  Ferromagnetic-ferroelectric nanocomposites are of interest for realizing
strong strain mediated coupling between electric and magnetic subsystems due to
high surface area-to-volume ratio. This report is on the synthesis of nickel
ferrite (NFO) -barium titanate (BTO) core-shell nano-fibers, magnetic field
assisted assembly into superstructures, and studies on magneto-electric (ME)
interactions. Electrospinning techniques were used to prepare coaxial fibers of
0.5-1.5 micron in diameter. The core-shell structure of annealed fibers was
confirmed by electron microscopy and scanning probe microscopy. The fibers were
assembled into discs and films in a uniform magnetic field or a field gradient.
Studies on ME coupling in the assembled films and discs were done by magnetic
field H induced polarization, magneto-dielectric effects at low frequencies and
at 16-24 GHz, and low frequency ME voltage coefficients (MEVC). We measured 2~
2-7% change in remnant polarization and in the permittivity for H = 7 kOe, and
a MEVC of 0.4 mV/cm Oe at 30 Hz. A model has been developed for low-frequency
ME effects in an assembly of fibers and takes into account dipole-dipole
interactions between the fibers and fiber discontinuity. Theoretical estimates
for the low-frequency MEVC have been compared with the data. These results
indicate strong ME coupling in superstructures of the core-shell fibers.
