Electronic structure and magnetic properties of the N monodoping and (Li, N)-codoped ZnO

Using first-principles calculations based on density functional theory, we investigated systematically the electronic structures and magnetic properties of N monodoping and (Li, N) codoping in ZnO. The results indicate that monodoping of N in ZnO favors a spin-polarized state with a magnetic moment of 0.95 μ_B per supercell and the magnetic moment mainly comes from the unpaired 2p electrons of N and O atoms. In addition, it was found that monodoping of N in ZnO is a weak ferromagnet and it is the spin-polarized O atoms that mediate the ferromagnetic exchange interaction between the two N atoms. Interestingly, by Li substitutional doping at the cation site (Li_Zn), the ferromagnetic stability can be increased significantly and the formation energy can be evidently reduced for the defective system. Therefore, we think that the enhancement of ferromagnetic stability should be attributed to the accessorial holes and the lower formation energy induced by Li_Zn doping.     

Theoretical study of the magnetic interaction of Cr-doped ZnO with and without vacancies

First-principles density-functional theory (DFT) calculations have been performed to study the magnetic properties of ZnO:Cr with and without vacancies. The results indicate that the doping of Cr in ZnO induces obvious spin polarization around the Fermi level and a total magnetic moment of 3.77μ_B. The ferromagnetism (FM) exchange interaction between Cr atoms is short-ranged and decreases with increasing Cr separation distance. It is suggested that the FM state is not stable with low concentration of Cr. The presence of O vacancies can make the half-metallic FM state of the system more stable, so that higher Curie temperature ferromagnetism may be expected. Nevertheless, Zn vacancies can result in the FM stability decreasing slightly. The calculated formation energy shows that V_Zn+Cr_Zn complex forms spontaneously under O-rich conditions. However, under Zn-rich conditions, the complex of V_O+Cr_Zn forms more easily. Thus, ZnO doped with Cr may exhibit a concentration of vacancies that influence the magnetic properties.     

Structure, electronic and magnetic properties of Cr-doped (ZnS)12 clusters: A first-principles study

We have studied the structural, electronic, and magnetic properties of (ZnS)₁₂ clusters doped with one (monodoped) and two (bidoped) Cr atoms in terms of a first-principles method. Substitutional, exohedral, and endohedral doping are considered. The substitutional isomer is found to be most favorable in energy for monodoped clusters, while the exohedral isomers are found to be most favorable for bidoped clusters. The magnetic coupling between the Cr atoms is mainly governed by the competition between direct Cr-Cr antiferromagnetic (AFM) interaction and the ferromagnetic (FM) interaction between two Cr atoms via S atom due to strong p-d hybridization. Finally, we show that the exohedral bidoped (ZnS)₁₂ clusters favor the FM state, which has potential applications in nanoscale quantum devices.     

Mn和N共掺ZnO稀磁半导体薄膜的研究

使用对Zn₂N₃:Mn薄膜热氧化的方法成功制备了高含N量的Mn和N共掺ZnO的稀磁半导体薄膜.在没有N离子共掺的情况下,ZnO:Mn薄膜的铁磁性非常微弱;如果进行N离子的共掺杂,就会发现ZnO:Mn薄膜在室温下表现出非常明显的铁磁性,饱和离子磁矩为0.23 μ_B—0.61 μ_B.这说明N的共掺激发了ZnO:Mn薄膜中的室温铁磁性,也就是受主的共掺引起的空穴有利于ZnO中二价Mn离子的铁磁性耦合,这和最近的相关理论研究符合很好. 关键词： 磁性半导体 受主掺杂 空穴媒介的铁磁性     

An alternative approach to investigate the origin of p-type conductivity in arsenic doped ZnO

P-type conductivity in MOCVD grown ZnO was obtained by directional thermal diffusion of arsenic from semi-insulating GaAs substrate. The films were single crystalline in nature and oriented along (002) direction. Ab initio calculations in the framework of density functional theory have been carried out with different chemical states of arsenic in ZnO. Present calculations suggested As_Zn–2V_Zn defect is a shallow acceptor and results in ferromagnetism in ZnO. The magnetic measurements of the samples indeed showed ferromagnetic ordering at room temperature. X-ray photoelectron spectra confirmed the presence of As_Zn and V_Zn. The core level chemical shift in binding energy of As_Zn indicated the formation of As_Zn–2V_Zn. Diffused arsenic substitutes zinc atom and creates additional zinc vacancies. The zinc vacancies, surrounding the oxygen atoms, result in unpaired O 2p electrons which in turn induce ferromagnetism in the samples.     

