Magnetoelectric Properties Of The Multiferroic Materials

Ferroelectric/Ferromagnetic composite is a novel functional material, which possesses not only ferroelectric and ferromagnetic properties but also magnetoelectric coupling effect, so that it has very broad range potential applications. In this thesis, the magnetoelectric effect of the multiferroic material was studied by developing several popular models of micro-mechanics, which can enable us to analyze the macroscopical behavior from the micro-structure of the composition. We mostly consider matrix-based two-phase multiferroic composites, matrix-based three-phase multiferroic composites and magnetoelectric polycrystals. These numerical results provide guidelines for the experimental design of the multiferroic material.A mean field Mori-Tanaka model was used to calculate the effective magnetoelectroelastic moduli of matrix-based two-phase multiferroic composites, emphasizing the effects of particular shape of second phase particles. The results show a strong influence of shape aspect ratios of constituent phases on the effective magnetoelectric coefficients, while the orientation is parallel and perpendicular about the pole polarization of matrix and magnetization of the second phase particles. A mean field Mori-Tanaka model was developed to calculate the effective magnetoelectroelastic moduli of matrix-based three-phase multiferroic composites.The change trend of magnetoelectric coefficients in the three-phase particulate composites which are made of rare-earth-iron alloys, piezoelectric ceramics and polymer (Terfenol-D/PZT/PVDF) were obtained. The variations of magnetoelectric coefficients of the composites with respect to the microstructural features have been investigated. The results show a strong influence of volume fraction and shape aspect ratios of constituent phases on the effective magnetoelectric coefficients. The optimal volume fraction and shape aspect ratios of particles for the strongest magnetoelectric coupling are identified. It is observed that lamellar particles are optimal for ME coefficient a11, while fibrous particles are optimal for a33.we developed an effective medium approximation to calculate the macroscopic effective coefficients of magnetoelectric polycrystals, which are annealed in perpendicular electric and magnetic fields, and emphasized the effects of temperature as well as shape and orientation distribution of grains on the macroscopic magnetoelectric coefficients of polycrystalline Cr2O3. Through a systematic study, it is observed that the effective magnetoelectric coefficients on the diagonal of magnetoelectric coefficient moduli are zero, and only non-diagonal magnetoelectric coefficients appear, and that calculated from the effective medium approximation is higher than simple volume averaging and agrees with experimental data better. It is also noted that polycrystals with randomly oriented grains are optimal for a12 while those with appropriate texture coefficients can cause nonzero a13 and with texture coefficient around 0.5 are optimal for this.