Ni基金属间化合物第一性原理研究

稀土(R)-过渡族(T)金属间化合物具有优异的物理和化学性质.本研究考虑电子自旋极化作用,基于第一性原理的全电子投影缀加平面波赝势法理论,采用局域自旋密度近似(LSDA),对Ni₁₃Nd₃B₂金属间化合物进行结构优化,计算体系晶格常数、电子结构和磁性能.结果表明,Ni₁₃Nd₃B₂为带隙很小的金属导体.LSDA近似下体系原子间存在复杂作用类型,Nd原子与近邻Ni,B原子以离子键作用为主,Ni原子与近邻Ni原子间表现共价作用情形.体系存在Nd-Ni铁磁耦合,总磁矩约8.4329μB,主要由Nd原子磁矩提供,自旋极化引起的体系Nd-4f,Ni-3p,Nd-5p电子自旋劈裂为体系表现磁性的根本原因.     

Intramolecular dynamics in small radicals with anisotropic magnetic interactions

The ESR lineshape of one electron spin coupled by anisotropic hyperfine interaction to two nuclei is calculated when two different configurations of the molecule interchange by a simple reaction scheme. The spectrum calculation includes the anisotropic electron Zeeman interaction and the nonsecular terms of the electron spin. The lineshape, which simulates both a nonsaturated CW or a pulse experiment, is calculated numerically by using a density matrix theory within the Liouville formalism. The theory is tested against the X-band spectra of-CH₂ in ZnAc single crystal for three different orientations of the magnetic field. The present theoretical lineshape reproduces all the experimental “forbidden” transitions and predicts that they are important for certain crystal orientations. The calculated reorientation barrier of the methylene in the present work is 7.17 kcal/mol which is closer to an estimated minimum value of 9 kcal/mol, than the previous value of 5–6 kcal/mol obtained by an analysis with the modified Bloch equations.     

Invariant exchange perturbation theory for multicenter systems and its application to the calculation of magnetic chains in manganites

The formalism of exchange perturbation theory is presented with regard to the general principles of constructing an antisymmetric vector with the use of the Young diagrams and tableaux in which the coordinate and spin parts are not separated. The form of the energy and wave function corrections coincides with earlier obtained expressions, which are reduced in the present paper to a simpler form of a symmetry-adapted perturbation operator, which preserves all intercenter exchange contributions. The exchange perturbation theory (EPT) formalism itself is presented in the standard form of invariant perturbation theory that takes into account intercenter electron permutations between overlapping nonorthogonal states. As an example of application of the formalism of invariant perturbation theory, we consider the magnetic properties of perovskite manganites La_1/3Ca_2/3MnO₃ that are associated with the charge and spin ordering in magnetic chains of manganese. We try to interpret the experimental results obtained from the study of the effect of doping the above alloys by the model of superexchange interaction in manganite chains that is constructed on the basis of the exchange perturbation theory (EPT) formalism. The model proposed makes it possible to carry out a quantitative analysis of the effect of substitution of manganese atoms by doping elements with different electron configurations on the electronic structure and short-range order in a magnetic chain of manganites.     

Ni2+:RbMgF3的电子顺磁共振谱的研究

含有过渡金属离子的晶体的光学吸收谱、零场分裂D值和g因子与晶体结构有密切关系,应用Ni^2 的参量化d轨道和三角晶场中d^8电子组态的强场能量矩阵,通过建立完全对角化方法,精确地计算了具有C3ν和D3d两种对称的Ni^2 ：RbMgF3的电子顺磁共振谱,分析了Ni^2 ：RbMgF3的零场分裂D值和g因子与晶体结构参量R和θ的依赖关系。理论值与实验值符合得很好。     

Ni^2＋：CsMgCl3的光学吸收谱和EPR的研究

应用三角晶场中d^2(d^8)电子组态包括静电相互作用和自旋－轨道耦合作用的强场能量矩阵,采用完全对角化方法,精确地计算了具有D3d对称的Ni^2 :CsMgCl3的光学吸收谱和EPR谱,理论结果与实验值符合得很好。     