First-principles prediction of the electronic and magnetic properties of nitrogen-doped ZnO nanosheets

The electronic and magnetic properties of N-doped ZnO nanosheets are investigated by density functional theory using local spin density approximation. The results show that in an isolated N-doped ZnO nanosheet, there is a clear spontaneous polarization of N 2p state with a magnetic moment 1.0 μ_B/N. We also find that the doped nitrogen atoms in ZnO nanosheets have a clustering tendency with ferromagnetic coupling between them, and thus a high room-temperature ferromagnetic nature is expected. The ferromagnetic coupling in N-doped ZnO nanosheets can be attributed to the hole-mediated double-exchange mechanism through strong p–d interaction between nitrogen and zinc atoms.     

亚铁磁Heusler合金Mn2CoGa和Mn2CoAl掺杂Cr,Fe和Co的局域铁磁结构

通过实验和计算的方法研究了Mn₂CoM_xGa_1-x 和Mn₂CoM_xAl_1-x (M=Cr, Fe, Co)掺杂系列合金样品. 研究发现, 在共价作用的影响下, Fe和Co原子占A位, 使被取代的MnA (-2.1 μ_B)变成MnD (3.2 μ_B), 在最近邻的强交换作用下亚铁磁基体中形成了MnB-CoC-MnD局域铁磁性结构, 使分子磁矩的增量最高可达6.18 μ_B. Fe, Co 掺杂后建立同样的局域铁磁结构, 居里温度的变化趋势却不同. 实验观察到Mn₂Co_1+xAl_1-x中掺杂容忍度高达x=0.64, 远高于在Mn₂CoGa中(x=0.36)的结果; 以及随着Al的减少, 合金由B2有序向A2混乱转变等现象, 为共价作用对合金结构稳定的影响提供了证据. 磁测量中发现Cr掺杂后磁矩增量高达3.65 μ_B以及居里温度快速上升的反常现象, 意味着对占位规则的违背.     

Electronic and magnetic properties of basic nanosystems of early 3d transition metals (Sc,Ti, V,Cr, and Mn)

This detailed and systematic theoretical study on the behavior of basic low dimensional (one- and two-dimensional) systems of early 3d transition metals should serve as a guideline to experimentalists as well as to theoreticians. We find that, lowering of dimensionality is favorable for emergence of magnetic ordering in all the systems studied, except Ti monolayers (MLs). For Ti MLs, both nonmagnetic and ferromagnetic states are degenerate within the numerical limits. For such a case, the interactions with substrate would play a decisive role in the magnetic ordering of the atoms in the ML. The total energy calculations show that the nonmagnetic and ferromagnetic states are almost degenerate for Cr and V MLs too; however, anti-ferromagnetic ordering is favored in these. The ferromagnetic ordering in Sc linear chains and anti-ferromagnetic ordering in MLs of Mn and Cr are found to be favored by a relatively larger margin showing good stability. Some low dimensional systems, showing electrons with only one kind of spin available at Fermi energy, may be suitable for spintronics related applications. The linear chains of Cr and Mn, and MLs of Sc are likely to form stable magnetic nanosystems as these exhibit almost saturated magnetic moment per atom around the equilibrium separation. The magnetic moment strengthens considerably as one goes from two- to one-dimension. Our results are supported qualitatively by available experimental results and offer a good insight into these nanosystems.     

Co掺杂(ZnO)12团簇的结构和磁性质

采用第一性原理密度泛函理论系统地研究Co原子单掺杂和双掺杂(ZnO)₁₂团簇的结构和磁性质.考虑三种掺杂方式:替代掺杂,外掺杂和内掺杂.首先比较各种掺杂团簇的稳定性.结果表明,不管是单掺杂还是双掺杂,外掺杂团簇都是最稳定结构.在结构优化的基础上,对掺杂的(ZnO)₁₂团簇进行磁性计算.发现团簇磁矩主要来自Co-3d态的贡献,4s和4p态也贡献了一小部分磁矩.由于轨道杂化,相邻的Zn和O原子也产生少量自旋.Co原子之间的磁性耦合由直接的Co-Co反铁磁耦合和Co和O原子之间通过p-d杂化产生的铁磁耦合这两种相互作用的竞争决定.研究发现外双掺杂团簇存在铁磁耦合,在纳米量子器件有潜在的应用价值.     

Ni掺杂ZnO团簇的结构和磁性质

采用第一性原理方法研究Ni原子单掺杂和双掺杂（ZnO）₁₂团簇的结构和磁性质.考虑替代掺杂、外掺杂和内掺杂三种掺杂方式.研究发现：不管是单掺杂还是双掺杂,外掺杂团簇都是最稳定结构.Ni原子之间的磁性耦合由直接的Ni-Ni反铁磁耦合和Ni和O原子之间通过p-d杂化产生的铁磁耦合这两种相互作用的竞争来决定.最重要的是,外双掺杂和内双掺杂团簇都存在铁磁耦合,在纳米量子器件有潜在的应用价值.     

