Rinciples Study On Spin Non Coupling Property Of Multiferroic Material

Complex transition metal oxides span a wide range of crystalline structures and play host to an incredible variety of physical phenomena. High dielectric permittivities, piezoelectricity, and ferroelectricity are just a few of the functionalities offered by this class of materials, while the potential for applications of the more exotic properties like high temperature superconductivity and colossal magnetoresistance is still waiting to be fully exploited. With recent advances in deposition techniques, the structural quality of oxide heterostructures now rivals that of the best conventional semiconductors, taking oxide electronics to a new level. Such heterostructures have enabled the fabrication of artificial multifunctional materials. At the same time they have exposed a wealth of phenomena at the boundaries where compounds with different structural instabilities and electronic properties meet. Multiferroics, defined for those multifunctional ma-terials in which two or more kinds of fundamental ferroicities coexist, have become one of the hottest topics of condensed matter physics and materials science in recent years. The coexistence of several order parameters in multiferroics brings out novel physical phenomena and offers possibilities for new device functions. Multiferroics is the coexistence of ferroelectricity (electric dipole order) and magnetism (spin order) in one system. Besides ferroelectricity and magnetism coexist,even more important is the coupling between magnetic and polarization orders, such a magnetoelectric coupling represents the basis for multi-control of the two orders by either an electric field or magnetic field,so Multiferroics is a new type multi-functions material.This thesis consists of the following parts. Chapter1introduces the properties and application potentials of multiferroic materials, along with some of the research progress and breakthroughs in such field. Chapter2introduces theories and computational meth-ods including density functional theory (DFT), Berry phase theory, maximally localized Wannier function (MLWF) and random phase approximation (RPA) methods, which can compute the screen coulomb and Hund exchange interactions. Chapter3introduces how the epitaxial strain and artificial interface affect or change properties of solids, along with the applications of film and supe lattice heterostrucrure. Chapter4includes two parts, the first of which introduces the interface effect of (CaMnO3)m/(BaTiC>3)n supperlattice, which changed the instability of antiferrodistortive (AFD) mode and fer-roelectric (FE) mode, thus realized the giant spin phonon coupling effect. The break-through provided a new approach to obtain strong megnetoelectric coupling effect in multiferroic field. In second part of chapter4introduces the spontaneous electric polar-ization caused by the interface effect of (CaTcO3)1/(BaTcO3)1superlattice interface ef- fect. Since the Neel temperatures of antiferromegnetic transitions are already very high for CaTcO3and BaTcO3, the (CaTcO3)1/(BaTcO3)1can potentially be used as room temperature multiferroic material. Moreover, the spontaneous electric polarization orig-inate from the contribution of both proper multiferroic effect and improper multiferroic effect, which is a relatively new physical phenomenon in multiferroic field. Chapter5mainly introduces the physics mechanism of spin phonon coupling effect. Works done by other researchers, which explain the mechanisms from the macro and phenomeno-logical point of view, are briefly introduced in the first place. Then we give a clear and intuitive explanation on the mechanism from the micro and electronic structural level, using the maximally localized Wannier function along with the Kugel-Khomskii model. In chapter6we introduce our work on high temperature multiferroic material hexago-nal LuFeO3. The origin of the magnetic order is explained by using Kugel-Khomskii model, meanwhile, the physical mechanism for h-LuFeO3to transfer from antiferro-magnetic to weak ferromagnetic in low temperature is discovered, which builds a solid foundation for LuFeO3to become a room temperature ferroelectric and ferromagnetic material. In the appendix, we introduces some important computational details includ-ing the model Hamiltonian based on density functional theory, the derivation of the magnetic interchange constant by using Kugel-Khomskii model, and the decomposi-tion of the structural defect by phonon mode, which are omitted in Chapter6.