Magnonics: Experiment to prove the concept

An experimental scheme for studying spin wave propagation across thin magnetic film samples is proposed. The scheme is based upon the creation of picosecond pulses of strongly localized effective magnetic field via ultrafast optical irradiation of a specially deposited exchange bias or exchange spring layer. The spin waves are excited near the irradiated surface before propagating across the thickness of the sample. They are then detected near the other surface either within the finite optical skin depth using the linear magneto-optical Kerr effect in metallic samples or by the magnetic second harmonic generation. The experiment can facilitate investigations of propagating spin waves with wavelengths down to several nanometers and frequencies in excess of hundreds of Gigahertz. An experiment upon a periodically layered nanowire (a finite cross-section magnonic crystal) is numerically simulated, although the sample might equally well be a continuous film or an array of elements (e.g. nanowires) that either have uniform composition or are periodically layered as in a magnonic crystal. The experiments could be extended to study domain wall-induced spin wave phase shifts and can be used for the creation of spin wave magnetic logic devices.     

The spin state of manganese atoms in Sr2MnSbO6 complex oxides

The magnetic susceptibility of Sr₂MnSbO₆ solid solutions in a Sr₂AlSbO₆ diamagnetic diluent is investigated. Variation in the magnetic characteristics with temperature and the solution concentration indicates the coexistence of two spin states of manganese atoms: low-spin and high-spin. The possibility of spin transitions is due to temperature-dependent local distortions in the crystal structure of the solutions.     

Automatic procedure for stable tetragonal or hexagonal structures: application to tetragonal Y and Cd

A simple effective procedure (MNP) for finding equilibrium tetragonal and hexagonal states under pressure is described and applied. The MNP procedure finds a path to minima of the Gibbs free energy G at T=0 K (G=E+pV, E=energy per atom, p=pressure, V=volume per atom) for tetragonal and hexagonal structures by using the approximate expansion of G in linear and quadratic strains at an arbitrary initial structure to find a change in the strains which moves toward a minimum of G. Iteration automatically proceeds to a minimum within preset convergence criteria on the calculation of the minimum. Comparison is made with experimental results for the ground states of seven metallic elements in hexagonal close-packed (hcp), face- and body-centered cubic structures, and with a previous procedure for finding minima based on tracing G along the epitaxial Bain path (EBP) to a minimum; the MNP is more easily generalized than the EBP procedure to lower symmetry and more atoms in the unit cell. Comparison is also made with a molecular-dynamics program for crystal equilibrium structures under pressure and with CRYSTAL, a program for crystal equilibrium structures at zero pressure. Application of MNP to the elements Y and Cd, which have hcp ground states at zero pressure, finds minima of E at face-centered cubic (fcc) structure for both Y and Cd. Evaluation of all the elastic constants shows that fcc Y is stable, hence a metastable phase, but fcc Cd is unstable.     

Ab initio studies on magnetism and hybridization of the ternary germanide

We perform a theoretical investigation on the magnetism and orbital hybridization in ternary germanide Ce₃Ni₂Ge₇ using the full-potential linearized augmented plane wave method (FP_LAPW) based on the density functional theory (DFT). The calculation with local spin density approximation (LSDA) predicts that there are two states for the Ce atoms due to the different environment: one (Ce₁) is near the nonmagnetic state and the other (Ce₂) is localized and magnetic. The orbital hybridization plays a key role in determining the state of Ce. On adding on-site Coulomb potential to the localized Ce₂-4f orbit, the magnetic moment obtained from our calculation fits well with the experimental value.     

Neutron-proton transfer reactions leading toT=1 particle-hole states of56Co

Zero range DWBA analysis for two-particle (neutron and proton) transfer reactions is carried out, using simple shell model structure wave functions for⁵⁴Fe,⁵⁶Co and⁵⁸Ni, with⁵⁶Ni inert core. In this structure calculation, a microscopic set of two-body interaction matrix elements derived from the non-local separable potential of Tabakin are employed. These matrix elements include in the perturbation theory two corrections (i) the second-order Börn term and (ii) the appropriate core excitations. Unlike the situation in many two-particle transfer reactions, the fragmentation of the reaction strengths to the excited states with respect to the lowest states of same spin and parity in the above transfer processes is satisfactorily borne out from this analysis.     

