三角晶格反铁磁CuFeO2的磁性和电子结构

基于广义梯度近似(GGA)的密度泛函理论(DFT), 通过构造铁磁(FM), 阻挫的三角非共线反铁磁(FAFM)、上上下下型共线反铁磁(↑↑↓↓AFM)三种不同磁性构型, 从非共线磁性结构计算出发, 优化了低温铜铁矿CuFeO2晶体材料的几何结构, 研究了磁性结构对电子结构、能隙和磁矩等的作用. 计算发现上上下下型反铁磁自旋排列能促进能隙形成, 总能降低, 磁矩增大. 由于上上下下型反铁磁与阻挫三角非共线反铁磁相能量接近, 外场的作用容易导致磁性结构相变到阻挫的三角反铁磁态, 其电子态密度分布与X光发射光谱测得的结果一致, 即具有高自旋的Fe离子3d电子自旋向上的子带中心位于Cu 3d能态之下, O 2p能态以上, 而且配位场理论分析表明Fe离子3d态自旋向下的空轨道为铁电极化提供了有利的化学环境.     

六角多铁性HoMnO3的电子和能带结构

基于密度泛函理论(DFT)结合投影缀加平面波(PAW)方法, 运用广义梯度近似(GGA), 在考虑电子基态自旋阻挫非共线的磁性结构基础上, 研究了具有六角钙钛矿结构HoMnO₃材料的磁性、电子和能带结构, 并解释了相关实验结果. 结果表明: 当考虑Mn³⁺离子的电子自旋在平面内呈阻挫的三角非共线反铁磁(NAFM)排列时, 六角HoMnO₃的总能降低、能隙变大、磁矩增大、各原子的位置更接近于实验值, 电子态密度(DOS)分布具有与X光吸收谱测量更为一致的结果. 对非共线磁性结构计算得到的电子态密度和能带结构的分析发现, 实验中观察到的1.7和2.3 eV两个光学吸收峰都源于Mn³⁺离子3d与平面内O(3, 4) 2p形成的杂化态与Mn [3d_3z²-r²]之间的电子跃迁, 而Ho 5d空轨道与平面上的O(3, 4) 2p轨道之间在z方向的强烈杂化驱动HoMnO₃产生垂直于平面方向的铁电极化.     

Fe/Cr超晶格的电子结构和磁性质

应用密度泛函全势线性缀加平面波(FLAPW)方法研究了Fem/Crn (m=3, 4; n=1, 3, 4)超晶格的电子结构和磁性质. 结果表明, Fe3/Cr1和Fe3/Cr3体系的基态中, Fe层间存在铁磁耦合; 而Fe4/Cr4体系基态中, 存在反铁磁耦合; Cr层的磁矩方向交替变化, 交界面上的Fe和Cr间存在反铁磁耦合.     

La0.5Ba0.5CoO3中Y的替代效应

用固相反应法制备了La0.5Ba0.5CoO3多晶材料，系统研究了Y的替代对材料磁性和输运特性的影响，结果发现，Y的掺入主要产生了两种效应，一是Y向Co的3d轨道产生了电荷转移，使分子磁矩下降，二是出现了Co离子的反铁磁交换作用，当Y含量少于或等于0.3时，材料中出现了自旋的非共线结构。当Y含量大于0.3时，材料从铁磁态为主转变为反铁磁态为主，对不同Y含量的材料，其导电机制都属于极化子的变程跳跃导电，随Y含量增加，材料电经迅速增大。     

Fe原子吸附对单层WS_2结构和性质的影响

本文采用基于密度泛函理论(DFT)的第一性原理方法研究了Fe原子吸附对单层WS_2结构和性质的影响。研究结果表明:Fe原子吸附在W原子的顶位最稳定,相应的原子吸附能为1.84 eV。Fe与衬底间的相互作用削弱了紧邻W―S键,使其键长增大0.011 nm。由于衬底原子的影响,Fe原子d轨道的电子重新分布,形成了2μB左右的局域原子磁矩。在低覆盖度下(0.125和0.25 ML),磁性作用以超交换作用为主,铁磁序不稳定。而在高覆盖度下(0.5和1.0 ML),Fe原子间距减小,磁性作用以RKKY作用为主,铁磁序稳定。电子结构的计算结果显示,在高覆盖度下,Fe/WS_2结构在费米能级处的电子自旋极化率等于100%。自旋向上与向下通道分别为间接带隙的半导体和金属。在1.0 ML覆盖度下,自旋向上的禁带宽度约为0.94 eV。这说明Fe原子吸附可以将直接带隙的WS_2半导体转变成半金属,形成一种潜在的自旋电子器件材料。     

