导　　师： 张建民

学科专业： 070207

授予学位： 硕士

作　　者： ;

机构地区： 陕西师范大学

摘　　要： 20世纪60年代,人们发现了双钙钛矿型化合物,由于其特殊的结构和各种奇异的电子特性,最近又吸引了许多科研人员做了大量有意义的基础性与应用性研究,包括处于磁场中时磁电阻显著下降,掺杂对材料能带的影响等等。我们目睹了人们对双钙钛矿这种过渡金属氧化物的研究兴趣。有序双钙钛矿结构为A2BB'O6/(其中A是碱土元素Mg、Ca、Sr、Ba或稀土元素La、Ce、Nd、B//B‘为不同的过渡金属元素,如B=Gr、Mn、Fe、Co和Ni,B'=Mo、W、Re/)。在双钙钛矿Sr2FeRe06和Ba2FeRe06这两种材料中观察到了大型室温隧道磁电阻效应,以及低场磁电阻效应,这些新的有吸引力的性质已经被证实可以有益地应用在未来的自旋电子器件上。 为了更详细的了解双钙钛矿的电子结构与磁交换,人们通常会在双钙钛矿中金属//磁性离子/(如A,B和B'/)的位置进行掺杂或者用电子掺杂来作深入研究。前人已经做过的工作有：用Cr、Nb和Ta等等来掺杂Sr2FeRe06结构进行磁学性质的研究。就我们所知,还没有人研究过Tc与Ba来掺杂双钙钛矿Sr2FeRe06材料。在本文中,我们研究Sr2FeRe1-xTxO6/(x=0,0.25,0.50,0.75和1/)及Sr2-xBaxFeReO6/(x=0,0.25,0.50,0.75,1,1.25,1.50,1.75,2/)的结构、电子属性及磁性。计算是在广义梯度近似下,通过基于密度泛函理论框架下的投影缀加波赝势来描述电子和原子核的相互作用。结果证明以上所有的化合物均是在自旋向上轨道上呈现绝缘性,而在自旋向下轨道呈现金属性。这导致在费米能级处,电子是完全自旋极化的。因此可以被应用于自旋电子器件来产生极化电流。得到的主要结论如下： 1/)随着Tc浓度的增加,晶格常数变小,晶胞的体积也缩小,这是由于Tc离子的半径比Re原子的小。 2/) Sr2eRe1-xTcxO6/(x=0,0.25,0.50,0.75和1/)这五个化合物都具有铁磁性基态,并且都是B位原子与B'位原子反铁磁性耦合。Fe的磁矩从3.705μB增加到3.794μB,而Re/(Tc/)原子的磁矩从-0.864μB/(-1.495μB/)变到了-0.288μB/(-1.063μB/)。因此,随着Tc掺杂浓度的增加,总磁矩变小,且减小的速度越来越快。 3/)掺杂后的Sr2FeRe1-xTcxO6/(x=0.25,0.50,0.75和1/)均与双钙钛矿Sr2FeReO6具有半金属性,可以被应用于一维自旋电子学器件中,用来产生100/%的极化电流。更重要的是,我们可以通过调节Tc原子的掺杂浓度来调节带隙的大小。 4/)最近邻的Fe-Fe, Re-Re, Tc-Tc原子对并不存在直接的相互作用,相反在沿每个Fe-O-Re-O-Fe链或Fe-O-Tc-O-Fe链上,Fe-3d,O-2p和Re-5d /(Tc-4d/)轨道之间的杂交非常强。 5/) Sr2-xBaxFeReO6/(x=0,0.25,0.50,0.75,1,1.25,1.50,1.75,2/),所有这些系统都是半金属,可应用在自旋电子学器件领域产生100/%的自旋极化电流。 6/)随着Ba掺杂浓度的增加Sr2-xBaxFeReO6/(x=0,0.25,0.50,0.75,1,1.25,1.50,1.75,2/)的总磁矩从3.0325μB增加到3.05μB。 7/) Sr2-xBaxFeReO6/(x=0,0.25,0.50,0.75,1,1.25,1.50,1.75,2/)的自旋通道带隙可以通过掺杂Ba原子来调整。 8/)相比于Sr和O原子之间,Ba原子和O原子之间有较强的相互作用。Fe原子和O原子之间相互作用在Sr2FeReO6材料中比在Ba2FeReO6里强。 Perovskite compounds, which were discovered in the1960s, have recently attracted a great deal of interest in basic and applied research due to their extraordinary structure and a variety of exotic electronic properties that include a spectacular decrease of electrical resistance in a magnetic field, the effect of doping on the band of perovskite compounds. In recent years we have witnessed an increased interest in the study of transition metal oxides with an ordered double perovskite structure A2BB'O6/(where A are alkaline earth elements Mg, Ca, Sr and Ba or rare earth elements La, Ce and Nd, B//B' are heterovalent transition metal elements such as B=Cr, Mn, Fe, Co and Ni, B'=Mo, W and Re/). Novel and attractive properties that can be useful in future spintronic device applications have been demonstrated, such as the large tunneling type magnetoresistance observed at room temperature, as well as the low field magnetoresistance found in Sr2FeReO6and Ba2FeReO6. In order to understand the electronic structure and magnetic exchange of the double perovskites in details, all the metallic//magnetic ions on the A, B and B' sites as well as the electron doping studies have been intensive researched recently. There has been considerable work on the structural and magnetic properties