A First-Principles Study Of The Structures And Properties Of Multiferroic Double-perovskite Materials

With the development of modern computer technology, people put forward to the integration devices which can satisfiesd the high demand of the miniaturization and integration. It needs to explore and research on new devices and materials which have varieties of functions. Single-phase multiferroic materials possess ferroelectricity (anti-ferroelectricty) and ferromagnetism (ferrimagnetism and antiferromagnetism). The interaction of electric property and magnetic property produces many additional features.Therefore, based on the high-tech era, multiferroic materials have great potentials. At present, the theoretical study of multiferroic materials concentrates on the the property of intrinsic and structure. The practical application focuses on searching good comprehensive multiferroic materials. In this thesis, we focus on the double perovskite multiferroic materials in theory and application, and do some beneficial exploration and attempt. Main comments are summarized as follows:1. Under the ground state, the monoclinic phase Bi2NiMnO6 were studied, through comparative analysis the Bi2NiMnO6’s ferromagnetic state and the ferrimagnetic state under 0 GPa and 30 GPa. It is concluded that:Bi2CoMnO6 is exist in the state of ferromagnetic, and it has nothing to do with the outside pressure. Pressure canenhance the magnetic of Bi2NiMnO6, but it makes the stability lower.2. We study of double-perovskite oxide multiferroic materials Bi2NiMnO6 doped with substitution Co at the B-site.The doping rate of Bi2Ni (1-x) CoxMnO6 (x=0.0625; 0.125; 0.25; 0.5) were 6.25%、12.5%、25% and 50% respectively, and study of the electronic structure, band structure and magnetic properties were investigated in detail. It was found that, Bi2Ni(1-x)CoxMnO6 is half-metallicity state and direct bandgap semiconductor with the priority of ferromagnetic state. The doping system Bi2Ni(1-x)CoxMnO6 has strong stability and it tends to possess best comprehensive performance when x=0.25. Magnetic properties of Bi2Ni(1-x)CoxMnO6 are derived from the ferromagnetic super-exchange interactions between Ni2+-O-Mn4+-O-Ni2+-Co2+-O-Mn4+-O-Ni2+-and-Co2+-O-Mn4+-O-Co2+. The existence of Co makes the electrons migrate away easily, so magnetic of Mn moment is reduced. But the effect of Co-O-Mn superexchange is not only to make up for the decrease of the magnetism moment of Mn, but also to make overall crystal magnetic moment increases. So the magnetism of dropping system Bi2Ni(1-x)CoxMnO6 increase with the increase of the concentration of the Co. Because Bi2Ni(1-x) CoxMnO6 close correlation between the electrical properties and magnetic properties, so that makes the system very great prospects of application in the field of magnetoelectric devices and spintronics.3. After the B position of Bi2NiMnO6was completely doped by Co, we got the BCMO. After optimizing the ferromagnetic state of Bi2CoMn06 system, the author mainly calculates the structure of crystal, the energy band, state density, and compare respectively with Bi2NiMnO6 and Bi2Ni(1-x)CoxMnO6 about magnetic properties. It was found that, BCMO is a kind of indirect band gap semiconductor, with half-metal property.Its distortion is more serious, stability of crystal and the ferromagnetism are weaker than Bi2NiMnO6 and Bi2Ni(1-x)CoxMnO6. It is because, aftering the ferromagnetic superexchange, Co2+ and Mn4+ ions in BCMO happens the second exchange. There are ferromagnetic superexchange function and double exchange function between Mn4+, Co2+ and Ni2+. That makes the magnetism moment of Co decreasing dramatically. It results in the decrease of the magnetism of BCMO system. So it can increase the magnetism of BNMO system only when the Co2+, Mn4+ and Ni2+ three ions exsist at the same time.This article mainly calculates the electronic structure, magnetic properties of multiferroic-magnetic perovskite materials with the frist-principle. From the theoretical design to calculation and analysis, we get some useful conclusions, which can make a positive impact on the future of multiferroic materials in research and application.