Coupling interaction in multiferroic BaTiO₃-CoFe ₂O₄ nanostructures

Recent experimental results (Zheng et al 2004 Science 303 661) have shown that a 1-3-type BaTiO₃-CoFe₂O₄ composite film exhibited a strong coupling between the ferroelectric and ferromagnetic order parameters. The coupling interaction in the nanostructured multiferroic film could be different from that in a bulk composite because there exists a giant residual stress/strain in the film. In this paper, by considering the influence of the three-dimensional residual strain in the film, the magnetically induced polarization of the nanostructured BaTiO₃-CoFe₂O ₄ multiferroic films has been studied using the Landau-Ginsberg- Devonshire thermodynamic theory. The calculation results show that the 1-3-type nanostructured composite film could produce a ferroelectric-ferromagnetic coupling larger than that of its bulk counterpart which is in broad agreement with the experimental results. The 2-2-type nanostructured composite films were found to produce much less coupling interaction in comparison with the 1-3-type film. The dependence of multiferroic coupling on composition and temperature was analysed on the basis of the calculated results.