Dielectric Behavior and Electro-Magnetic Coupling at Room Temperature in BiFeO3/PVDF and CoFe2O4/PVDF Composites

The dielectric properties of composites formed by dispersions of two types of ceramic particles, BiFeO3 (multiferroic) and CoFe2O4 (magnetic), in poly(vinylidene fluoride), PVDF, (which can present a ferroelectric phase) were studied at room temperature as a function of particle concentration. The characterization studies include determination of dielectric polarization curves in the presence of external magnetic fields, H (that is, P-E plots in the presence of H, where P is the dielectric polarization and E the external applied electric field). Impedance spectroscopy analysis shows the presence of several distributions of dipolar relaxation processes, some of which are assigned to charge rearrangements at the different interfaces of the system, for example at the ceramic-polymer interfaces. Negative magneto-electric coupling was observed at low ceramic proportion (where leakage currents are lower than 20 nA/cm(2)), that is, decrease of P when applying H, (partial derivative P/partial derivative H)(E) < 0. The negative coupling is opposite to previously observed for BiFeO3 dispersed in a nonferroelectric matrix (styrene-butadiene-rubber, SBR) where (partial derivative P/partial derivative H)(E) < 0 . These results suggest a strong polymer-particle interaction, where the interfaces between the two components play a central role. The observation of negative magneto-electric suggests a matrix-mediated coupling, possibly through a magneto-striction mechanism at the ceramic-polymer interfaces. The description of magneto-electric coupling on the bases of magnetostriction effects leads to the conclusion that the results in PVDF and SBR are in agreement with positive magnetostriction, that is positive strains (elongation) when a magnetic field is applied.