【作者】 林秋宝；

【导师】 朱梓忠；

【作者基本信息】 厦门大学 ， 理论物理， 2008， 博士

【摘要】 磁性材料的研究一直是很重要的一个领域,特别是材料在纳米尺度下的磁性,它们在电子学领域有非常重要的应用。纳米磁性在未来半导体材料,高集成化元件中都有重要应用,使它成为目前的一个非常活跃的研究领域。在同一种材料上同时使用电子的电荷和自旋的所谓自旋电子学也成为一个引人注目的研究方向。理论上,对磁性和结构的关系的研究可以揭示一些神奇的基本物理原理和发现一些重要的潜在应用。密度泛函理论的发展就为物质电子结构性质的理论研究打开了一个广阔的天地,提供了关键的方法步骤。本博士论文的研究工作应用基于密度泛函理论的第一原理方法,并在计算中引入轨道-自旋相互作用进行极化非共线计算,系统地对W的团簇（零维）、W的原子链（一维）、W的原子薄片（二维）以及过渡金属掺杂的稀磁半导体的结构稳定性、电子结构性质及磁性进行了计算,获得了一些有价值的信息,得到了一些重要的结果:1)对零维的W_n团簇（n=3～27）的结构性质进行了研究。通过对结构的优化计算,得到了W_n团簇（n=3～7）的最低能量结构和（n=8～27）的局域能量极小的典型结构。使用凝胶模型提出了团簇的电子组态,并很好地解释了幻数团簇的电子数和结构稳定性;得到并分析了团簇的HOMO-LUMO能隙,团簇的结合能、能量的一阶和二阶差分随原子数n变化;得到了团簇中原子的成键特点随团簇原子数的变化特征,指出了W团簇随着原子数的增加很快便出现金属性等性质。2)系统地研究了W的一维原子链的磁性。我们计算了一维线性链的铁磁、反铁磁和螺旋磁性（自旋波）三种结构的稳定性和磁性的变化。研究中,分别作了考虑轨道自旋相互作用和不考虑轨道自旋相互作用两种情况的计算。结果显示,稳定的一维W线性原子链具有磁性,而且反铁磁链的相对稳定性高。原子间距很近时没有磁矩,而随着原子间距的增加,原子磁矩在一个很小的变化范围内跃升,最终随着原子间距的增大趋于单原子磁矩。轨道磁矩在有近邻原子作用时出现,且极化方向与自旋极化方向相反。稳定的W一维反铁磁链具有磁性且具有金属性,磁矩主要是d-电子的贡献。随着原子链的原子间距的增加,逐渐从金属性向半导体性变化,电子的局域程度增加,原子链上原子的磁矩变大,同时,s电子对磁矩的贡献增大。3)对二维的W原子面的结构与磁性进行了计算。对于平面结构,进行了从斜方、六角、长方、到正方结构的计算,不同结构下也进行了不同的晶格常数下性质的计算。计算的磁性结构包含铁磁,反铁磁以及部分近邻反铁磁性结构。我们对不同结构的稳定性、磁性、电荷密度、电子态密度以及磁密度进行了比较。结果发现,不管是铁磁,反铁磁以及部分近邻反铁磁性的W平面结构,近六角（由于Jahn-Teller效应使其相对于六角结构产生了一些畸变）的平面结构最稳定;反铁磁的结构比铁磁的结构稳定;铁磁结构的磁性在某些结构上出现奇异;而稳定的W平面结构没有磁性或磁性很弱;其中正方结构的反铁磁稳定结构表现出弱磁性,其输运电子也表现出弱极化;稳定的W原子薄片是金属性的。在晶格被拉伸的情况下W原子面出现磁性,反铁磁结构比铁磁结构更早出现磁性,正方结构比斜方结构更快出现磁性。此外,还比较了W原子面在六角结构和正方结构下的电荷密度、电子态密度和磁密度,讨论了它们的电子结构性质,并对自旋轨道作用和非共线耦合的作用进行了讨论。4)对3d过渡金属（TM=V、Cr、Mn、Fe、Co和Ni）掺杂的Ⅲ-Ⅴ半导体（GaAs和GaP）的电磁性质进行了第一原理的计算。结果发现:V、Cr和Mn掺杂的Ⅲ-Ⅴ将出现铁磁状态（FM）,Fe掺杂的Ⅲ-Ⅴ将出现反铁磁状态（AFM）,而Co和Ni掺杂时,其磁性不稳定。其中Cr掺杂的Ⅲ-Ⅴ将可能是具有较高居里温度Tc的稀磁半导体（DMS）。在TM掺杂的Ⅲ-ⅤDMS中,当TM-3d自旋朝上的t_2g轨道全空时,TM离子的磁矩将大于理论期待值;当TM-3d自旋朝上的t_2g轨道全满时,TM离子的磁矩将小于理论期待值:当TM-3d自旋朝上的t_2g轨道部分填充时,TM离子的磁矩与期待值的差距取决于晶体的对称性和磁性离子的能带情况。Si与Mn对Ⅲ-ⅤDMS进行共掺杂时,将有利于DMS出现FM状态,并使得TC提高。更多还原

