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1、烟台大学硕士学位论文磁性纳米结构的量子调控姓名：徐成福申请学位级别：硕士专业：无线电物理指导教师：姜明20100401I摘 要 纳米自旋电子学是目前凝聚态物理和材料科学中的重要活跃领域。磁性纳米结构具有许多新奇的特性，而对其磁性的调控已经成为当前研究的热点。 本论文简要回顾了纳米磁性材料的研究背景和纳米结构磁性调控的研究现状；详细地介绍了本论文工作中涉及到的电子结构理论和计算方法基于密度泛函理论的离散变分方法和基于离散变分方法的ADF。 我们采用第一性原理计算系统研究了棒型C4v对称Fe（Co）-GaN纳晶和方型Oh对称Fe（Co）-GaN纳晶的特性与调控，详细考察了纳晶磁结构与对称性和表面原
2、子吸附构型间的关系。 我们以112棒型纳晶团簇Ga22N23为蓝本，采用过渡金属离子（Fe或Co离子）代替Ga离子来完成掺杂，掺杂浓度为9.1。保持C4v对称性，同时改变表面H原子的吸附情况来考察体系的特性和磁性调控。 我们的计算结果表明，(1) Ga20Fe2N23和Ga20Fe2N23H4(N位)团簇均有两个稳态，基态总磁矩为5B，亚稳态总磁矩为1B；与此相反，Ga20Fe2N23H2团簇的基态总磁矩为1B处，亚稳态总磁矩为5B。Ga20Fe2N23H4(Ga位)团簇有三个稳态，基态总磁矩为0B，亚稳态总磁矩为3B， 次亚稳态总磁矩为5B；而Ga20Fe2N23H8团簇只有一个稳态，总磁矩
3、为1B。 (2) Ga20Co2N23团簇有两个稳态，基态总磁矩为3B，亚稳态总磁矩为0B；而Ga20Co2N23H8团簇的基态总磁矩为1B，亚稳态总磁矩为3B。Ga20Co2N23H2和Ga20Co2N23H4(Ga位)团簇均只有一个稳态，总磁矩为3B；Ga20Co2N23H4(N位)纳晶团簇则有三个稳态，基态总磁矩为1B，亚稳态总磁矩为3B，次亚稳态总磁矩为5B。通过电子结构分析，我们发现了铁磁，反铁磁，亚铁磁，类自旋密度波型等多种纳晶磁结构。 方型Oh对称TM-GaN纳晶掺杂浓度为7.7，计算结果表明，(1) Ga12FeN14H32团簇具有总磁矩为4B的亚铁磁基态；增加H原子覆盖度后，
4、Ga12FeN14H48团簇有两个稳态：总磁矩为4B铁磁基态和总磁矩为2B亚铁磁亚稳态。(2) Ga12CoN14H32团簇的铁磁基态总磁矩为5B；Ga12CoN14H48团簇的亚铁磁基态总磁矩为2B。 关键词：密度泛函理论，磁性纳晶团簇，电子结构，量子调控 IIAbstract Nanospintronics is one very active field in the present condensed matter physics and material science. Magnetic nanostructures have many novel features, while
5、the studies of manipulating their magnetism have become very hot recently. In this thesis we have briefly reviewed the research background of magnetic nanomaterials and the manipulations. In detail we have introduced the electronic structure theories and calculation methods involved in this thesis -
6、 the Discrete Variational Method (DVM) based on the Density Functional Theory(DFT) and ADF based on DVM. Using first-principles calculations we systematically study the characteristics and manipulations of rod-type Fe(Co)-GaN nanocrystals with C4vsymmetry and cubic Fe(Co)-GaN ones with Ohsymmetry. T
7、he relations among magnetic structures in these nanocrystals, point symmetry and surface atom adsorption are investigated in detail. Based on 1 1 2 rod-type nanocrystalline cluster Ga22N23, we replace the Ga ions with TM ions (Fe or Co ions), the doping concentration is about 9.1%. Holding C4vsymmet
8、ry, we change the number and sites of the adsorbed H atoms to achieve manipulations. Our results show that, (1) Ga20Fe2N23and Ga20Fe2N23H4(N site) clusters have two stable states, the ground state with the total magnetic moment (TMM)of 5 B, the metastable state with the TMM of 1 B; Contrarily, Ga20F
