烧绿石型铱氧化物中的阻挫磁性及电子结构研究
发布时间：2018-09-01 13:43
【摘要】：近几年,人们发现自旋轨道耦合将导致一种新的物质形态：拓扑绝缘体,拓扑绝缘体成为了研究强自旋轨道耦合的热点。5d过渡金属氧化物中存在复杂的电子关联和自旋轨道耦合作用,这其中具有烧绿石结构的铱基化合物被认为是研究物质中电子相互作用、自旋-轨道耦合以及拓扑形态的理想材料之一。烧绿石型铱氧化物(A2Ir207,其中A为Y,Bi或者镧系元素)属于面心立方体系,空间点群为Fd-3m。实验发现随着A位元素离子半径的增加,体系从具有磁性的绝缘体转变到无磁序的非常规金属态。Bi2Ir207中铋元素与钇元素离子半径不同,且不包含具有磁矩的稀土元素以及f轨道电子,有助于区分稀土元素以及铱元素对材料物理性质的影响。在本文中,首先我们简要的介绍了烧绿石型铱化物的相关理论概念以及研究进展。然后我们研究了A=Y、Eu以及Bi不同A位元素烧绿石型铱化物多晶材料的晶体结构、电输运性质、磁性质以及热力学性质。另外我们取多晶Bi2Ir207作为研究对象,通过对其Bi位和Ir位分别进行Ca离子和Mn离子掺杂的方式,对其自旋轨道耦合、阻挫效应以及电、磁性质等物理性质做了详细的介绍。主要取得以下研究成果：通过对三种不同A位元素的多晶样品的晶体结构,磁性,电输运以及比热的对比研究发现：Y2Ir2O7和Eu2Ir2O7具有强磁阻挫和长程磁有序,而Bi2Ir2O7表现出弱的磁阻挫和短程有序,这种A位元素离子半径的变化不仅会影响物质的导带宽度,而且影响Ir-Ir之间的相互作用和能量交换,进而影响材料的电输运性质以及基态的磁性质。同时我们也了解到,在所研究的这几种烧绿石铱化物中存在具有阻挫结构的反铁磁性,强自旋轨道耦合基础上的各向异性对于这种阻挫结构的形成具有重要的作用。成功获得了一系列Ca和Mn掺杂的Bi2Ir2O7多晶样品,研究表明Bi2Ir2O7本身表现出较强的磁不稳定性和金属导电行为。利用半径较小的Ca离子替代Bi离子,体系出现了金属-绝缘体转变现象,转变温度Tc随着Ca离子掺杂量的增加而提高。在磁性研究中并没有发现与金属-绝缘体转变相对应的磁相变,因此我们认为这里出现的金属-绝缘体转变是由于引入离子半径较小的Ca离子造成了电子之间的散射增强,晶格无序度增大。另外,通过计算分析表明Ca离子掺杂减小了Ir-O-Ir的键角以及Ir-5d与Bi-6p轨道的电子杂化进而提高了样品的反铁磁性,影响了体系原本的磁阻挫结构。利用3d5结构的Mn离子替代Ir离子,掺杂体系同样也出现了不同温度Tc的金属绝缘体转变,我们认为这里出现的金属绝缘体转变主要有两个方面的原因：一方面Mn离子掺杂引入了无序,另一方面3d元素的离子半径元素的离子半径更小,能带宽度更窄,电子受到的屏蔽作用较小,电子间的库伦相互作用较大,因此当利用Mn离子代替Ir离子时,导致Ir-O-Ir键角变小。Mn离子掺杂并没有引起磁相变,但是从居里外斯拟合的结果分析发现,随着掺杂量的增加体系的反铁磁性增强。掺杂体系的电阻随外加磁场的变化趋势与母体相反,电阻随外加磁场的增加而减弱。这种现象的出现与掺杂体系中引入的Mn离子之间以及Mn离子与Ir离子之间地相互作用有关。
[Abstract]:In recent years, it has been found that spin-orbit coupling will lead to a new form of matter: topological insulators, which have become a hot spot in the study of strong spin-orbit coupling. Pyrochlore-type iridium oxides (A2Ir207, in which A is Y, Bi or Lanthanide) belong to the face-centered cubic system and the space point group is Fd-3m. It is found that the system changes from a magnetic insulator to a non-magnetic one with the increase of the ionic radius of the A-site element. Bi_2Ir_207 has different ionic radius from yttrium and does not contain rare earth elements with magnetic moments and f-orbital electrons. It is helpful to distinguish the effects of rare earth elements and iridium on the physical properties of materials. Then we studied the crystal structure, electrical transport properties, magnetic properties and thermodynamic properties of pyrochlore-type iridium compounds with different A-sites A=Y, Eu and Bi. In addition, we took polycrystalline Bi2Ir207 as the research object, and doped the Bi and Ir sites with Ca and Mn ions respectively to study their spin orbits. The physical properties such as channel coupling, frustration effect and electrical and magnetic properties are introduced in detail. The main achievements are as follows: The crystal structure, magnetism, electrical transport and specific heat of polycrystalline samples with three different A-site elements are compared. It is found that Y2Ir2O7 and Eu2Ir2O7 have strong magnetoresistance frustration and long-range magnetic ordering, while Bi2Ir2O7 exhibits strong magnetoresistance frustration and long-range magnetic ordering. Weak magnetoresistance frustration and short-range ordering are observed. The change of ion radius at position A not only affects the conduction bandwidth, but also the interaction and energy exchange between Ir and Ir, thus affecting the electrical transport properties of the materials and the magnetic properties of the ground state. A series of Ca and Mn doped Bi_2Ir_2O_7 polycrystalline samples have been successfully obtained. The results show that Bi_2Ir_2O_7 exhibits strong magnetic instability and metal conductivity. Metal-insulator transition occurs when Bi ions are replaced by ions, and the transition temperature Tc increases with the increase of Ca ions doping. The magnetic phase transition corresponding to the metal-insulator transition has not been found in the magnetic study. Therefore, we believe that the metal-insulator transition occurs here is due to the introduction of Ca ions with smaller ion radius. In addition, Ca ion doping reduces the bond angle of Ir-O-Ir and the electronic hybridization of Ir-5d and Bi-6p orbitals, thereby improving the antiferromagnetism of the sample and affecting the original magnetoresistive frustration structure of the system. There are also metal insulator transitions at different temperatures. We think there are two main reasons for the metal insulator transitions: on the one hand, disorder is introduced by Mn ion doping, on the other hand, the ionic radius of the 3D element is smaller, the bandwidth is narrower, the shielding effect of the electron is smaller, and the electricity is lower. The Coulomb interaction between the substitutes is large, so the bond angle of Ir-O-Ir decreases when Mn ion is used instead of Ir ion. Mn ion doping does not induce magnetic phase transition, but the analysis of Curie fitting results shows that the antiferromagnetism of the system increases with the increase of the doping amount. The resistance of the doped system changes with the applied magnetic field and the parent. On the contrary, the resistance decreases with the increase of applied magnetic field, which is related to the interaction between Mn ions and Ir ions.
