To Improve The Ferromagnetic Properties Of BiFeO₃ By Doping From First-principles Study

Perovskite structure is one of the most common structures and also has very important status of science and technology in the solid-state material. It has a general chemical formula of ABO3 in the field of scientific research. The perovskite oxides have attracted much attention in the field of modern electronic information due to their multiple crystal structures and abundant physical properties. Due to their abundant physical and chemical properties like ferroelectricity, piezoelectric, electromagnetic effect, giant magnetoresistance ation reduction, hydrogen solution, rization and electrocatalysis, effect, electrooptical isome effect, acoustic-optical effect, high oxid and so on, they have attracted extensive attention in the field of physics, material science and chemistry. As a kind of important inorganic non-metallic material, this transition metal perovskite materials is a kind of important magnetic resistance materials and catalytic materials, with a unique crystal structure. The formation of crystal defect structures and properties after A- or B- doping, such as anionic defects or different valence state of B ion, perovskite oxides can be widely applied to the field of solid fuel cell, solid electrolyte, sensors, and high temperature heating materials, solid resistor and precious metal redox catalyst. Meanwhile through A- and B- cationic replacement, changing its cationic type and proportion, it is very easy to adjust the physical properties of this kind of oxide. For example choosing appropriate alloy（e.g., choose different valence state, atoms of different ionic radius） has an obvious influence on the material structure, electronic, magnetic and polarization properties. Thus perovskite type oxide is a kind of important functional materials with excellent performance and wide application. In recent years, the researches of ferroelectric, ferroelectric/ferromagnetic composites and ferroelectric/ferromagnetic preparation have become an important research direction of condensed matter physics and materials physics.In the thesis, the first-principles calculation method based on density functional theory has been used to calculate and analysis the A- or B- doping and intrinsic defects in single-phase multiferroic material BiFeO₃. The relative stability of the doped system, electronic structure and magnetic properties are disscused in this thesis. Through detailed analysis and discussion, the following results have been obtained:（1）The electronic structural and magnetic properties of the perovskite Co-doped BiFeO₃ and intrinsic defects have been investigated using the first-principles calculation method based on density functional theory. The detailed disscusion based on the results of calculation shows that the substitution of a Co atom for Fe atom in BiFeO₃ introduces spin polarized impurity states in the band gap, generates a magnetic moment of 1.0 μB（absoltue）, and leads to a half-metallic property. When two Co atoms substitute for Fe atoms in BiFeO₃, the system shows weakly ferrimagnetic and half-metallic property. The ferrimagnetism may attribute to the different local magnetic moment of Fe3+and Co3+ at different sublattice sites and the strong hybridization between the 3d orbits of Co and the 2p orbits of O, which breaks the balance between antiparallel sublattice magnetization of Fe3+. At the same time, we have found that the intrinsic defects VFe and VBi in BiFeO₃ generate 2.0 μB and 1.0 μB magnetic moment respectively, whereas the intrinsic defects VO has no effect on magnetism in BiFeO₃ and Co-doped BiFe O3 system. However the intrinsic defect VO transforms the Co-doped BiFeO₃ system from half-metallic to metallic. The magnetic moment of intrinsic defect VFe in BiFeO₃ system may attribute to charge compensation and spin skipping of Fe atom. Electrons release from intrinsic defect VO form coupled impurites state in the gap of BiFeO₃ system, so that there is no magnetic moment in the BiFeO₃ system with intrinsic defect VO.（2） Based on first-principles spin-polarized density functional theory calculations, the relative stability, electronic structures, and magnetic properties of B-, C-, N-, and F-doped BiFeO₃ are investigated. Meanwhile we also calculated the doped system with intrinsic defect of VO. The substitution of B, C, N, and F for O produces a magnetic moment of 3.0, 2.0, 1.0, and 1.0 μB per dopant, respectively. The net magnetic moments are from the break of the symmetry of the AFM spin ordering network. The results show that the doped BiFeO₃ presents different electronic properties and the magnetic properties. The C-doped system shows a half-metallic property with a C-type spin alignment. The B- and N-doped BiFeO₃ are ferrimagnetic semiconductors, and show an A-type and a G-type spin alignment, respectively. As for F-doped case, system becomes metallic in its G-type spin alignment. The dangling bond-related state of intrinsic defect VO form a state in the doped BiFeO₃ system, and the coulped electrons of a state are just spin antiparallel, so that the intrinsic defect VO has no effect on magnetism in doped BiFeO₃ system. However the intrinsic defect VO makes the doped BiFeO₃ system into metallic nature. It is a little more complicated in the doped BiFeO₃ system with charge. Due to the charge compensation mechanism, one injected electron may extinct one defect in B-, N-, C-doped BiFeO₃ system, so that the magnetic moment of the doped BiFeO₃ system decrease 1.0 μB accordingly. As for the case of F-doped BiFeO₃ system with charge, the injected electron and the electron inspired by F substituting O in BiFeO₃ form a VO-like state, so the net magnetic moment in F-doped BiFeO₃ system returns to zero.（3） Based on first-principles spin-polarized density functional theory calculations, the electronic structures, and magnetic properties of Cu and Zn-doped BiFeO₃ are investigated. The calculated formation energies show that Cu prefers to occupy Fe site, while Zn prefers to occupy Bi site. All the doped BiFeO₃ systems turn out to be favorable for G-type antiferromagnetic arrangement. The substitution of Cu and Zn for Fe produces a magnetic moment of 3.0 and 4.0 μB per dopant, respectively. The net magnetic moments are from the broken symmetry of the AFM spin ordering network. For the substitution of Cu and Zn for Bi, the net magnetic moment originates from Cu/Zn itself and holes produced by Cu/Zn. Two-Cu/Zn-doped cases show various magnetic behaves. If O vacancy is far away from dopants, the O vacancies don’t affect the net magnetic moment of the substitution of Cu and Zn for Fe, but have notable effect on Bi site doping. The O vacancies result in metallicity in all doped cases. Our study has demonstrated that the nonmagnetic Cu and Zn doping will lead to the diversity and complexity of magnetic properties depending on doping sites, distance between dopants, intrinsic defect, and so on, which could be responsible for the observed various magnetic behaviors in Cu/Zn-doped BiFeO₃ samples.