Ab initio investigation of local magnetic structures around substitutional 3d transition metal impurities at cation sites in III-V and II-VI semiconductors

The local magnetic structures around substitutional 3d transition metal impurities at cation sites in zinc blende structures of III-V (GaN, GaAs) and II-VI (ZnTe) semiconductors are investigated by using a spin-polarized density functional theory. We find that Cr-, Co-, Cu-doped GaN, Cr-, Mn-doped GaAs and Cr-, Fe-, Ni-doped ZnTe are half metallic with 100% spin polarization. The magnetic moments due to these 3d transition metal (TM) ions are delocalized quite significantly on the surrounding ions of host semiconductors. These doped TM ions have long range interactions mediated through the induced magnetic moments in anions and cations of host semiconductors. For low impurity concentrations Mn in GaAs also has zero magnetic moment state due to Jahn-Teller structural distortions. Based upon half metallic character and delocalization of magnetic moments in the anions and cations of host semiconductors these above mentioned 3d TM-doped GaN, GaAs and ZnTe seem to be good candidates for spintronic applications.     

First-principles study on the magnetism in ZnS-based diluted magnetic semiconductors

The electronic and magnetic properties of wurtzite ZnS semiconductor doped with transition metal (Cr, Mn, Fe, Co, and Ni) atoms are studied by using the first-principle’s method in this paper. The ZnS bulk materials doped with Cr, Fe, and Ni are determined to be half-metallic, while those doped with Mn and Co impurities are found to be semiconducting. These doped transition metal ions have long range interactions mediated through the induced magnetic moments in anions and cations of host semiconductors. These doped ZnS-based diluted magnetic semiconductors seem to be good candidates for the future spintronic applications.     

A Green＇s function model for ferromagnetism and spin excitations of （Ga,Mn）As diluted magnetic semiconductors

We study (Ga, Mn)As diluted magnetic semiconductors in terms of the Ruderman--Kittel--Kasuya--Yosida quantum spin model in Green's function approach. Random distributions of the magnetic atoms are treated by using an analytical average of magnetic configurations. Average magnetic moments and spin excitation spectra as functions of temperature can be obtained by solving self-consistent equations, and the Curie temperature T_C is given explicitly. T_C is proportional to magnetic atomic concentration, and there exists a maximum for T_C as a function of carrier concentration. Applied to (Ga, Mn)As, the theoretical results are consistent with experiment and the experimental T_C can be obtained with reasonable parameters. This modelling can also be applied to other diluted magnetic semiconductors.     

Spin-fluctuation mediated high-temperature ferromagnetism in Si:Mn dilute magnetic semiconductors

We discuss a possible route to explain high-temperature ferromagnetism in Si:Mn dilute magnetic semiconductors. We argue that most Mn atoms are segregated within nanometer-sized regions of magnetic precipitate and form the alloy, or compound, MnSi_{2 -z} with z ≈ (0.25?\div0.30), whereas a small minority of Mn atoms forms ?ngstr?m-sized magnetic defects embedded in the host. Assuming that MnSi_{2 -z} is a weak itinerant ferromagnet which supports sizable spin fluctuations (paramagnons) far above the intrinsic Curie temperature, we show that the Stoner enhancement of the exchange interaction between the local magnetic moments of the defects occurs. As a result, a significant increase of the temperature of global ferromagnetic order in the system is achieved. We develop a phenomenological approach, to qualitatively describe this effect.     

EPR and ferromagnetism in diluted magnetic semiconductor quantum wells

Motivated by recent measurements of electron paramagnetic resonance spectra in modulation-doped CdMnTe quantum wells [Phys. Rev. Lett. 91, 077201 (2003)]], we develop a theory of collective spin excitations in quasi-two-dimensional diluted magnetic semiconductors. Our theory explains the anomalously large Knight shift found in these experiments as a consequence of collective coupling between Mn-ion local moments and itinerant-electron spins. We use this theory to discuss the physics of ferromagnetism in (II,Mn)VI quantum wells and to speculate on the temperature at which it is likely to be observed in n-type modulation-doped systems.     