【学位授予单位】：安徽工程大学
【学位级别】：硕士
【学位授予年份】：2016
【分类号】：O611.3
本文编号：2217387
[Abstract]:In recent years, it has been found that spin-orbit coupling will lead to a new form of matter: topological insulators, which have become a hot spot in the study of strong spin-orbit coupling. Pyrochlore-type iridium oxides (A2Ir207, in which A is Y, Bi or Lanthanide) belong to the face-centered cubic system and the space point group is Fd-3m. It is found that the system changes from a magnetic insulator to a non-magnetic one with the increase of the ionic radius of the A-site element. Bi_2Ir_207 has different ionic radius from yttrium and does not contain rare earth elements with magnetic moments and f-orbital electrons. It is helpful to distinguish the effects of rare earth elements and iridium on the physical properties of materials. Then we studied the crystal structure, electrical transport properties, magnetic properties and thermodynamic properties of pyrochlore-type iridium compounds with different A-sites A=Y, Eu and Bi. In addition, we took polycrystalline Bi2Ir207 as the research object, and doped the Bi and Ir sites with Ca and Mn ions respectively to study their spin orbits. The physical properties such as channel coupling, frustration effect and electrical and magnetic properties are introduced in detail. The main achievements are as follows: The crystal structure, magnetism, electrical transport and specific heat of polycrystalline samples with three different A-site elements are compared. It is found that Y2Ir2O7 and Eu2Ir2O7 have strong magnetoresistance frustration and long-range magnetic ordering, while Bi2Ir2O7 exhibits strong magnetoresistance frustration and long-range magnetic ordering. Weak magnetoresistance frustration and short-range ordering are observed. The change of ion radius at position A not only affects the conduction bandwidth, but also the interaction and energy exchange between Ir and Ir, thus affecting the electrical transport properties of the materials and the magnetic properties of the ground state. A series of Ca and Mn doped Bi_2Ir_2O_7 polycrystalline samples have been successfully obtained. The results show that Bi_2Ir_2O_7 exhibits strong magnetic instability and metal conductivity. Metal-insulator transition occurs when Bi ions are replaced by ions, and the transition temperature Tc increases with the increase of Ca ions doping. The magnetic phase transition corresponding to the metal-insulator transition has not been found in the magnetic study. Therefore, we believe that the metal-insulator transition occurs here is due to the introduction of Ca ions with smaller ion radius. In addition, Ca ion doping reduces the bond angle of Ir-O-Ir and the electronic hybridization of Ir-5d and Bi-6p orbitals, thereby improving the antiferromagnetism of the sample and affecting the original magnetoresistive frustration structure of the system. There are also metal insulator transitions at different temperatures. We think there are two main reasons for the metal insulator transitions: on the one hand, disorder is introduced by Mn ion doping, on the other hand, the ionic radius of the 3D element is smaller, the bandwidth is narrower, the shielding effect of the electron is smaller, and the electricity is lower. The Coulomb interaction between the substitutes is large, so the bond angle of Ir-O-Ir decreases when Mn ion is used instead of Ir ion. Mn ion doping does not induce magnetic phase transition, but the analysis of Curie fitting results shows that the antiferromagnetism of the system increases with the increase of the doping amount. The resistance of the doped system changes with the applied magnetic field and the parent. On the contrary, the resistance decreases with the increase of applied magnetic field, which is related to the interaction between Mn ions and Ir ions.
【学位授予单位】：安徽工程大学
【学位级别】：硕士
【学位授予年份】：2016
【分类号】：O611.3
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