First Principle Study For Magnetic Properties And Polarization Of Some Ferroelectrics

Ferroelectrics have been attractted for fundamental research interest because of the rich superior physical properties. We studied the ferroelectricity of the new type non-oxide ferroelectric to know the comprehensive properties of all kinds of ferroelectrics. In the actual experiment research and production practice of ferroelectric, it is inevitable to form the intrinsic defect such as vacancy. The defect will influence significantly their performance, and may be lead to appearing the novelty properties such as magnetism and half-metallic properties. Constructing the ferroelectric/ferromagnetic multilayer is another effective method that gained the magnetism from ferroelectrics. In this thesis, we have systematically investigated the electronic structure, chemical bonding, spontaneous polarization of new type fluoride ferroelectric, the vacancy formation energy, magnetic properties of persvokite ferroelectric containing the single vacancy and the Ti-O ferroelectric displacements, magnetic properties in multiferro tunnel junction Co/BaTiO₃/Co by using the first principle method. The main results are as follows:1. We have calculated the electronic structure and spontaneous polarization of fluoride ferroelectric BaZnF₄. The Born effective charges of BaZnF₄ have been calculated by using berry phase method show the dynamic charges of Ba and F(3) is larger than normal iron charge. There is weak covalence between Ba and F. The covalence between Zn and F are shown obviously. The both are in favour of the spontaneous polarization of BaZnF₄. In addition, we evaluated the value of spontaneous polarization about 12.6μC /cm2which is agreement with experimental report.ÎœcμC/cm²2. The vacancy formation energy, density of state, spin charge density and Bader charge of both cubic and tetragonal BaTiO₃ containing the Ba, Ti and O vacancy have been calculated, respectively. The results manifested the Ti, O vacancy can induce magnetism and Ba vacancy can not induce the magnetism. The mechanism of magnetism induced by titanium vacancy is the partial polarized O 2p orbital. Furthmore, Ti vacancy in cubic BaTiO₃ can induce novelty half-metallic properties. The mechanism of magnetism induced by oxygen vacancy in BaTiO₃ is the unpaired electrons of the Ti atom nearest vacancy occupy the t_2g orbital.3. The Ti-O ferroelectric displacements, electronic structure, magnetic properties in multiferro tunnel junction Co/BaTiO₃/Co have been analyzed. We find that the ferroelectricity of the MFTJ can be maintained until below a critical thickness of 1.6 nm for the BaTiO₃ films. The induced magnetic moment of Ti and O atoms and the change of magnetic moment localized in Co atoms are because the polarization in ferroelectric barrier breaks the symmetry equilibrium lead to the charge redistribution of interfacial atoms. The surface magnetoelectric coupling coefficient has been estimated about 1.40×10^-10 G cm²/V.