Theoretical studies on the electronic structure and magnetic properties of Cu(2,5-dmpz)Cl2

First-principles calculations have been performed to study the electronic structure, the metallic and magnetic properties of Cu(2,5-dmpz)Cl₂. The calculations are based on the density functional theory (DFT) with the generalized gradient approximation (GGA) and the full-potential-linearized-augmented plane wave (FPLAPW) method. The total energy, magnetic moment, density of states (DOS) and electronic band structure are calculated. The results reveal that the compound has a stable semiconductive antiferromagnetic (AFM) ground state and a semiconductive ferromagnetic (FM) metastable state, which is in good agreement with the experimental results. Based on the spin distribution and the DOS, it is found that the spin magnetic moment is mainly from the Cu²⁺, and with relative small contribution from Cl, N atoms.     

Temperature effects on the two-magnon Raman-scattering of antiferromagnets

In this first of two papers we investigate the temperature effect on the spin wave energy and the two-magnon Raman scattering cross section for an antiferromagnet with rutile structure in the spin wave scheme. In the Heisenberg hamiltonian we consider the exchange interaction between magnetic neighbors up to the third order, an effective anisotropy field, and an external magnetic field. In the effective Raman hamiltonian an anisotropy factor, allowed by the crystal symmetry, is taken into account.The theory employs a Green's function method, where the Green's functions are obtained from equations of motion. The results are similar to published ones obtained with diagrammatic techniques.Extract from thesis, Munich, 1974     

磁性金属材料中交换耦合作用和自旋波的研究

基于交换耦合理论通常使用的近似分析的一般原理, 严格的分析了没有特定假设情况下的磁序范围或有关磁化密度的形式, 及在任何近似下提出一种关于耦合参数的计算方法. 并结合铁磁系统(磁性金属材料Gd, Fe, Ni), 定量的讨论了这种关系的适用范围, 也对自旋波和交换耦合进行了相关分析. 分析表明: 对于近邻磁性原子之间的交换耦合的计算以及在有限波矢量情况下对自旋波谱的计算都得到较为有意义的改进. 提出的交换耦合近似及自旋波谱的关系, 应用于铁磁系统时对近邻原子之间相互作用能给出较好的描述, 或对任何磁体中非完全局域磁化的自旋波谱较大波矢部分给出较合理的描述. 从磁性理论来看, 按照本文模型应用于磁学系统计算得到的结果与实验结果较好的符合.     

Magnetic structure of Er<Subscript>5</Subscript>Ge<Subscript>3</Subscript> at 4.2 K

A symmetry analysis of the possible magnetic structures of Er₅Ge₃ in the ground state is performed using the results of measurements of elastic magnetic neutron scattering at 4.2 K. It is shown that the minimum discrepancy factor R _m ≈9.5% corresponds to a modulated collinear magnetic structure in which the magnetic moments of erbium atoms are oriented along the a ₃ axis of the unit cell of the crystal structure and induce an antiferromagnetic longitudinal spin wave (AFLSW). The magnetic structure is characterized by the wave vector k=2π(0, 0, μ /a ₃) (where μ≈0.293) and the modulation period λ≈3.413a ₃. The magnetic ordering temperature T _N ≈38 K is determined from the temperature dependence of the intensity of magnetic reflections. __________ Translated from Fizika Tverdogo Tela, Vol. 45, No. 9, 2003, pp. 1653–1659. Original Russian Text Copyright ? 2003 by Vokhmyanin, Dorofeev.     