NiO表面磁性对分子吸附的影响的第一性原理研究

过渡金属氧化物广泛应用在当今能源与环境相关的催化领域,理解其表面化学性质以及结构-反应活性之间的关系对于先进催化材料的进一步发展以至理性设计至关重要.3d后过渡系金属(Mn,Fe,Co,Ni)的氧化物以其中金属离子独特的自旋状态和由此产生的铁磁/反铁磁性为典型特征.研究过渡金属氧化物的自旋状态以及磁性对表面化学的影响将使我们更加完整了解这些材料的表面化学.以NiO为代表的后过渡系金属岩盐结构一元氧化物具有反铁磁性,被经常作为反铁磁研究的模型体系.尽管在低温(低于其Neel温度)下NiO体相的完整晶体具有确定的反铁磁序,但是一系列最新研究表明,在条件变化时NiO表面的Ni离子可以产生不同的磁序.以此为背景,本工作以NiO为模型体系,采用DFT+U的第一性原理方法研究了NiO表面磁序对表面的小分子吸附活性的影响,包括表面吸附活性对各磁性相的表面取向以及吸附物种磁性的依赖关系.我们考察了NiO的5种反铁磁相和一种铁磁相,两个晶面NiO(001)和NiO(011),顺磁性分子NO和非顺磁性分子CO.我们发现表面能受磁性的影响较轻微,NiO(001)面上从49到54 meV/?2,NiO(011)面上从162到172 meV/?2.在NiO(001)面上,CO与NO都倾向于在Ni离子的顶位吸附.对于不同的体相磁序与表面取向,CO吸附能的变化范围为-0.33~-0.37 eV,NO吸附能的变化范围为-0.42~-0.46 eV.在NiO(011)表面,两种分子都倾向于吸附在由两个Ni离子构成的桥位.我们发现相对于NiO不同磁性相的体相长程磁序,吸附位点处构成桥位的两个Ni离子的局部磁矩相对取向对于分子的吸附具有更加显著的影响.计算得到NO在局部磁矩相对取向反平行(↑↓)吸附位点处的吸附能为-0.99~-1.05 eV,在局部磁矩相对取向平行(↑↑)吸附位点处吸附会增强,吸附能为-1.21~-1.30 eV.对于CO,尽管计算的吸附能在(↑↓)吸附位点(-0.73~-0.75 eV)与在(↑↑)吸附位点(-0.71~-0.72 eV)非常接近,两种吸附位点处的CO吸附时分子轨道杂化方式以及吸附后CO的局域电子态密度却具有明显不同的特征.本工作突出揭示了分子在过渡金属氧化物表面的多重吸附位点上吸附时吸附位点的局域磁矩相对取向对吸附性能的影响.     

NiO表面磁性对分子吸附的影响的第一性原理研究（英文）

过渡金属氧化物广泛应用在当今能源与环境相关的催化领域,理解其表面化学性质以及结构-反应活性之间的关系对于先进催化材料的进一步发展以至理性设计至关重要.3d后过渡系金属(Mn,Fe,Co,Ni)的氧化物以其中金属离子独特的自旋状态和由此产生的铁磁/反铁磁性为典型特征.研究过渡金属氧化物的自旋状态以及磁性对表面化学的影响将使我们更加完整了解这些材料的表面化学.以NiO为代表的后过渡系金属岩盐结构一元氧化物具有反铁磁性,被经常作为反铁磁研究的模型体系.尽管在低温(低于其Neel温度)下NiO体相的完整晶体具有确定的反铁磁序,但是一系列最新研究表明,在条件变化时NiO表面的Ni离子可以产生不同的磁序.以此为背景,本工作以NiO为模型体系,采用DFT+U的第一性原理方法研究了NiO表面磁序对表面的小分子吸附活性的影响,包括表面吸附活性对各磁性相的表面取向以及吸附物种磁性的依赖关系.我们考察了NiO的5种反铁磁相和一种铁磁相,两个晶面NiO(001)和NiO(011),顺磁性分子NO和非顺磁性分子CO.我们发现表面能受磁性的影响较轻微,NiO(001)面上从49到54 meV/~2,NiO(011)面上从162到172 meV/~2.在NiO(001)面上,CO与NO都倾向于在Ni离子的顶位吸附.对于不同的体相磁序与表面取向,CO吸附能的变化范围为-0.33~-0.37 eV,NO吸附能的变化范围为-0.42~-0.46 eV.在NiO(011)表面,两种分子都倾向于吸附在由两个Ni离子构成的桥位.我们发现相对于NiO不同磁性相的体相长程磁序,吸附位点处构成桥位的两个Ni离子的局部磁矩相对取向对于分子的吸附具有更加显著的影响.计算得到NO在局部磁矩相对取向反平行(↑↓)吸附位点处的吸附能为-0.99~-1.05 eV,在局部磁矩相对取向平行(↑↑)吸附位点处吸附会增强,吸附能为-1.21~-1.30 eV.对于CO,尽管计算的吸附能在(↑↓)吸附位点(-0.73~-0.75 eV)与在(↑↑)吸附位点(-0.71~-0.72 eV)非常接近,两种吸附位点处的CO吸附时分子轨道杂化方式以及吸附后CO的局域电子态密度却具有明显不同的特征.本工作突出揭示了分子在过渡金属氧化物表面的多重吸附位点上吸附时吸附位点的局域磁矩相对取向对吸附性能的影响.     

