反Heusler合金Ti_2Ru_(1-x)Fe_xSn半金属磁性的第一性原理研究

本文基于第一性原理,通过对反Heusler合金Ti_2RuSn的Y位进行Fe元素掺杂,来探究其掺杂前后的相关特性及掺杂机理,以便寻求半金属性更稳定的Heusler合金材料,为后续相关理论研究及实验提供一定参考.在掺杂过程中随着Fe元素掺杂浓度的增加,反Heusler合金Ti_2RuSn的半金属性并未受到破坏,其带隙反而随掺杂浓度逐渐变宽,从未掺前的0.451 eV展宽到了全掺杂的0.711 eV.为分析掺杂体系的稳定性,我们计算了它们相对于理想反Heusler合金Ti_2RuSn块体的形成能,结果表明,对反Heusler合金Ti_2RuSn的Y位进行Fe元素掺杂可以展宽其带隙,并且掺杂浓度越低,体系相对较容易形成.     

Electronic properties of the magnetocaloric compound GdAl2: A Compton scattering study

We present Compton profiles of the GdAl₂ compound and its constituents using a 20Ci ¹³⁷Cs Compton spectrometer. The experimental Compton data have been analysed using theoretical data obtained from the spin polarised relativistic Korringa–Kohn–Rostoker (SPR-KKR) method and also the charge transfer on the formation of the compound. Both the experimental and the SPR-KKR theoretical Compton data support a charge transfer from Al→Gd in GdAl₂, which is in accordance with the conclusions drawn from the partial, total and integrated density of states of GdAl₂ and its constituents.     

The scattering of electrons moving in a magnetic field

The problem investigated here is that of the scattering of relativistic electrons by a spherically symmetric force center. In contrast to the usual formulation of the problem it is assumed that the electrons are traveling in a constant and uniform magnetic field.     

The effect of 4d states based full Heusler alloy on the electronic and magnetic properties of new half metallic ferromagnetism: DFT+U study

A full heusler alloy Ru₂MoSb is studied using the full potential linearized augmented plane wave (FP-LAPW) method based on density functional theory (DFT). Structural, electronic and magnetic properties are investigated for this material. We have used the generalized gradient approximation of Perdew, Burke, and Ernzerh (GGA-PBE), the GGA+U and the modified Becke–Johnson potential of GGA+U (mBJ-GGA+U) to model exchange correlation potential. The Hubbard on-site Coulomb interaction correction U is calculated by constraint local density approximation for both 4d elements Ru and Mo.For both approximations GGA+U and mBJ- GGA+U, density of states and band structure reveal that our compound has a half-metallic character. Magnetic properties show that Ru₂MoSb is ferromagnetic with an integer magnetic moment of 3 µB which is in good agreement with the Slater–Pauling rule. The half-metallicity of Ru₂MoSb is stable under lattice constant changes which makes it a potential contender for spintronic applications.The negative values of the cohesion energy and the formation energy indicate that our Heusler alloy can be synthesized and stabilized experimentally.     

Raman scattering study of two-dimensional magnetic van der Waals compound VI_3

The layered magnetic van der Waals materials have generated tremendous interest due to their potential applications and importance in fundamental research. Previous x-ray diffraction(XRD) studies on the magnetic van der Waals compound VI_3, revealed a structural transition above the magnetic transition but output controversial analysis on symmetry. In this paper we carried out polarized Raman scattering measurements on VI_3 from 10 K to 300 K, with focus on the two Ag phonon modes at ～ 71.1 cm~(-1) and 128.4 cm~(-1). Our careful symmetry analysis based on the angle-dependent spectra demonstrates that the crystal symmetry can be well described by C_(2h) rather than D_(3d) both above and below structural phase transition. We further performed temperature-dependent Raman experiments to study the magnetism in VI_3. Fano asymmetry and anomalous linewidth drop of two Ag phonon modes at low temperatures, point to a significant spin–phonon coupling. This is also supported by the softening of 71.1-cm~(-1) mode above the magnetic transition. The study provides the fundamental information on lattice dynamics and clarifies the symmetry in VI_3. And spin–phonon coupling existing in a wide temperature range revealed here may be meaningful in applications.     

