【作者】 王琼；

【导师】 谢文辉；

【作者基本信息】 华东师范大学 ， 凝聚态物理， 2010， 硕士

【摘要】 自上世纪60年代起建立的密度泛函理论包括随后出现的Kohn-Sham方程,已经成为凝聚态物理研究领域中的有力工具。而基于密度泛函理论的第一性原理计算方法,则是计算材料科学的重要基础和核心技术。近年来,由于大型、高速电子计算机的应用,第一性原理在计算和设计材料方面的优越性显得越来越突出,运用变得越来越广泛。钙钛矿氧化物材料的研究一直是磁性材料研究中的重要领域之一。而在钙钛矿氧化物中,双钙钛矿La2NiMnO6和La2CoMnO6(本文分别简写为LNMO和LCMO)是居里温度接近室温的铁磁性半导体材料,在理论和实验中具有重要的价值,一直引起科学工作者的关注,并已经有许多研究报道,但是依然有一些有价值的问题值得我们更进一步仔细研究。比如,本论文的工作中,我们通过对A位离子(La位)掺入一定量替代原子Ba,Sr,Ca,发现其磁性质会发生变化,从磁性铁磁半导体转变成了铁磁半金属。为了仔细了解掺杂La位原子对晶体结构和磁性质的影响,本文首先运用第一性原理计算方法研究了LNMO和LCMO的电子结构,从分析了它们的电子结构特点和磁性质等,然后重点讨论了La位掺杂对这两类材料的电子结构和磁性质的影响。主要工作有以下几个方面：1,用第一性原理计算研究了LNMO和LCMO的结构性质,电子结构和磁性质。我们的计算结果显示LNMO和LCMO的单斜相P21/n应当是铁磁绝缘体,这和实验的报道是吻合的,显示了我们计算结果的可靠性。我们发现对于LNMO,GGA计算可以得到与实验相符的结果,计算出的总磁矩为5μB,其中Ni原子和Mn原子的磁矩分别为1.47μB和2.65μB。对于LCMO,GGA的计算结果与实验不符,LDA+U计算可以得到与实验相符的结果。计算得到LCMO的总磁矩为6μB,Co原子和Mn原子的磁矩分别为2.63μB和3.16μＢ,这表明LCMO的磁性质受内部电子关联的影响较大。我们还进一步分析了这两类材料的内部的化学成键特点和价态,对于LNMＯ通过对电子结构的分析判断出Ni原子的价态为+2价,Mn原子的价态为+4价,而对于LCMO则得到Co原子的价态为+2价,Mn原子的价态为+3价。这些结果有助于进一步理解这些材料的性质。2,讨论了La位原子不同程度的掺杂对LNMO和LCMO的单斜相P21/n结构和磁性质的影响,并对通过计算得到的化合物的电子结构的分析解释了磁性变化的原因。我们计算发现对于LNMO,对于La位原子的25%,50%,75%的Sr掺杂,都会出现半金属特性,但是对于LCMO,只有25%的Sr原子掺杂显示了半金属特性。在论文中,我们对相关化合物的电子结构变化和结构变化进行了详细地讨论,分析了半金属性质的形成原因。此外,我们还发现随着掺杂浓度的递增,两个主要八面体NiO6(CoO6)和MnO6之间的夹角呈增加的趋势,更符合Goodenough-Kanamori定则关于过渡金属元素之间成键磁性规律的描述。更多还原

【Abstract】 The Density Functional Theory has become most powerful tool to investigatge electronic structure and material characteristics in condensed matter physics and material physics, since it was established in 1960s and followed by its famous export equation---Kohn-Sham equation. The first-principle calculation, which is based on the Density Functional Theory, is playing important role in computational materials. In recent years, the superiority of the first-principle calculation is becoming more and more outstanding to explore and design novel material due to the application of the large and high-speed computer.Perovskite oxide is very important subjects in magnetic material, among of which double perovskite La2NiMn06 and La2CoMnO6(hereafter called LNMO and LCMO) are both kinds of ferromagnetic semiconductor with high Curie temperature (～270°),which has attracted much more attention in both theory and experiment. It is very interesting to investigate their special electronic structure and magnetic propertise using first-principle calculation. In this thesis, we focus on changes of property when Ba, Sr, Ca atoms are dopped instand of the A-site atom.The electronic structure and crystal structure of LNMO and LCMO are investigated carefully. In order to understand the effect of La-site dopping on crystal structure and magnetic property, we studied the electronic structure and magnetic properties of both LNMO and LCMO.Our results serve as a first quantitative theoretical prediction for future study, and the main work are listed as follows:1,The structural, electronic and magngetic properties of monoclinic P21/n double perovskite LNMO and LCMO were studied using first-principle desity functional calculations, and the internal bonding regularity and valence state are also analysed.We found LNMO and LCMO are both ferromagnetic insulator in P21/n monoclinic crystal strucuture. For LNMO, our calculations show that the GGA calculation results are coincide with the experiment. The magnetic moment of LNMO are 5μB, and 1.47μBand 2.65μB for Ni and Mn, respectively. But for LCMO, the LDA+U calculation results are coincide with the experiment. The magnetic moment of LCMO are 6μB, and 2.63μB and 3.16μB for Co and Mn, respectively. Above all, the valence state of Ni, Co and Mn are+2 and+4, respectively.2, Discussing the different degree effects of doping La on its structure and magnetic properties, and explaining the reseaon for the change of every kinds of atoms’ magnetic moment when dopping La-site by electronic structure and band structure.25%,50% and 75% La replaced by Sr results in half-metal ferromagnetism in LNMO. In contrast, only 25% Sr dopping in LCMO behave half-metal ferromagnetism. We analyzed how the electronic structure and crystal structure changed with Sr dopped concentration in details.Our results reveal that the M-O-Mn angle (M is Co or Ni) of the two octahedrons(NiO6/CoO6 and MnO6)is increasing with Sr concentration, The related change trendency of chemical bonding and magnetic are consistent with famous GK rules.更多还原