Electronic structure and phase transitions of MnAs

On the basis of a two center tight binding approach the recursion method of Haydock et al. was applied for MnAs to calculate non-polarized and spin polarized local densities of states, magnetic moments and band energies for two different B8₁ crystal structures and two B31 structures. The itineracy of the Mn 3d-states was implied by allowing for hybridization of p-states centred at the As-sites with d-states centred at the Mn-sites. Self-consistency procedures were applied to determine the relative position of the paramagnetic p- and d-band centers (E_p - E_d = - 1 eV), and to calculate the magnetic moments (3.27 μ_B for the low temperature B8₁ phase and 3.14 μ_B for the corresponding B31 phase). Because of the similarities of the electronic structures of both phases it is argued that for the first-order B8₁ [rlarr2] B31 phase transition the usually applied model of a high spin-low spin transition of the Mn³⁺ ion is basically wrong. Instead of such an ionic model it is suggested to adopt an itinerant d-electron concept within that both phases are closely related but the B8₁ phase is favoured above a certain critical magnetic moment as supported by a very recent study of Kato et al. For the high temperature B8₁ phase occurring after the second-order B31 → B8₁ phase transition a magnetic moment of 3.37 μ_B was obtained.     

Recent advances in the theory of magnetism of NiAs-type transition-metal chalcogenides and pnictides

Magnetism of NiAs-type transition-metal compounds is studied from the viewpoint of the itinerant electron. Electronic nonmagnetic bands of MAs and MSb (M = Cr, Mn, Fe, Co, Ni) and ferromagnetic bands of MnAs and MnSb are calculated by the self-consistent APW method. The obtained ferromagnetic moments are in good agreement with observations. The anomalous temperature dependence of the paramagnetic susceptibility observed in MnAs, MnAs_1−xP_x, FeAs and CoAs is well explained on the basis of spin fluctuation theory, by taking account of characteristic behavior of the density of states obtained for each compound.     

第一性原理研究闪锌矿结构CrSe和CrAs半金属铁磁性的稳定性

运用第一性原理的全势能线性缀加平面波方法对闪锌矿结构CrSe和CrAs的电子结构进行自旋极化计算。闪锌矿结构CrSe和CrAs处于晶格平衡时都具有半金属性,它们自旋向下的电子能带带隙分别为3.38eV 和1. 79eV,同时,它们的自旋总磁矩分别为4.00和3.00μB/formula。自旋总磁矩主要来源于Cr的原子磁矩,Se和As的原子磁矩对总磁矩的贡献很小而且为负值,因而它们具有明显的铁磁性特征. 使晶体晶格在±10%的范围内发生各向同性形变,对闪锌矿结构CrSe和CrAs的电子结构进行计算. 计算结果表明,当晶格各向同性形变分别为-4 % ~ 10 %和-2 % ~10 %时,闪锌矿结构CrSe和CrAs仍然保持半金属铁磁性,并且总磁矩都稳定于4.00和3.00μB/formula.     

第一性原理研究闪锌矿结构CrSe和CrAs半金属铁磁性的稳定性

运用第一性原理的全势能线性缀加平面波方法对闪锌矿结构CrSe和CrAs的电子结构进行自旋极化计算.闪锌矿结构CrSe和CrAs处于晶格平衡时都具有半金属性,它们自旋向下的电子能带带隙分别为3.38 eV和1.79 eV,同时,它们的自旋总磁矩分别为4.00和3.00μ_B/formula.自旋总磁矩主要来源于Cr的原子磁矩,Se和As的原子磁矩对总磁矩的贡献很小而且为负值,因而它们具有明显的铁磁性特征.使晶体晶格在±10%的范围内发生各向同性形变,对闪锌矿结构CrSe和CrAs的电子结构进行计算.计算结果表明,当晶格各向同性形变分别为-4%～10%和-2%～10%时,闪锌矿结构CrSe和CrAs仍然保持半金属铁磁性,并且总磁矩都稳定于4.00和3.00μ_B/formula.     