ZnS(111)表面掺杂Mn的电子结构和磁性的第一性原理研究

应用基于密度泛函理论的第一性原理,研究Mn原子掺杂在ZnS(111)表面的电子结构和磁性.对于单原子的掺杂组态,替位表面第一层的Zn原子时体系形成能最低,说明该层是最稳定的掺杂位置.体系总磁矩取决于Mn原子的局域环境.而对于双掺杂组态,当Mn与Mn之间呈短程铁磁耦合作用时体系最稳定.这可由Mn原子和近邻S原子的p-d杂化作用解释.此时,体系的居里温度估算值为469 K,明显高于室温,具有理论指导意义.Mn原子和受主半导体之间的相互作用是自旋极化产生的主要原因.计算结果表明,该掺杂材料可以很好的用来制作稀磁半导体,具有良好的应用前景.     

Mn掺杂ZnS(110)表面的电子结构和磁性

采用基于密度泛函理论的第-性原理方法,研究Mn掺杂ZnS(110)表面的电子结构和磁性.计算分析不同掺杂组态的几何参数、形成能、磁矩、电子态密度以及电荷密度.结果表明:单个Mn原子掺杂,替位于表面第二层的Zn原子时体系形成能最低,说明该层是最稳定的掺杂位置.对于两个Mn原子的掺杂,当Mn与Mn之间呈反铁磁耦合时体系最稳定.体系的总磁矩和自由Mn原子的磁矩差别很小,但是Mn原子的局域磁矩却依赖于Mn原子的3d态和近邻S原子的3p态的杂化作用,即受周围S原子环境的变化影响较大.此外,分析电荷密度图得出Mn原子替换Zn原子后与S原子形成了更强的共价键.     

Ab initio study of electronic structures and magnetism in ZnMnTe and CdMnTe diluted magnetic semiconductors

In this work, we aimed to examine the spin-polarized electronic band structures, the local densities of states as well as the magnetism of ZnMnTe- and CdMnTe-diluted magnetic semiconductors (DMSs) in the ferromagnetic phase, and with 25% of Mn. The calculations are performed by the recent ab initio full potential augmented plane waves plus local orbitals (FP−L/APW+lo) method within the spin-polarized density-functional theory and the local spin density approximation. We have determined the exchange splittings produced by the Mn d states: Δ_x(d) and Δ_x(pd), and we found that the effective potential for the minority spin is more attractive than that for the majority spin. Also, we show the nature of the bonding from the charge spin-densities calculations, and we calculate the exchange constants N₀α and N₀β, which mimics a typical magneto-optical experiment. The calculated total magnetic moment is found to be equal to 5μ_B for both DMSs. This value indicates that every Mn impurity adds no hole carriers to the perfect ZnTe and CdTe crystals. Furthermore, we found that p–d hybridization reduces the local magnetic moment of Mn and produces small local magnetic moments on the nonmagnetic Te, Zn and Cd sites.     

第一原理研究Mn掺杂LiNbO3晶体的磁性和光吸收性质

采用基于密度泛函理论和局域密度近似的第一性原理分析了Mn掺杂LiNbO₃晶体的结构, 磁性, 电子特性和光吸收特性. 文中计算了Mn占据Li位和Nb位体系的形成焓, 对应的形成焓分别为-8.340 eV/atom和-8.0062 eV/atom, 也就意味着Mn 原子优先占据Li位. 这也就意味着Mn原子占据Li位的掺杂LiNbO₃晶体结构更稳定. 磁性分析的结果显示, 其对应磁矩也比占据Nb位的高. 进一步分析磁性的来源, 自旋态密度结果显示: Mn掺杂LiNbO₃晶体的磁性主要源于掺杂原子Mn, Mn原子携带的磁矩高达 4.3 μ_B, 显示出高自旋结构. 由于Mn-3d与近邻O-2p及次近邻Nb-4d 轨道的杂化作用, 计算表明: 诱导近邻O原子及次近邻Nb原子产生的磁矩对总磁矩的贡献较小. 通过光学吸收谱的分析, 得出在可见光区Li位被Mn原子替代以后显示出更好的光吸收响应相比于Nb位. 本文还分析了O空位对于LiNbO₃晶体磁性与电子性质的影响, 结果显示O空位的存在可以增加Mn掺杂LiNbO₃体系的磁性.     

Transition metal encapsulated hydrogenated silicon nanotubes: Silicon-based half-metal

One-dimensional hydrogenated silicon nanotubes (H-SiNTs) with transition metal atom encapsulated were systematically studied by using density functional theory. The band structures and magnetic properties of the H-SiNTs can be tailored by doping transition metal (TM) (TM = Cr, Mn, Fe, Co) atoms within the tube. The hydrogenated silicon nanotubes are semiconductors with wide band gaps. TM doping turns H-SiNTs to be metals or semiconductors with a very small gap, and TM atoms at the center of the tubes keep large magnetic moments. Robust half-metallicity is observed in Mn-doped H-SiNTs and it is free from Peierls distortion. Thus, H-SiNTs with encapsulated magnetic elements may find important applications in spintronic devices.     