Inelastic neutron scattering study of magnetic excitations in the helimagnetic and antiferromagnetic phases of NiBr2

The spin wave dispersion in NiBr₂ has been studied by medium and long wavelength inelastic neutron scattering in the [1 1 0], [1 0 0] and [0 0 1] directions at 4.2 and 30 K, i.e. in the incommensurate helical and collinear antiferromagnetic phases. The values of the intralayer Heisenberg exchange constant J_ij and XY anisotropy constant D at 4.2(30) K are J₀₁ 0.379(1)(0.379(1)), J₀₂ 0.0036(50)(0.0036(50)), J₀₃ - 0.105(5) (?0.105(5)), J′ - 0.0423(50)(?0.389(50))D 0.0364(50)(0.0290(50)), where J′; is the interlayer exchange constant. In fitting the 4.2 K data account is taken of the co-existence of three equivalent domains and of intensity arising from $\text{ω(}\text{q}\text{)}$ and $\text{ω(}\text{q ± k}{}_0\text{)}$ where $\text{k}{}_0$ is the wavevector of the helix. In the low frequency region of the dispersion curve such peaks are resolved. The results reinforce the hypothesis that in zero-field the commensurate-incommensurate phase transition is driven by fluctuations.     

How itinerant are 3d electrons in transition metals? A study of magnetic circular dichroism in 2p photoemission

We show how the spectral structure of the magnetic circular dichroism (MCD) in photoemission (PE) can be analysed using a sum rule for the sublevel orbital moment. For itinerant 3d metals, such as Co, Fe and Ni, the MCD in the 2p PE does not agree with a single-particle model. On the other hand, a final-state impurity model calculation gives an excellent agreement with the experimental results for the Ni 2p PE of nickel metal. This indicates that inter-configurational mixing must be taken into account to understand the detailed MCD structure in metallic systems. Vice versa, MCD-PE can help to explain the complicated behavior of electrons in correlated materials. Received: 23 May 2001 / Accepted: 4 July 2001 / Published online: 5 October 2001     

Doping-induced temperature evolution of a helicoidal spin structure in the MnGe compound

The helicoidal magnetic structure of a MnGe compound doped with 25% Fe is studied by means of small-angle neutron scattering in a wide temperature range of 10–300 K. Analysis of the scattering-function profile demonstrates that magnetic structures inherent to both pure MnGe and its doped compounds are unstable. The doping of manganese monogermanide is revealed to lead to higher destabilization of the magnetic system. In passing from MnGe to Mn_0.75Fe_0.25Ge, the magnetic-ordering temperature T _N decreases from 130 to 95 K, respectively. It is demonstrated that, at temperatures close to 0 K, the intensity of the contribution to scattering from stable spin helices decreases and the intensity of scattering by spin helix fluctuations increases with increasing impurity-metal concentration. An increased intensity of anomalous scattering caused by spin excitations existing in the system is observed. Helicoidal fluctuations and spin excitations corresponding to low temperatures indicate the quantum nature of the instability in the doped compound. However, MnGe doping with Fe atoms has no influence on the compound’s magnetic properties at temperatures of higher than T _N. The temperature range of short-range ferromagnetic correlations is independent of concentrations and is restricted by temperatures T ranging from 175 to 300 K.     

Neutron scattering study of the magnetoelectric compound GaFeO 3

We present a neutron scattering study carried out on single-crystals of the magnetoelectric compound GaFeO₃, prepared by the floating-zone (FZ) method. The refinement of the crystal structure indicates the existence of a large amount of iron on the gallium site, corresponding to a strong internal substitution disorder. We show that the Ga/Fe disorder and the magnetic transition temperature are weakly dependent on the growth conditions. The disorder also affects the magnetic excitations; our measurements evidence damped spin waves, well defined only close to the Brillouin zone centre.     

f Electron contribution to the change of electronic structure in CeRu2Si2 with temperature: a Compton scattering study

High resolution Compton profiles have been measured in the single crystal of CeRu(2)Si(2) above and below the Kondo temperature to elucidate the change of the Ce-4f electron from localized to itinerant states. Two-dimensional electron occupation number densities projected on the first Brillouin zone, which are obtained after a series of analyses, clearly specify the difference between itinerant and localized states. The contribution of Ce-4f electrons to the electronic structure is discussed by contrast with a band calculation.     

