Thus the results of this study showing a giant ME coupling in discs of core-shell fibers are indicative of their potential use for sensors and energy harvesting applications.<br/> The research at Oakland University was supported by a grant from AFOSR and the National Science Foundation (DMR- 1808892).<br/> References<br/>1. Ying Liu, et.al., MRS Communications 2020, 10, 230.<br/>2. Ying Liu, et.al., J. Compos.Sci. 2021, 5, 268.