Ferromagnetic transition in GaAs/Mn/GaAs/In x Ga1 − x As/GaAs structures with a two-dimensional hole gas

The transport properties of GaAs/Mn/GaAs/In _x Ga_{1 ? x} As/GaAs structures with a layer that is separated from the quantum well and contains Mn impurities in the concentration range 4–10 at % corresponding to the reentrant metal-insulator transition observed in the bulk GaMnAs material [17] have been investigated. The hole mobility in the objects under investigation is more than two orders of magnitude higher than the known values for the GaMnAs semiconductor and GaMnAs-based magnetic heterostructures. This makes it possible to observe Shubnikov-de Haas oscillations, which confirm a two-dimensional character of the hole energy spectrum. The calculated Curie temperature for heterostructures with indirect exchange interaction through a two-dimensional hole channel is in good agreement with the position of the maximum (at 25–40 K) in the temperature dependences of the electrical resistance of the channel. This suggests that two-dimensional holes play an important role in ferromagnetic ordering of the Mn layer under these conditions. The observations of a negative spin-dependent magnetoresistance and an anomalous Hall effect, whose magnitude correlates well with the results of theoretical calculations for two-dimensional ferromagnetic systems based on III-Mn-V, also indicate a significant role of the two-dimensional channel in ferromagnetic ordering.     

Noncollinear ferromagnetism in (III,Mn)V semiconductors

We investigate the stability of the collinear ferromagnetic state in kinetic exchange models for (III,Mn)V semiconductors with randomly distributed Mn ions. Our results suggest that noncollinear ferromagnetism is common to these semiconductor systems. The instability of the collinear state is due to long-range fluctuations involving a large fraction of the localized magnetic moments. We address conditions that favor the occurrence of noncollinear ground states and discuss unusual behavior that we predict for the temperature and field dependence of its saturation magnetization.     

热退火对Mn离子注入非故意掺杂GaN微结构、光学及磁学特性的影响

通过Mn离子注入非故意掺杂GaN外延层制备了GaN:Mn薄膜,并研究了退火温度对GaN:Mn薄膜的微结构、光学及磁学特性的影响.对不同退火温度处理后的GaN:Mn薄膜的拉曼谱测试显示,出现了由与离子注入相关的缺陷的局域振动(LV)和(Ga,Mn)N中Mn离子的LV引起的新的声子模.在GaN:Mn薄膜的光致发光谱中观察到位于2.16,2.53和2.92 eV处的三个新发光峰(带),其中位于2.16 eV处的新发光带不能排除来自Mn相关辐射复合的贡献.对GaN:Mn薄膜的霍尔测试显示,退火处理后样品表现出n型体材料特征.对GaN:Mn薄膜的振动样品磁强计测试显示,GaN:Mn薄膜具有室温铁磁性,其强弱受Mn相关杂质带中参与调节磁相互作用的空穴浓度的影响.     

Mn/GaAs(001)的表面再构与Mn的磁矩研究

通过第一性原理计算，系统地研究了Mn/GaAs(001)表面的各种再构和相应的局域电子态密度分布，以及表面上Mn的磁矩与各种再构间的对应关系.结果发现，Mn的行为类似电荷施主，将向GaAs表面提供电子，数量依表面的需求而定；直接与Mn的磁矩相联系的d轨道，既可以向GaAs表面施予电子，以弥补Mn的s电子的不足，又可以吸纳因GaAs表面饱和而富余的s电子.这些概念可有效地简化对金属引起的半导体表面再构的理论描述. 关键词： 表面再构 Mn/GaAs(001) 第一性原理计算     

Local atomic structure around Mn ions in GaN:Mn thin films: Quantitative XANES analysis

GaN:Mn dilute magnetic semiconductors with zinc-blende type of lattice and room temperature ferromagnetism were investigated by the X-ray absorption near edge structure (XANES) with a high accuracy approach of the multidimensional interpolation, which makes it possible to determine the nanoscale local atomic structure around Mn impurities. It is found that Mn atoms are substantially incorporated into the GaN lattice and Jahn-Teller distortion around Mn atom is observed. Our results show that symmetry changes around Mn atom influence on XANES spectrum significantly. Furthermore, the possible impact of local distortions on the magnetic properties is discussed.     

