First Principles Study Of Doping The Bifeo <sub> 3 </ Sub> Nature

The multiferroic material, in which the ferroelectricity and ferromagnetism coexist, may become one kind of new functional materials and has a broad application prospect. It makes the mutual control of the magnetic and electric properties to be possible due to its magnetoelectricity coupling effect. As the only room-temperature multiferroic material, BiFeO₃ attracts more and more attention and becomes the primary material for the investigation to multiferroism. For BiFeO₃ film, the large drain conductance problem has been solved, and the film of large remnant polarization has been made. But its ferromagnetism is too weak to be used at room temperature. So, the enhancement of its magnetism is a key problem waiting to be solved.Owing to the development of density functional theory and computer technique, the first-principle calculation becomes a regular method to study multiferroic materials. By using the VASP (Viena ab-initio simulation package), which bases on the density functional theory combined with the projector augumented wave (PAW), we have conducted the first-principle investigation to BiFeO₃ and the B-site doped ones.This thesis consists of two parts. There are four chapters in the first part which involve mainly in the background and significance of research, multiferroism and its realization, the ferroelectricity and ferromagnetism of BiFeO₃, and the density functional theory. The second part consists of two chapters. The magnetic, electronic, and energy band structures of BiFeO₃ are studied in the fifth chapter. To improve its ferromagnetism, BiFeO₃ is doped on B-site. Those structures of BiFe_0.5Mn_0.5O3, BiFe_0.5Ni_0.5O3, and BiFe0.75Co0.25O3 are investigated in the sixth chapter.The results in the fifth chapter show that the ferroelectrism of BiFeO₃ mainly comes from the relative displacement of Bi atoms. The ferroelectricity of BiFeO₃ could be kept as long as the relative positions of Bi and other atoms are almost unchanged. Fe atom has a great magnetic moment, but the lattice has almost no ferromagnetism because of its antiferromagnetic structure. In the sixth chapter, the B-site doping to BiFeO₃ is studied. The numerical results show that the impurities of Co, Ni and Mn can't significantly change the geometrical structure so that BiFe_0.5Mn_0.5O3, BiFe_0.5Ni_0.5O3, BiFe0.75Co0.25O3 keep the obvious ferroelectricism. The ferromagnetism can be significantly improved since the doping changes the G-type antiferromagnetic order into the ferromagnetic one. But the doping lowers the insulativity. The doping of Mn or Ni makes the material to be conductor, and the impurity of Co will weaken its insulativity.