过渡元素掺杂Fe_3O_4(001)表面磁电性能的理论研究

采用基于密度泛函理论的第一性原理方法,计算Fe_3O_4,Fe_3O_4(001)表面以及过渡元素掺杂表面的电子结构和磁性。结果表明Fe_3O_4的半金属性主要来源于B位Fe离子,并且Fe的3d轨道发生强烈自旋极化;比较(001)表面不同终端A和B终端的表面能和电子结构,得出两种终端稳定性存在差异且A终端较稳定同时表现半金属性;由过渡元素V、Cr、Mn、Co、Cu和Zn取代Fe_3O_4(001)表面A终端A位Fe进行掺杂,形成的6种新表面结构都保持了半金属性。对比它们的表面能和磁矩,Mn掺杂的表面结构最稳定并且磁矩明显增大。     

多铁性铬氧化物YCrO3体系的磁特性及比热研究

系统研究了低温条件下铬氧化物YCrO3体系的磁特性及比热特点.实验结果表明,在高温区(T＞140 K)直流磁化率随温度的变化遵从居里-外斯定律,YCrO3体系处于顺磁状态,样品的有效平均磁矩μefr=3.99μB、顺磁居里-外斯温度TCw =-340 K,即体系具有反铁磁性.在TN =140 K附近,YCrO3体系经历了由顺磁态向倾角反铁磁态的相转变,其自旋磁结构一直保持为Г4(Gx,Ay,Fz;FRz);低温条件下(T＜140 K),体系具有完整闭合的磁滞回线,表明YCrO3样品具有反铁磁性的同时,具有明显的弱铁磁性,且随温度的降低铁磁性逐渐增强.对应于磁相变温度TN,比热曲线在140 K附近出现尖锐的λ形反常突起,由于Y离子没有磁性,显示了Cr3+磁矩亚晶格的顺磁-反铁磁相变对比热的贡献.     

TinO2和TinO2-(n=1-10)团簇的几何结构、电子和磁性质

基于密度泛函理论(DFT)的B3LYP方法,研究了TinO2和TinO2-(n=1-10)团簇的几何结构、电子结构以及磁性.结果表明,两个氧以分离的原子状态吸附在金属团簇的表面,呈现出以一个钛原子为中心的O-Ti-O的相邻吸附形式.中性团簇和阴离子团簇的能量最低结构相似.稳定性分析表明TinO2具有很高的稳定性,特别是TiO2和Ti7O2.此外,详细讨论了团簇的电离势、电子亲和能、电子解离能和能隙.基于最低能量结构,讨论了团簇的磁性,发现电荷从Ti原子向O原子转移,并且电荷转移主要发生在TinO2的Ti-3d、Ti-4s和O-2p轨道.磁性团簇中反铁磁序占据主导,磁矩主要来源Ti-3d电子的贡献,而两个氧原子的贡献非常小.     

