Theoretical Study Of Magnetoelectric Coulping Properties In Multiferroic Nanocomposite Thin Films

With the rapid development of micro-and nano-information technology, the exploration of multifunctional smart nanofilm materials with electric, magnetic and light properties has become a hot point of research in the fields of advanced manufacture, material engineering, applied physics and microelectronics. Multiferroic nanocomposite thin films are composed of ferroelectric/piezoelectric materials and magnetostrictive materials. The ferroelectric-ferromagnetic coupling can be realized by the stress-strain transfer between the two phases, resulting in the magnetoelectric coupling effect which is a physical property from a change in polarization (P) with applied magnetic field (H) or a change in magnetization (M) with applied electric field (E). However, the nanocomposite films deposited on substrates are extremely constrainted by the substrate, and the large strain will obviously affect their magnetoelectric properties. The traditionally theoretical formulas of magnetoelectric coupling based on the block composites are no longer suitable to the nanofilm materials, and the current theory based on magnetoelectric properties of multiferroic nanocomposite thin film is not perfect. Therefore, further theoretical research and analysis should be carried out.In this dissertation, the expressiones of Helmholz free energy of laminated and vertical epitaxial ferroelectric thin films were deduced by using the Lauda-Ginsburg-Devenshire themodynamic theory, and the ferroelectric properties and piezoelectric properties of the ferroelectric films were investigated. Combining with the corresponding themodynamic potential and the elastic theory, the magntoelectric voltage coefficients of the laminated (2-2type) and vertical (1-1and1-3types) heteroepitaxial nanocomposite thin films were theoretically studied. The main research contents and results were displayed as follows:(1) The impact of mismatch strain between the substrate and the thin film on the phase transition of ferroelectric thin films was analyzed for the laminated single-domain BaTiO3and PbTiO3ferroelectric thin films epitaxially grown on a cubic substrate according to the thermodynamic theory. The dependences of piezoelectric properties on the in-plane misfit strain of ferroelectric thin films were discussed.(2) For the laminated epitaxial multiferroic nanocomposite thin films with2-2type, the in-plane mechanical equilibrium equation was introduced due to the consideration of the substrate clamping effect. Combining with the corresponding themodynamic potential and the elastic theory, the influence of the ferroelectric phase content, substrate/film thickness ratio and in-plane strain of ferroelectric films on the magnetoelectric voltage coefficient of nanocomposite thin films were analyzed. The magnetoelectric coupling of2-2-type laminated multiferroic nanocomposite thin films was transferred by the in-plane strain. Therefore, the substrate clamping obviously affected the magnetoelectric properties of nanocomposite films, namely, the magnetoelectric voltage coefficient sharply decreased with the increase of film thickness. Compared with the block materials, the in-plane strain greatly affected the magnetoelectric property of nanocomposite films. Therefore, the suitable substrate materials provided an effective method to modulate the magnetoelectric voltage coefficients of nanocomposite thin films. The calculated results obtained by our method were consistent with the experimental ones when compared with the calculated results obtained from previous other models.(3) The single-domain BaTiO3and PbTiO3ferroelectric thin films in the vertical epitaxial multiferroic nanocomposite thin films were constrained not only in plane with substrate but also out of plane with ferromagnetic phase. The modified expressiones of Helmholz free energy for three-demensional constrained ferroelectric films were proposed with the aid of corresponding mechanical boundry conditions. The elaborate analysis of in-plane and out-of-plane strain effects on ferroelectric properties and piezoelectric properties of ferroelectric thin films in the vertical nanocomposite structures was carried out by using the nonlinear thermodynamic theory. The research results show that strain states of vertical epitaxial ferroelectric thin films will result in the phase transition in the films, then have an effect on the ferroelectric properties and piezoelectric properties. (4) The influences of the volume fraction of ferromagnetic phase and different substrates on the magnetoelectric voltage coefficient of the vertical epitaxial multiferroic nanocomposite thin films with1-1and1-3types were studied by using the corresponding themodynamic potential combined with elastic theory with the consideration of out-of-plane elastic effect between ferroelectric and ferromagnetic films. With the aid of the elastic theory, it demonstrates that the substrate clamping and volume fraction of ferromagnetic phase affected the out-of-plane strain of ferromagnetic films. Therefore, the choices of substrate materials with different lattice constants and ferromagnetic phase fraction could effectly modulate and control the magnetoelectric voltage coefficients of vertical epitaxial multiferroic composite films.