Cr掺杂ZnO纳米线电子性质和磁性质

本文采用第一性原理密度泛函理论系统的研究了Cr原子单掺杂和双掺杂两种尺寸ZnO纳米线的电子性质和磁性质.所有掺杂纳米线的形成能都比纯纳米线的形成能低,表明掺杂增强了纳米线的稳定性.研究发现Cr原子趋于替代纳米线表面的Zn原子.所有掺杂纳米线都显示了金属性.纳米线的总磁矩主要来源于Cr原子3d轨道的贡献.由于杂化,相邻的O原子和Zn原子也产生了少量自旋.在超原胞内,Cr和O原子磁矩反平行排列,表明它们之间是反铁磁耦合.表面双掺杂纳米线铁磁态能量比反铁磁态能量低149 meV,表明Cr掺杂ZnO纳米线可能获得室温铁磁性.     

Fe掺杂ZnTe团簇结构和磁性质

本文采用第一性原理密度泛函理论系统的研究了Fe原子单掺杂和双掺杂(ZnTe)12团簇的结构和磁性质。我们考虑了替代掺杂和间隙掺杂。不管是单掺杂还是双掺杂,间隙掺杂团簇都是最稳定结构。团簇磁矩主要来自Fe-3d态的贡献,4s 和4p 态也贡献了一小部分磁矩。由于轨道杂化,相邻的Zn和Te原子上也产生少量自旋。最重要的是,我们指出间隙双掺杂团簇是铁磁耦合,在纳米量子器件有潜在的应用价值。     

Fe掺杂ZnTe团簇结构和磁性质

本文采用第一性原理密度泛函理论系统的研究了Fe原子单掺杂和双掺杂（ ZnTe）12团簇的结构和磁性质。我们考虑了替代掺杂和间隙掺杂。不管是单掺杂还是双掺杂,间隙掺杂团簇都是最稳定结构。团簇磁矩主要来自Fe－3d态的贡献,4s和4p态也贡献了一小部分磁矩。由于轨道杂化,相邻的Zn和Te原子上也产生少量自旋。最重要的是,我们指出间隙双掺杂团簇是铁磁耦合,在纳米量子器件有潜在的应用价值。     

Cr掺杂ZnS纳米管结构和磁性质

采用第一性原理密度泛函理论系统研究Cr原子单掺杂和双掺杂单壁ZnS纳米管的结构和磁性质.研究发现掺杂纳米管的形成能比纯纳米管的形成能低,说明掺杂过程是放热的.单掺杂纳米管的总磁矩主要来自Cr原子3d态的贡献.结果表明Cr原子掺杂单壁ZnS纳米管趋向于铁磁态.但铁磁态和反铁磁态的能量差仅为0.036 eV.为获得室温铁磁性,我们用一个C原子替代掺杂体系中的一个S原子.计算发现铁磁态的能量比反铁磁态低0.497eV.表明此掺杂体系可能获得室温铁磁性.     

Enhanced ferromagnetism in nano-sized Zn0.95Mn0.05O grains

The present work reports ferromagnetism by doping magnetic Mn atoms in the diamagnetic ZnO matrix and the ferromagnetism has been extended up to 640 K in nano-grained Zn_0.95Mn_0.05O samples. The bulk and nano-grained samples were stabilized in hexagonal crystal structure with space group p63mc. The grain size and lattice strain of the samples were estimated from room temperature XRD spectrum. Surface morphology of the samples was examined at room temperature using SEM picture and EDX spectrum. The ferromagnetism of the bulk material shows enhancement in nano-grained samples, which was mainly due to the solution of Mn atoms into the lattice sites of ZnO by mechanical milling. The enhancement of magnetic moment and ferromagnetic ordering temperature with reduction in grain size has been understood in terms of the core-shell structure and existing theoretical models. The present work also demonstrated the role of surface spin disorder on the enhancement of ferromagnetism in Zn_0.95Mn_0.05O nanograins.     