三元化合物ZnVSe2半金属铁磁性的第一性原理计算

采用基于密度泛函理论(DFT)框架下的第一性原理平面波赝势(PWP)方法,结合广义梯度近似(GGA),对三元化合物ZnVSe₂晶体的电子结构进行了计算,分析了ZnVSe₂晶体自旋极化的能带结构、电子态密度、电荷布居、磁矩等.计算结果表明,三元化合物ZnVSe₂会产生自旋极化状态,能带结构和态密度显示为半金属特征,表现出显著的铁磁性行为,具有高达近100%的传导电子自旋极化率,其半金属能隙为0.443eV,理论预测其可能是一种具有一定应用潜能 关键词： 2')" href="#">ZnVSe₂ 平面波赝势方法 半金属铁磁性 第一性原理     

Microscopic R-matrix theory in a generator coordinate basis: (III). Multi-channel scattering

The microscopic R-matrix theory presented in two previous papers is extended to the multichannel scattering case. An antisymmetrized wave function is built in the two-centre harmonic oscillator model. This wave function is proved to be equivalent to the resonating group one even if the nuclei have non-zero spin. The method only requires the calculation of numerical values of matrix elements between Slater determinants. The microscopic R-matrix theory may be applied to study both reactions and inelastic scattering.     

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.     

基于闪锌矿晶体中受激电磁耦子散射产生可调谐太赫兹波的理论研究

基于受激电磁耦子散射原理,采用已报道的利用非线性光学参量振荡方法产生可调谐太赫兹波的实验条件作为理论分析的实验模型,以GaAs,GaP, InP,ZnTe晶体为代表,计算分析了在闪锌矿晶体中参量振荡产生太赫兹波的吸收、增益特性,对输出THz波的调谐特性给出了详尽分析.分析太赫兹波高效耦合输出的腔型结构,并与掺氧化镁铌酸锂晶体组成的太赫兹波参量振荡器做对比. 关键词： 太赫兹波 太赫兹波参量振荡 电磁耦子 闪锌矿晶体     

The electronic structure and magnetic properties of metallic antiferromagnetic Co[(CH3PO3)(H2O)] studied by first-principles calculations

The electronic structure, the metallic and magnetic properties of metal phosphonate Co[(CH₃PO₃)(H₂O)] have been studied by first-principles calculations, which were based on the density-functional theory (DFT) and the full potential linearized augmented plane wave (FPLAPW) method. The total energy, the spin magnetic moments and the density of the states (DOS) were all calculated. The calculations reveal that the compound Co[(CH₃PO₃)(H₂O)] has a stable metallic antiferromagnetic (AFM) ground state and a half-metallic ferromagnetic (FM) metastable state. Based on the spin distribution obtained from calculations, it is found that the spin magnetic moment of the compound is mainly from the Co²⁺, with some small contributions from the oxygen, carbon and phosphorus atoms, and the spin magnetic moment per molecule is 5.000μ_B, which is in good agreement with the experimental results.     

Effect of Si substitution on electronic structure and magnetic properties of Heusler compounds Co2TiAl1−xSix

The electronic structures of Co-based Heusler compounds CoTiAl_1−xSi_x (x=0, 0.25, 0.5, 0.75 and 1) are calculated by first-principles using the full potential linearized augmented plane wave (FP-LAPW) method within GGA and LSDA+U scheme. Particular emphasis was put on the role of the main group elements. In recent years, the GGA calculations of Co₂TiAl (x=0) and Co₂TiSi (x=1) indicated that they are half-metallic, but the electronic structure of this compound with x=0.25, 0.5 and 0.75 has not been reported yet, neither theoretically nor experimentally. The calculated results reveal that these are half-metallic and exhibit an energy gap in the minority spin state and also show 100% spin polarization. The substitution of Al by Si leads to an increase in the number of valence electrons, with increasing x. Our calculated results clearly show that with the Si doping, the lattice parameter linearly decreases; bulk modulus increases, and the total magnetic moment increases. The calculated energy gap in the minority spin state, using GGA scheme, was smaller than that obtained by using LSDA+U scheme. The outcomes of this research also show that the Co-3d DOS and therefore, the magnetic properties of compounds are dependent on electron concentration of the main group elements and it will affect the degree of p-d orbital occupation.