TM掺杂的Ⅲ-Ⅴ族稀磁半导体电磁性质的第一原理计算

使用基于自旋局域密度泛函理论的第一性原理方法对3d过渡金属(TM=V，Cr，Mn，Fe，Co和Ni)掺杂的Ⅲ-Ⅴ族半导体(GaAs和GaP)的电磁性质进行了计算.结果发现：用V，Cr和Mn掺杂时体系将出现铁磁状态，而Fe掺杂时将出现反铁磁状态，Co和Ni掺杂时，其磁性则不稳定.其中，Cr掺杂的GaAs和GaP将可能是具有较高居里温度的稀磁半导体(DMS).在这些DMS系统中，V离子的磁矩大于理论期待值，Fe，Co和Ni离子的磁矩小于理论期待值，Cr和Mn离子的磁矩与期待值的差距取决于晶体的对称性以及磁性离子的能带分布.此外，使用Si和Mn共同对Ⅲ-Ⅴ族半导体进行掺杂，将有利于DMS表现为铁磁状态，并可以使体系的T_C进一步提高. 关键词： 稀磁半导体 过渡金属 掺杂 共掺杂     

Diluted magnetic semiconductors and spintronics

Classification of concentrated and diluted magnetic semiconductors is given and their physicochemical properties that are interesting for spintronics are characterized. The electronic structure of magnetic impurities in semiconductors and the nature of indirect exchange interactions between impurity spins in diluted magnetic semiconductors are considered. On the basis of the proposed theory of kinematic exchange, the Curie temperature T _C for bulk diluted magnetic semiconductors (In,Mn)Sb are estimated.     

Reorientation,multidomain states and domain walls in diluted magnetic semiconductors

The competition between surface/interface and intrinsic anisotropies yields a number of specific reorientation effects and strongly influences magnetization processes in diluted magnetic semiconductors as (Ga,Mn)As and (In,Mn)As. We develop a phenomenological theory to describe reorientation transitions and the accompanying multidomain states applicable to layers of these magnetic semiconductors. It is shown that the magnetic phase diagrams of such systems include a region of four-phase domain structure with four adjoining areas of two-phase domains as well as several regions with coexisting metastable states. We demonstrate that the parameters of isolated domain walls in (Ga,Mn)As nanolayers are extremely sensitive to applied magnetic field and can vary in a broad range. This can be used in microdevices of magnetic semiconductors with pinned domain walls. For (Ga,Mn)As epilayers with perpendicular anisotropy the geometrical parameters of domains have been calculated.     

Phenomenological band structure model of magnetic coupling in semiconductors

A unified band structure model is proposed to explain the magnetic ordering in Mn-doped semiconductors. This model is based on the p-d and d-d level repulsions between the Mn ions and host elements and can successfully explain magnetic ordering observed in all Mn doped II-VI and III-V semiconductors such as CdTe, GaAs, ZnO, and GaN. The model can also be used to explain the interesting behavior of GaMnN, which changes from ferromagnetic ordering to antiferromagnetic ordering as the Mn concentration increases. This model, therefore, is useful to provide a simple guideline for future band structure engineering of magnetic semiconductors.     

Optical manipulation of coupled spins in magnetic semiconductor systems: A path toward spin optoelectronics

Experimental results based on the optical excitations in the III–V-based ferromagnetic semiconductors are reviewed. On the bases of results obtained by both cw- and femto-second-pulse optical excitation, we point out the feasibility of magnetization rotation in the hole-mediated ferromagnetic semiconductor (Ga,Mn)As via the angular momentum and photon energy of light. Here, p–d exchange interaction is the effective channel that transmits a small change in spin axis of the valence band to the ferromagnetically coupled Mn spin sub-system. Within the limit of this picture, we also discuss a hole–Mn spin complex for which hole and Mn spins rotate and relax together upon optical excitation. Partial magnetization reversal observed in the experiments of the electrical current injection in (Ga,Mn)As-based magnetic-tunnel-junction devices is also reviewed in view of the effects caused by the spin-polarized holes. Here, we point out that a spin current of 10⁵ A/cm² may be reduced further if spin injection efficiency can be improved by the optimal designs of the device structure.