【Abstract】 The studies on the magnetism of materials,especially in the nano-structured scale,have been an important field,because of their very important applications in electronics and other realm of science.The nano-magnetism has become a stirring region of study because of their potential important applications in future semiconductors and highly integrated devices.Also,the spintronics,with the study of spontaneously employing the charge and spin of an electron in some bulk semiconductor materials,has become a very attractive research realm.Theoretically, the investigation of the relationship between the structure and the magnetism can also reveal some basic physical principles and discover important potential applications. The development of density functional theory（DFT） has opened a wide realm for the studies of electronic structures,and provided a pivotal scheme.In this dissertation,by using the first-principles calculations based on the density functional theory and by introducing the non-collinear calculations with the spin-orbital coupling,the structural stabilities,electronic structures and magnetic properties of zero-dimensional W clusters,one-dimensional W atomic chain,two-dimensional W atomic sheet and some TM-doped semiconductor materials have been systematically studied.As a result, some valuable information has been obtained and some important results have been achieved,i.e.:（1） The structural properties of W_n clusters（n = 3～27） has been studied.The most stable structures of clusters（n = 3～7） with global energy minimum and typical structures of clusters（n = 8～27） with local energy minimum are determined by the ab initio calculations of optimizing the structures.Based on the jellium model,the electronic configuration is proposed which can explain well the electronic magic numbers and the relative stabilities of W clusters.The binding energies,the first and second differences of binding energies and the HOMO-LUMO gaps versus the number of atoms in the clusters are obtained and analyzed.The character of the variety of the properties of bonding between the atoms versus the number of atoms in the cluster has been obtained,indicating that W clusters become metallic very quickly with the increase of cluster size.（2） The magnetism of free standing W atomic chains is studied.The calculations have been performed on the diversification of structural stability and the magnetism of ferromagnetic,anti-ferromagnetic and spiral polarized（spin wave） states.The chains with and without the spin-orbit（L-S） coupling are taken into account, respectively.It is shown that the stable W atomic chains are magnetic,and the anti-ferromagnetic is shown to be the most stable one.There is no magnetism in the chains when the atomic distance is small,however,the magnetic moment increases rapidly within a small region of atomic distance when the distance is larger than a certain value,and then approaches the value of a single atom.The orbital magnetic moment emerges when the inter-atomic exchange interaction emerges,and the orbital polarized direction is opposite to the spin polarized one.The stable anti-ferromagnetic W chain is metallic besides magnetic,and the magnetic moment is primarily contributed by d-electrons.When the atomic distance increases,the chain transforms gradually from metal to semiconductor,the atomic magnetic moment increases and the electrons are more localized,in the meantime,the s-electrons contributes more to the magnetism of the chain.（3） The structural and magnetic properties of two-dimensional W atomic sheet are studied.The calculations are performed for the plain structures with rhombic, hexagonal,rectangle and square and with variant lattice constants.The calculated magnetic structures include ferromagnetism,anti-ferromagnetism and partial anti-ferromagnetism.The structural stability,magnetism and electronic structure properties of different structures are compared.The main results are:the near-hexagonal structure（a structure with small distortion from hexagonal structure due to the John-Teller effect） is the most stable one in all the plain structures;the anti-ferromagnetism is more stable than ferromagnetism;the magnetism of some ferromagnetism structure appears oddity;the stable atomic sheets are nonmagnetic or with week magnetism;the stable square-structure appears week magnetism,and the transportation electrons also appears dilute polarization;the stable W atomic sheets are metallic.The magnetism appears in the sheets when the lattice constant is elongated,moreover,the magnetism appears more early in anti-ferromagnetism structures and more quickly in the square structure.The charge densities,the electronic density of states,and the magnetization density of hexagonal and square structures are also compared.The electronic structure properties and the effect of spin-orbit non-collinear coupling are discussed.（4） The magnetic properties ofⅢ-Ⅴsemiconductor（GaAs,GaP） doped by 3d-TM（TM=V,Cr,Mn,Fe,Co and Ni） are studied.It is shown that the ferromagnetic state（FM） will be realized in V,Cr and Mn doped GaAs and GaP;the antiferromagnetic（AFM） state is favored when doped by Fe,whereas,the materials show unstable magnetism when doped by Co and Ni.TheⅢ-Ⅴsemiconductor doped by Cr is a candidate with high Curie temperature（T_C）.Furthermore,it shows that the magnetic moment of TM is larger than theoretical expected value when the spin-up t_2g-orbitals of TM are empty,whereas,the magnetic moment of TM become smaller than expected value when the spin-up t_2g-orbitals are fully occupied.When the spin-up t_2g-orbitals are partially occupied,the difference between TM’s magnetic moment and expected value leans heavily on the crystal symmetry and the band structures of the magnetic ions.Finally,GaAs and GaP co-doped by Si and Mn are studied,it shows that co-doping will show better stability of FM state and higher T_C.更多还原