【作者】 刘俊；

【导师】 王新强；

【作者基本信息】 重庆大学 ， 凝聚态物理， 2008， 博士

【摘要】 可将电子荷电性和自旋性从材料内部进行有机融合的自旋电子学已成为国际研究的新热点,自旋电子学的发展必将开启新的电子时代。半金属材料是自旋电子学的重要材料载体之一。在目前所发现的半金属中,高温相尖晶石结构半金属材料因居里温度高、室温自旋极化率大、制备简单等优点被认为极具应用潜力。然而,对尖晶石结构半金属材料的研究仍处于初步探索阶段。例如,它们的磁电阻仍不够大而难以满足应用要求,所发现的半金属材料仍极为有限等。因此,极有必要发现更多具有优良物理性能的尖晶石结构半金属材料,并对它们的磁电性能及其微观机制作深入探讨。本文通过基于密度泛函理论的第一性原理计算,预言了多种新高温相尖晶石结构半金属材料如ScFe₂O₄、LaFe₂O₄和LiPr₂O₄等和高自旋极化材料Fe₃F₄,并详细计算、分析了它们的半金属性、电荷分布、分子磁矩等磁电性能;然后利用配位场理论分析了这些材料磁电性能的微观机制和电子结构;最后预测了它们的应用前景。主要研究工作和成果概况如下:（1）对不同含量的非磁过渡元素（TM=Sc、Ti、V、Cr、Mn）取代Fe₃O₄中A位或B位Fe离子后所得的尖晶石化合物进行了几何结构优化,并计算了它们的自旋极化态密度和能带结构,预言了新半金属材料ScFe₂O₄、Fe2ScO4、TiFe₂O₄和CrFe₂O₄。通过计算和分析发现,ScFe₂O₄、Fe2ScO4、TiFe₂O₄和CrFe₂O₄的分子磁矩明显大于Fe₃O₄,而电阻率则可能小于Fe₃O₄。在相同外磁场中,ScFe₂O₄、Fe₂ScO₄、TiFe₂O₄和CrFe₂O₄的磁电阻可能大于Fe₃O₄,从而在自旋电子器件中更有应用前途。ScFe₂O₄、Fe₂ScO₄、TiFe₂O₄为弱铁磁性耦合化合物,CrFe₂O₄和Fe₃O₄为亚铁磁性耦合化合物。耦合差异的原因在于在配合物ML₄和ML₆中,中心离子TM与周围O配体间既有离子键作用也有共价键作用,孤立原子Sc和Ti的外层电子数少,故Sc和Ti离子与O配体的离子键比重高于Fe和Cr离子。（2）将稀土元素La和Pr取代Fe₃O₄的A位或B位Fe离子后得到尖晶石化合物（RexNM1-x）A（ReyNM2-y）BO4,其中Re=La、Pr,表示稀土元素;NM=Li、Co、Mn、Fe,表示非稀土元素。然后对它们的几何结构进行了优化,并计算了它们的自旋极化态密度和能带结构,预言了新稀土半金属材料FeLa₂O₄、CoLa₂O₄、MnLa₂O₄和LiPr₂O₄。通过计算和分析发现,FeLa₂O₄和CoLa₂O₄与Fe₃O₄类似,为ⅡB型半金属材料,而MnLa₂O₄和LiPr₂O₄与Fe₃O₄不同,为ⅡA型半金属材料。四种稀土半金属材料的A位和B位中心离子中,只有一个位置具有磁性,因此它们具有弱铁磁性耦合,可用间接交换作用模型（RKKY模型）进一步解释。这些稀土半金属材料的分子磁矩有较大的变化范围,最小为1.0μB,最大为5.0μB,从而具有比较广泛的应用前景。FeLa₂O₄、CoLa₂O₄、MnLa₂O₄的磁性主要来源于过渡金属离子,即过渡离子的3d轨道受较强的晶体场作用发生分裂,导致一种自旋子带处于费米面附近,并与O配体的2p轨道形成杂化轨道,而另一自旋子带的能量高于费米能级而形成空带。La离子没有4f电子,从而对材料的磁性几乎没有贡献。LiPr₂O₄的磁性主要来源于Pr离子。原因在于,处于晶体场中的Pr4f轨道的自旋向上子带移到费米面处,而向下子带则移到费米面之上形成空带,而且因受到外层电子的屏蔽作用,Pr4f轨道不能与其他轨道杂化而局域在离子内部。（3）将不同含量的F元素取代Fe₃O₄的O配体构成含F配体的高温相尖晶石结构材料,对它们的几何结构进行了优化,并计算了优化后材料的自旋极化态密度和能带结构,基于配位场理论分析其磁电性能的微观机制和电子结构。结果发现,F取代A位Fe离子的O配体不利于材料半金属性的出现。F取代B位Fe离子的配体时,F含量（x）对材料的半金属性及其稳定性有明显影响。x≤0.5时,材料没有半金属性。x>0.5时,材料具有半金属性,且随F配体含量的增加,费米面整体相对向高能方向移动,材料的半金属性更加稳定。主要原因在于随F配体含量增加,原胞中电子密度增大,从而导致原胞中具有更强的库仑斥力（即晶体场）,该斥力导致了自旋向下和向上子带的分离加剧。Fe₃F₄的稳定相晶格常数约为0.643 nm,分子磁矩约为10.68μB,明显高于Fe₃O₄的4.0μB。Fe₃F₄的自旋极化率比Fe₃O₄稍小,但零磁场电导率比Fe₃O₄高很多,因而更具有应用潜力。更多还原