钙钛矿型复合稀土铁氧化物电子结构的第一性原理研究

采用基于赝势平面波基组的第一性原理方法对一系列具有钙钛矿结构的复合稀土铁氧化物RFeO₃ (R＝镧系稀土元素)的构型和电子结构进行了系统研究. 考察了铁磁(FM)和反铁磁(AFM)两种磁结构, 其中AFM型在常温下更为稳定, 且其构型优化结果更接近于实验测量值. 不论是FM还是AFM构型, R原子的磁距均与自由R^3＋离子相同; 对于Fe原子, 在FM结构中的磁距较AFM类型来得小; 随着R原子序数的增加, 两种磁结构中Fe原子的磁距均呈减小趋势; 对于相同类型的RFeO₃化合物, 具有非常类似的能带结构, 其中AFM构型在费米能级处存在明显的带隙, 而FM型化合物则具有金属特性.     

α-SrMnO3电子结构的第一性原理研究

采用平面波赝势方法对钙钛矿型锰酸盐氧化合物α-SrMnO3的电子结构进行了第一性原理研究. 六方钙钛矿型结构α-SrMnO3化合物为磁性绝缘体, 磁基态对应于共面八面体及共顶点八面体间的磁性交换作用均为反铁磁性(AFM), 其禁带宽度为1.6 eV; 费米能级附近的Mn3d态与O2p态存在很强的杂化作用, 属于共价绝缘体, 这种强共价性使得Mn4+的自旋磁矩偏离理想值. 采用Noodleman的对称性破缺方法, 根据α-SrMnO3不同磁有序态的总能量拟合出α-SrMnO3中的自旋交换耦合常数. α-SrMnO3的局部微结构(Mn—O—Mn)决定了整个体系的特殊磁性交换作用. 共面及共顶点的八面体间均存在AFM交换作用, 并且共顶点八面体间的AFM作用比较强.     

First-principles comparative study of multiferroic compound PbVO3

We report a comprehensive first-principles investigation of the structural, electronic, magnetic and phase transition properties in multiferroic compound PbVO₃ with systematic comparisons of various exchange-correlation (XC) functionals. The antiferromagnetic (AFM) insulating ground state of tetragonal phase has been obtained in the framework of the band theory, which is characterized by C-type two-dimensional AFM magnetic ordering in the ab plane. A first-order structural transformation from tetragonal phase to idea cubic perovskite structure takes place at 1.75 GPa, corresponding to the ferroelectric to paraelectric phase transition. Electronic structure calculations suggest that the ground state of the cubic paraelectric phase is a nonmagnetic orbital-disorder metal.     

Gapped ferromagnetic graphene nanoribbons

We theoretically design a graphene-based all-organic ferromagnetic semiconductor by terminating zigzag graphene nanoribbons (ZGNRs) with organic magnets. A large spin-split gap with a 100% spin polarized density of states near the Fermi energy is obtained, which is of potential application in spin transistors. The interactions among electron, spin and lattice degrees of freedom are studied using the first-principles calculations including non-collinear spin orientations. All of the calculations consistently demonstrate that although no d electrons existing, the antiferromagnetic π-π exchange together with the strong electron-lattice interactions between organic magnets and ZGNRs make the ground state ferromagnetic.     

Comparative electronic band structure study of the intrachain ferromagnetic versus antiferromagnetic coupling in the magnetic oxides Ca3Co2O6 and Ca3FeRhO6

In the (MM'O6)infinity chains of the transition-metal magnetic oxides Ca3MM'O6 the MO6 trigonal prisms alternate with the M'O6 octahedra by sharing their triangular faces. In the (Co(2O6)infinity chains of Ca3Co2O6 (M = M' = Co) the spins are coupled ferromagnetically, but in the (FeRhO6)infinity chains of Ca3FeRhO6 (M = Fe, M' = Rh) they are coupled antiferromagnetically. The origin of this difference was probed by carrying out spin-polarized density functional theory electronic band structure calculations for ordered spin states of Ca3Co2O6 and Ca3FeRhO6. The spin state of a (MM'O6)infinity chain determines the occurrence of direct metal-metal bonding between the adjacent trigonal prism and octahedral site transition-metal atoms. The extent of direct metal-metal bonding in the (Co2O6)infinity chains of Ca3Co2O6 is stronger in the intrachain ferromagnetic state than in the intrachain antiferromagnetic state, so that the intrachain ferromagnetic state becomes more stable than the intrachain antiferromagnetic state. Such a metal-metal-bonding-induced ferromagnetism is expected to occur in magnetic insulators and magnetic metals of transition-metal elements in which direct metal-metal bonding can be enhanced by ferromagnetic ordering. In the (FeRhO6)infinity chains of Ca3FeRhO6 the ferromagnetic coupling does not lead to a strong metal-metal bonding and the adjacent spins interact by the Fe-O...O-Fe super-superexchange, hence leading to an antiferromagnetic coupling.     