Structural and magnetic properties of Mn-doped ZnO powders

A mixture of bi(acetylacetonato) zinc(II)hydrate and tris(acetylacetonato) manganese(III) complexes was thermally co-dissociated to synthesize Mn-doped ZnO powders. In order to examine the effect of oxygen vacancies on the ferromagnetic coupling of Mn ions, two preparation routes were used: in route (I) the preparation was done in an open environment, whereas in route (II) the preparation was done in a closed environment. The X-ray diffraction (XRD) and the X-ray fluorescence (XRF) measurements indicate that the Mn content in the three samples are 3.9% (I), 3.3% (II) and 4.2% (II). The XRD results showed that the Mn ions were incorporated in the ZnO crystal and that a Zn_1−xMn_xO solid solution has formed. The magnetic characterization indicated that only samples prepared via route (II) exhibited a room temperature ferromagnetic component of magnetization. Furthermore, magnetic analysis showed that the magnetic moment per dopant ion in the samples examined was in the range of 4.2-6.1 μ_B/Mn. The percentages of coupled Mn atoms to the total number of Mn atoms were found to be extremely small (less than 0.1%), which by itself cannot explain the observed RT hysteresis loops. Thus, in order to produce long-range ferromagnetic order in these samples, the FM coupling has to be mediated via defects. The observed FM in this study may be attributed to the presence of oxygen vacancies, which mediate the ferromagnetic exchange between the coupled Mn ions. This is consistent with the bound magnetic polarons (BMP) model where defects like oxygen vacancies cause the polarons to overlap and give rise to a long-range ferromagnetic order in dilute magnetic semiconductors (DMS).     

Effect of codoping with C or N on electronic structures and magnetic properties of ZnO:Cu

Using first-principles calculations based on density functional theory, we investigated systematically the electronic structures and magnetic properties of ZnO:Cu. The results indicate that Cu-doped ZnO prefers a ferromagnetic ground state and behaves like a half-metallic ferromagnet. The magnetic moment mainly localizes at Cu atom and the rest mainly comes from the spin polarized O atoms. It has been found that the ferromagnetic stability can be enhanced slightly by substituting an oxygen atom with one N atom; while the ferromagnetic stability can be weakened by replacing one O atom with a C atom. Due to absence of magnetic ion and the 100% spin polarization of the carriers in ZnO:Cu, one can expect that Cu-doped ZnO would be a useful half-metallic ferromagnet both in practical application and in theoretical studies.     

Cr掺杂ZnO纳米线的电子结构和磁性

采用自旋极化密度泛函理论系统研究了Cr掺杂ZnO纳米线的电学、磁学以及光学属性.计算结果显示,Cr原子沿[0001]方向替代ZnO纳米线中的Zn原子时体系一般呈现铁磁耦合,沿[1010]和[0110]方向替代Zn原子时体系呈现反铁磁耦合,且磁性耦合状态在费米能级附近出现了明显的自旋劈裂现象,发生了强烈的Cr 3d和O 2p杂化效应.自旋态密度计算结果显示,磁矩主要来源于Cr原子未成对3d态电子的贡献,磁矩的大小与Cr原子的电子排布有关.光学性质计算结果显示,Cr掺杂ZnO纳米线在远紫外和近紫外都具有明显的吸收峰,吸收峰发生了明显的红移.这些结果都表明Cr掺杂ZnO纳米线也许是一种很有前途的稀磁半导体材料. 关键词： ZnO 纳米线 第一性原理 磁性     

氧空位对钴掺杂氧化锌半导体磁性能的影响

从实验和理论上阐述了氧空位对Co掺杂ZnO半导体磁性能的影响.采用磁控溅射法在不同的氧分压下制备了Zn_095Co_005O薄膜,研究了氧分压对薄膜磁性能的影响.实验结果表明,高真空条件下制备的Zn_095Co_005O薄膜具有室温铁磁性,提高氧分压后制备的薄膜铁磁性逐渐消失.第一性原理计算表明,在Co掺杂ZnO体系中引入氧空位有利于降低铁磁态的能量,铁磁态的稳定性与氧空位和Co之间的距离密切相关. 关键词： Co掺杂ZnO 稀磁半导体 第一性原理计算 氧空位缺陷     

First principles study the ferromagnetic properties and electronic structure of boron doped ZnSe

Using the full-potential linearized augmented plane wave method with generalized gradient approximation, the magnetic properties and the electronic structure of the boron-doped ZnSe (zinc blende phase) are investigated. Spin polarization calculations show the magnetic moment of the 64-atoms supercell containing one B_Se (B_Zn) is 3.00 (0.015) μ_B. The density of states indicates the magnetic moments of the B_Se doped configuration mainly come from the doped boron atoms and a few from its neighboring zinc atoms. The ferromagnetic and antiferromagnetic calculations for several doped configurations suggest B_Se could induce stable ferromagnetic ground state in ZnSe hosts and ferromagnetic couplings exist between the doped boron atoms. Electronic structures show that B_Se is p-type ferromagnetic semiconductor and hole-mediated double exchange is responsible for the ferromagnetism, while the B_Zn doped configuration is n-type semiconductor. Relative shallow acceptor and donor levels indicate boron-doped ZnSe is ionized easily at working temperatures.