9、e2N23H2has the ground state with the TMM of 1 Band the metastable state with the TMM of 5 B. Ga20Fe2N23H4(Ga site) has three stable states, the ground state with 0 B,the metastable state with 3B, the second metastable state with 5 B; while Ga20Fe2N23H8has only one stable state, the TMM is 1 B. (2) G
10、a20Co2N23cluster has one ground state with 3 Band one metastable state with 0 B; while Ga20Co2N23H8has the ground state with 1 Band the metastable state with 3 B. Ga20Co2N23H2and Ga20Co2N23H4(Ga site) clusters have only one ground state, the TMM is 3 B; Ga20Co2N23H4(N site) cluster has three stable
11、states, the ground state with 1 B, the metastable state with 3 B, the second metastable state with 5 B. A variety of magnetic structures including ferro, antiferro, ferri and spin density wave are found in these nanocrystals by electronic structure analysis. The doping concentration of cubic Ohsymme
12、tric TM-GaN nanocrystals is about 7.7%. Our calculated results show that, (1) Ga12FeN14H32cluster has the ferrimagnetic ground state with the TMM of 4 B; while Ga12FeN14H48 cluster has two stable states, the ferromagnetic ground state with the TMM of 4 Band the ferrimagnetic metastable state with th
13、e TMM of 2 B. (2) Ga12CoN14H32cluster has the ferromagnetic ground state with the TMM of 5 B; Ga12CoN14H48cluster has the ferrimagnetic ground state with the TMM of 2 B. Keywords: Density functional theory, Magnetic nanocrystals, Electronic structures, Quantum manipulations 烟 台 大 学 硕 士 学 位 论 文 1第一章
14、绪 论 1.1 纳米自旋电子学和纳米磁性半导体材料 目前，分子电子学（Molecular electronics）和自旋电子学（Spin electronics或Spintronics）是凝聚态物理中两个非常活跃的分支。分子电子学涉及分子中跨越不同分立能级的电荷输运，而这些分子的能级谱可以通过外加电场调控。自旋电子学涉及到通过电子的自旋自由度来调控电子输运。到目前为止这两个领域是独立地发展的，但一些研究者已经着手要将它们结合起来研究。纳米自旋电子学（Nano-spintronics）研究分子或纳米粒子中的量子输运同与自旋相关的现象之间的相互关系。目前自旋阀（Spin valve）已经是自旋电子
15、学中的一个基本器件，而如何利用纳米粒子构建的自旋场效应晶体管（Spin field effect transistor）以及如何调控单电子自旋（Manipulation of a single electronic spin）就成了纳米自旋电子学的前沿课题1。 稀磁半导体（Diluted Magnetic Semiconductor，DMS）同样是目前凝聚态物理研究中的一个热点2, 3。通常稀磁半导体的制备是采用少量3d过渡族元素（Mn，Fe，Ni，Co，Cr等）掺入到半导体材料中而产生铁磁性，但不过多地影响其半导体特性。由于稀磁半导体材料中的基质半导体与磁性杂质原子中电子之间或者不同磁性杂质
16、原子的电子之间的相互转移及相互作用而使得这类材料具有一些新奇的特性。程功等人4曾对近几年来稀磁半导体在纳米材料领域的研究进展作过综述。 -族化合物（如：GaAs、InAs、GaN、InN、AlP 等）半导体材料在光电子器件中已得到广泛的应用。GaN基稀磁半导体的研究也有多方面的报道。2005年，Heon-Jin Choi等人5报道了单晶稀磁半导体纳米线Ga1-xMnxN( x = 0.01-0.09)，这些纳米线直径为10-100 nm，长度长达几十个微米，呈现铁磁性（TC300K），且整根纳米线磁性粒子分布均匀，具有P型导电性，从而支持铁磁性源于空穴的理论。接着，Q. Wang等人6利用第一性原理计算预言了掺杂Cr的GaN纳米线的铁磁性不依赖于Cr原子占据的位置，每一个Cr原子的磁矩为2.5B，磁化轴垂直于线的中心轴，但各向异性能很小。 目前，稀磁半导体铁磁性的起源尚没有统一的解释。2007年，H. Katayama-Yoshida等人7