on the Sr2FeReO6with the Cr, Nb and Ta doping. To the best of our knowledge, there is no report on the electronic structure and magnetic properties of the Tc-doped and Ba-doped Sr2FeReO6. In this paper, the structure, electronic and magnetic properties of Sr2-xBaxFeReO6/(x=0to2/) have been studied by using the projector augmented wave potentials under generalized gradient approximation. All of the compounds are insulators in one of the spin channels, but a metal in the other. This leads to a complete spin polarization at the Fermi level and thus application as a source of spin polarized charge carriers in spintronic devices. The following conclusions are obtained: 1/) The results show that the cell volume shrinks as Tc concentration increases, while the lattice parameters have no obvious change. That results from the sites and the smaller ionic size of the Tc ions in the Sr2FeRe1-xTcxO6. 2/) The Sr2FeRe1-xTcxO6holds the same ferromagnetic groundstate as the undoped Sr2FeReO6. And the local magnetic moments of Fe ions increases from3.705μB to 3.794μB with the Tc doping, while the local magnetic moments of Re /(Tc/) ions decreases from-0.864μB /(-1.495μB/) to-0.288μB /(-1.063μB/) with the Tc doping. Therefore, doping Tc ions leads to a reduction in the Sr2FeRe1-xTcxO6 3/) The total magnetic moments, and the reduction has an increased tend with increasing the Tc ions component /(x/) in the Sr2FeRe1-xTcxO6. 4/) These Sr2FeRe1-xTcxO6systems are half-metallic and could be used in the field of quasi-one-dimensional spintronics devices for producing100/%spin polarized currents. More important, each Sr2FeRe1-xTcxO6with different x also has half-metallic character, this is to say, the half-metallic nature is not destroyed when doping Tc ions, and it has no relationship to the different x. 5/) More charges are accumulated around O atoms reflecting substantial charge transfer from Re and Fe atoms to O atoms. And the charge distributions show that there exists no direct interaction between two nearest Fe-Fe, Re-Re, Tc-Tc pairs, whereas along each Fe-O-Re-O-Fe chain or Fe-O-Tc-O-Fe chain, the hybridizations between Fe3d, O2p, and Re5d or Tc4d orbitals are fairly significant. 6/) For Sr2-xBaxFeReO6/(x=0,0.25,0.50,0.75,1.00,1.25,1.50,1.75,2/), all these systems are half-metallic and could be used in the field of spintronics devices for producing100/%spin polarized currents. 7/) The band gap of the total spin-up DOS at the Fermi level increases slightly with increasing the x for Sr2-xBaxFeReO6and can be adjusted by change the content of the Ba. 8/) The total magnetic moments of the Sr2-xBaxFeReO6increase from3.0325μB to3.μB with the Ba doping except for x=1.25. 9/) It can be seen that Ba and O atoms have stronger interaction than Sr and O atoms. And the interaction of Fe and O atoms is stronger in Sr2FeReO6than that in Ba2FeRe06.

关 键 词： 双钙钛矿 电子结构 磁性 第一性原理

分 类 号： [O469]

领　　域： [理学] [理学] [理学]