Electrical and magnetic properties of BiFeO$_{3}$-CoFe$_{2}$O$_{4}$ nanotube composite

We report the electrical and magnetic properties of BiFeO$_{3}$ and CoFe$_{2}$O$_{4}$ nanotube composite multiferroics. CoFe$_{2}$O$_{4}$ nanotubes were prepared on Pt coated Si substrates using a template assisted method, yielding nanotubes with 20-50 nm thick walls and an outer diameter of 200 to 400 nm. These nanotubes were then uniformly coated by a BiFeO$_{3}$ layer by a metal organic decomposition method to yield the composite multiferroics. We observed ferroelectric switching behavior with saturated hysteresis loops with $P_{r}$ and $E_{c}$ values of approximately 0.08 \textit{$\mu $}C/cm$^{2}$ and 15 kV/cm, respectively, for a maximum applied electric field of 50 kV/cm. For pure BiFeO$_{3}$ thin films the hysteresis curves do not show any saturating trend and the E$_{c}$ is three times smaller than that of the composite. The magnetic measurements show that the pure BiFeO$_{3}$ is non-ferrimagnetic, while the composite shows a clear hysteresis with saturation magnetization of $\sim $12 emu/cm$^{3}$. These composite BiFeO$_{3}$ -- CoFe$_{2}$O$_{4}$ structures provide an approach for studying magnetoelectric coupling at the interfaces between different ferroic materials.