Neutron diffraction study of crystal structure and antiferromagnetic ordering in the heavy fermion compound CePd2Al3

Nuclear and magnetic structures of an annealed polycrystalline sample of the heavy-fermion compound CePd₂Al₃ prepared by arc-melting were investigated by neutron powder diffraction. The chemical structure corresponds well to the ordered hexagonal PrNi₂Al₃-type structure. The antiferromagnetic structure of CePd₂Al₃ with an ordered magnetic moment _Ce=0.47(2) _B at saturation is remarkably similar to that in the heavy fermion superconductor UPd₂Al₃. The additional incommensurate magnetic structures reported previously both for UPd₂Al₃ and CePd₂Al₃ are not observed in the present sample of CePd₂Al₃. At 1.4 K the magnetoresistivity of CePd₂Al₃ measured up to 14 T indicates only one field-induced phase transition at 3.0 T.     

Dynamical interaction of antiferromagnetic subsystems: a neutron scattering study of the spinwave spectrum of the garnet Fe2Ca3(GeO4)3

The Fe³⁺ ions in the garnet Ca₃Fe₂Ge₃O₁₂ form two identical antiferromagnetic subsystems. The interaction between the two subsystems is vanishing within molecular field approximation forq=0. A coupling appears due to the spin fluctuations. The dynamics of the system is described by the Hamiltonian for a Heisenberg antiferromagnet. Symmetry requirements impose two exchange parameters between the sublattices (nearest neighbours)J ₁ in the direction of the 3-fold axis andJ' ₁ in the other three space diagonals. The interaction within each sublattice (second nearest neighbours) is described by the exchange parameterJ ₂. The measured spin wave dispersion curves for the three principal symmetry directions are very well reproduced by a model calculation withJ ₁=-0.909(9) K,J' ₁=-0.307(8) K andJ ₂=-0.615(2)K. The observed intensities are in agreement with predictions from the model. Forq0 the model predicts two acoustic branches going towards zero frequency. A calculation beyond linear spin wave theory forq=0 predicts a quantum gap for the lower acoustic branch. This gap has been found at 0.033(4) THz. An anisotropy gap of 0.007 THz has been taken from the literature.     

A first-principle study of half-metallic ferrimagnetism in the Ti2CoGa Heusler compound

Electronic structure calculations based on density functional (DFT) theory within the generalized gradient approximation (GGA) for the Ti₂CoGa Heusler compound have been performed using the self-consistent full-potential linearized augmented plane wave (FPLAPW) method. The electronic band structures and density of states of the Ti₂CoGa compound show that the spin-up electrons are metallic, but the spin-down bands have a gap of 0.5 eV, resulting in stable half-metallic ferrimagnetic behavior with a magnetic moment of 2μ_B.     

The polarization effect of a laser in multiphoton Compton scattering

The multiphoton Compton scattering in a high-intensity laser beam is studied by using the laser-dressed quantum electrodynamics(QED) method, which is a non-perturbative theory for the interaction between a plane electromagnetic field and a charged particle. In order to analyze in the real experimental condition, a Lorentz transformation for the cross section of this process is derived between the laboratory frame and the initial rest frame of electrons. The energy of the scattered photon is analyzed, as well as the cross sections for different laser intensities and polarizations and different electron velocities. The angular distribution of the emitted photon is investigated in a special velocity of the electron, in which for a fixed number of absorbed photons, the electron energy will not change after the scattering in the lab frame.We obtain the conclusion that higher laser intensities suppress few-laser-photon absorption and enhance more-laser-photon absorption. A comparison between different polarizations is also made, and we find that the linearly polarized laser is more suitable to generate nonlinear Compton scattering.