【Abstract】 Spintronics has been emphasized very recently because the charge and spin of electrons can be probably controlled in spintronic materials. A new electronic age will be opened with the quick development of spintronics. Half-metal materials are very important spintronic materials. The cubic spinel half-metal materials are thought to have larger application probability than other half-metal materials because they have high Curie temperature, large room-temperature spin-polarization and they can be prepared simply. However, researches on them have been carried on for short time so that their room-temperature magnetoresistance is still not enough to satisfy their application and there are few kinds of spinel half-metal materials. Therefore, it is very necessary to probe more new spinel half-metal materials with good physical properties and to study their magnetic and electric properties, and then their microscopic mechanism.In this paper, the high spin-polarized material Fe3F4 and many new spinel half-metal materials such as ScFe₂O₄, LaFe₂O₄ and LiPr₂O₄ are prefaced from the first-principle calculation based on the density function theories. And their magnetic and electric properties including half-metallicity, charge distribution and molecular magnetic moments are calculated or analyzed in system. Then their electronic structures and the microscopic mechanism of magnetic and electric properties are analyzed based on the ligand field theories. At last, application probability of these half-materials is prefaced. The main works and results involve:The geometric structures of cubic spinel compounds including transition metals （TM） on A-sites or B-sites are optimized,where TM=Sc、Ti、V、Cr、Mn. Then their spin-polarized state densities and energy band structures are calculated in system. Many new half-metal materials including ScFe₂O₄, Fe₂ScO₄, TiFe₂O₄ and CrFe₂O₄ are prefaced from calculation. The molecular moments of ScFe₂O₄, Fe₂ScO₄, TiFe₂O₄ and CrFe₂O₄ are all larger than those of Fe₃O₄, but their resistivity is lower than that of Fe₃O₄. Therefore, the magnetoresistance of ScFe₂O₄, Fe₂ScO₄, TiFe₂O₄ and CrFe₂O₄ is larger than that of Fe₃O₄ so that they have wider application probability in spintronics. ScFe₂O₄, Fe2ScO4 and TiFe₂O₄ have weak ferromagnetic coupling, but CrFe₂O₄ and Fe₃O₄ has ferrimagnetic coupling. The mechanism is that there are both covalent bonds and ionic bonds between the transition ions （TM） and their ligands in the compound ML₄ and ML₆. However, the proportion of ionic bonds in ScFe₂O₄ and TiFe₂O₄ is larger than those in Fe3O4 and CrFe₂O₄ because Sc and Ti atoms have fewer electrons than Fe and Cr atoms.Fe-ions on A-sites or B-sites of Fe3O4 are substituted by doped La-ions or Pr-ions so that the cubic spinel compounds （RexNM1-x）A（ReyNM2-y）BO4 are designed, where Re=La, Pr and NM=Li, Co, Mn, Fe. Their geometric structures are optimized and their spin-polarized state densities and energy band structures are calculated. New rare-earth spinel half-metal materials including FeLa₂O₄, CoLa₂O₄, MnLa₂O₄ and LiPr₂O₄ are prefaced from calculation. Calculated results show that FeLa₂O₄ is a kind ofⅡB type half-metal material, which is similar with CoLa₂O₄ and Fe3O4, but MnLa₂O₄ and LiPr₂O₄ are bothⅡA type half-metal materials. FeLa₂O₄, CoLa₂O₄, MnLa₂O₄ and LiPr₂O₄ are weak ferromagnetic coupling compounds because the centric ions on A-sites and B-sites do not have magnetic moments at the same time. Therefore, the coupling can be interpreted by the Rude-Gmann-Kittel-Kasuya-Yosida Model （RKKY）. Their molecular magnetic moments vary from 1.0μB to 5.0μB. Therefore, they have wider application area. The magnetic moments of FeLa₂O₄, CoLa₂O₄ and MnLa₂O₄ mainly come from transition ions. The mechanism is that the 3d-orbits of transition ions are splitted by strong crystal field in the tetrahedron compounds ML4 and octahedron compounds ML6. This results in that a kind of 3d sub-band are near fermi level, and then are mixed into hybrid orbits. However, the other kind of 3d sub-band are above the fermi level. La-ions have no contribution for the magnetism of half-metal materials because they have no 4f electrons, although there are hybrid orbits between O-ions and La-ions. The magnetic moments of LiPr₂O₄ mainly come from Pr-ions. The mechanism is that the Pr4f orbits are splitted by strong crystal field, and then there is relative moving between the up-spin and down-spin sub-band. This results in that the up-spin sub-band is near fermi face, but the down-spin sub-band is above the fermi level. On the other hand, the Pr4f orbits can not be mixed with other orbits, and then are localized in ions because there is static shielding of electrons around them.The O-ligands of Fe3O4 are substituted by F-ions, and then new cubic spinel materials are designed. Their geometric structures are optimized and their spin-polarized state densities and energy band structures are calculated in system. The mechanism of magnetic and electric properties and the electronic structures are analyzed based on the ligand field theories. Results show that spinel materials have no half-metallicity if their O-ligands around the Fe-ions on A-sites are substituted by F-ions. On the other hand, the concentration of F-ligand around Fe-ions on B-sites can cause important influence on the half-metallicity and stabilities of materials. Materials have no half-metallicity if x≤0.5. Materials have half-metallicity if x>0.5. The Fermi face move forward higher energy, and then the half-metallicity are more stable with increasing concentration of F-ions. The main mechanism is that the concentration of electrons in a primitive cell increases with the increasing of F-ions so that there are stronger crystal field, which cause stronger splitting of up-spin sub-band and down-spin sub-band. The stable crystal constant is about 0.643 nm and the molecular magnetic moment is about 10.68μB, which is higher than 4.0 of Fe3O4. The spin-polarization of Fe3F4 is a little lower than Fe3O4, but its conductance is much higher than Fe3O4 so that it has huger application potential than Fe3O4.更多还原