Electronic and magnetic properties of manganese and iron-doped Ga(n)As(n) nanocages (n=7-12)

The electronic and magnetic properties of Mn- or Fe-doped Ga(n)As(n) (n=7-12) nanocages were studied using gradient-corrected density-functional theory considering doping at substitutional, endohedral, and exohedral sites. When doped with one atom, the most energetically favorable site gradually moves from surface (n=7-11) to interior (n=12) sites for the Mn atom, while the most preferred doping site of the Fe atom alternates between the surface (n=7,9,11) and interior (n=8,10,12) sites. All of the ground-state structures of Mn@Ga(n)As(n) have the atomlike magnetic moment of 5mu(B), while the total magnetic moments of the most stable Fe@Ga(n)As(n) cages for each size are about 2mu(B) except for the 4mu(B) magnetic moment of Fe@Ga(12)As(12). Charge transfer and hybridization between the 4s and 3d states of Mn or Fe and the 4s and 4p states of As were found. The antiferromagnetic (AFM) state of Mn(2)@Ga(n)As(n) is more energetically favorable than the ferromagnetic (FM) state. However, for Fe(2)@Ga(n)As(n) the FM state is more stable than the AFM state. The local magnetic moments of Mn and Fe atoms in the Ga(n)As(n) cages are about 4mu(B) and 3mu(B) in the FM and AFM states, respectively. For both Mn and Fe bidoping, the most energetically favorable doping sites of the transition metal atoms are located on the surface of the Ga(n)As(n) cages. The computed magnetic moments of the doped Fe and Mn atoms agree excellently with the theoretical and experimental values in the Fe(Mn)GaAs interface as well as (Ga, Mn)As dilute magnetic semiconductors.     

Theoretical study of CeO2 and Ce2O3 using a screened hybrid density functional

The predicted structures and electronic properties of CeO(2) and Ce(2)O(3) have been studied using conventional and hybrid density functional theory. The lattice constant and bulk modulus for CeO(2) from local (LSDA) functionals are in good agreement with experiment, while the lattice parameter from a generalized gradient approximation (GGA) is too long. This situation is reversed for Ce(2)O(3), where the LSDA lattice constant is much too short, while the GGA result is in reasonable agreement with experiment. Significantly, the screened hybrid HSE functional gives excellent agreement with experimental lattice constants for both CeO(2) and Ce(2)O(3). All methods give insulating ground states for CeO(2) with gaps for the 4f band lying between 1.7 eV (LSDA) and 3.3 eV (HSE) and 6-8 eV for the conduction band. For Ce(2)O(3) the local and GGA functionals predict a semimetallic ground state with small (0-0.3 eV) band gap but weak ferromagnetic coupling between the Ce(+3) centers. By contrast, the HSE functional gives an insulating ground state with a band gap of 3.2 eV and antiferromagnetic coupling. Overall, the hybrid HSE functional gives a consistent picture of both the structural and electronic properties of CeO(2) and Ce(2)O(3) while treating the 4f band consistently in both oxides.     

Comparative study of magnetic and electronic properties of room-temperature polar magnets ScFeO3 and InFeO3

We performed calculations based on collinear and non-collinear spin density functional theory to investigate magnetic and electronic properties of new room-temperature polar magnetic compounds AFeO₃ (A = Sc, In). Exchange and correlation effects were simulated by standard local spin density approximation and the generalized gradient approximation. We determined that the canted G-type antiferromagnetic (G-AFM) structure of the Fe spin moments is the most stable magnetic structure for both compounds, in accordance with experimental facts. The calculated Fe spin magnetic moments and resulting weak ferromagnetic components perpendicular to the c-axis also agree well with the experiment. The magnitude of the Fe spin moment does not depend on the fact if the G-AFM structure is collinear or canted. The energy difference between these two structures is found to be much larger for InFeO₃, so as the exchange interaction constant J_nn between nearest-neighbor magnetic Fe ions. The ScFeO₃ and InFeO₃ electronic structures, which reproduce well their magnetic properties are presented and discussed. By comparing them with the band structures of some typical multiferroics, we conclude that different electronic mechanisms should be responsible for their ferroelectric distortion. In ScFeO₃, the cause of this distortion should be attributed to the hybridization between almost empty Sc 3d and occupied O 2p states along the hexagonal c-axis. For InFeO₃, however, the stereochemical activity of the In 5s localized electrons should be the principal driving force for ferroelectric distortion.