掺杂BiFeO₃性质的第一性原理研究

【作者】 张晖；

【导师】 刘拥军；

【作者基本信息】 扬州大学 ， 理论物理， 2009， 硕士

【摘要】 多铁性材料同时具有铁电性和铁磁性,并且存在磁电耦合效应,从而可以实现铁电性和磁性的相互调控,可能成为一种新型功能材料,有着广阔的应用前景。BiFeO₃作为几乎唯一的室温单相多铁材料,引起人们越来越多的关注,成为当前多铁性研究最主要的材料体系之一。在铁电性方面,BiFeO₃薄膜大的漏导问题已经基本解决,具有大的剩余极化的BiFeO₃薄膜已经被制备出来;但是,BiFeO₃材料室温下的铁磁性非常弱,离应用还有差距,因此如何增强BiFeO₃的磁性是目前需要解决的最主要的问题之一。随着密度泛函理论和计算机硬件的不断发展,基于第一性原理的计算已成为研究多铁材料的一个常规方法。本文采用密度泛函理论（DFT）结合投影缀加波PAW（projector augumented wave）方法的VASP（Vienna ab-initio simulation package）软件包,对BiFeO₃以及对其B位掺杂替代形成的材料进行第一性原理研究。本论文主要包括了两个部分,第一部分包含四个章节,主要内容包括:论文研究背景和意义,多铁性及实现多铁性的思路,BiFeO₃的铁电性、铁磁性及其不足,密度泛函理论。论文第二部分包含两个章节:第五章计算了多铁材料BiFeO₃的磁结构、电子结构、能带结构;在第六章中为提高BiFeO₃的铁磁性,对其进行了B位过渡元素替代掺杂,计算了BiFe_0.5Mn_0.5O3、BiFe_0.5Ni_0.5O3、BiFe0.75Co0.25O3材料的磁结构、电子结构、能带结构。本文第五章计算结果表明:BiFeO₃的铁电性主要来源于Bi原子的相对位移,保持Bi和其他原子的相对位移不变,就可以较好地保持BiFeO₃材料的铁电性;尽管Fe原子具有较大的磁矩,但由于体系的反铁磁结构导致原子磁矩大部分相互抵消,整体几乎不显示铁磁性。在第六章中研究了对BiFeO₃的B位过渡元素替代掺杂,计算结果表明:B位上Co、Ni、Mn杂质的掺入基本不会改变BiFeO₃原有的铁电结构,可以推断掺杂对材料铁电性影响不大;同时掺杂导致原有的G型反铁磁序发生变化,形成了亚铁磁序的磁结构,材料的铁磁性有了较大的提高;然而,杂质的掺入使材料的绝缘性发生了明显变化,Ni、Mn杂质的掺入使材料变为了导体,不再满足铁电材料对绝缘性质的要求,Co杂质掺入后材料的绝缘性也有所减弱。更多还原

【Abstract】 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.更多还原