Half-metallicity and onsite Hubbard interaction on d-electronic states: a case study of Fe2NiZ (Z = Al,Ga, Si,Ge) Heusler systems

DFT-based structural optimisations of Fe₂NiZ (Z?=?Al, Ga, Si, Ge) Heusler compounds confirm the stability of these alloys in F-43m phase. While defining the electronic structure, onsite Hubbard approximation scheme for exchange correlations predicted better results than the generalised gradient approximation. Calculated band structure and densities of states together with spin magnetic moments designate the half-metallic character of these alloys. Indirect band gaps, 1.2?eV for Fe₂NiAl, 0.98?eV for Fe₂NiGa, 1.3?eV for Fe₂NiSi and 1.1?eV for Fe₂NiGe in spin-down states are observed. The ferromagnetic spin moments amount to an integral value of 5μB for (Al, Ga) and 6μB for (Si, Ge) systems with a maximum contribution from transition metal atom (Fe). To forecast the possible turnout of the thermopower, Seebeck coefficients, electrical and thermal conductivities are calculated, which directly hints the thermoelectric response of these materials. This study creates a possibility of these alloys in thermoelectrics and spintronics.     

Electronic structure of MnSb

On the basis of a two center tight binding model the recursion method of Haydock et al. was applied to calculate spin polarized and non-polarized electronic densities of states and magnetic moments for the B8₁ crystal structure and a fictitious B32 structure of MnSb. The model implies the itinerant nature of the Mn d-states hybridizing with Sb p-states. For the non-polarized densities of states the self-consistent charge transfer amounts to 0.6 electrons from Mn to Sb with a band center difference of E_p ? E_d = - 0.75 eV. For the magnetic moments, which were also calculated self-consistently, 3.5 μ_B for the d-band and 3.34 μ_B in total were obtained for the B8₁ crystal structure. For the B31 structure bigger moments of 3.67 μ_B and 3.53 μ_B were obtained, correspondingly. The results for MnSb are also discussed with respect to very recent results for MnAs calculated within the same model. The local densities of states as obtained for MnSb are in good agreement with experimental XPS results.     

单层金属卤化物CoX_2(X=Cl、Br、I)可调能隙和磁性研究

本文采用密度泛函理论系统的研究了二维单层金属卤化物CoX_2(X=Cl,Br,I)的结构稳定性、电子性质和磁性质.三种卤化物的束缚能分别是9.01、8.04和6.95 eV,表明Co原子和卤素原子间存在强相互作用.三种材料的能带结构都显示了间接带隙半导体特性.三种材料的总磁矩都是3 μ_B,主要来源于Co原子的磁矩.为了实现对材料物性的调控,我们考虑了双轴应变.发现压缩应变不仅可以显著增强铁磁态的稳定性,还可以实现体系从间接带隙半导体向直接带隙半导体的转变.     

Self-consistent semi-relativistic energy band structure of fcc and tetragonal Ni metal

The electronic structure of paramagnetic and ferromagnetic fcc Ni and tetragonal Ni has been determined by means of self-consistent semi-relativistic linear muffin-tin orbital (LMTO) energy band studies. The paramagnetic studies used the local density exchange-correlation potential of Hedin et al. whereas the ferromagnetic spin-polarized calculations used the local spin-density exchange-correlation potential of Gunnarson et al. The magnetic moments (0.54 μ_B), magnetization density, Fermi contact hyperfine fields and Fermi surface areas are found to agree with experiment and with other theoretical calculations, notably those of Wang and Callaway. For tetragonal Ni, distorted along the [111] direction similar to the local strain of Ni layers between Cu layers in modulated structures, the magnetic moment (0.46 μ_B) and resultant hyperfine fields are reduced substantially from that determined for fcc Ni.     

Calculated magnetic moment of δ-manganese

Spin-density-functional theory is used to calculate the magnetic moment of δ-Mn whose ground state is assumed to be either antiferromagnetic or ferromagnetic. The band structure is given for paramagnetic, antiferromagnetic and ferromagnetic δ-Mn. The magnetic moment of antiferromagnetic δ-Mn is found to be 3μ_B while that of ferromagnetic δ-Mn is 2.7 μ_B. The total energy favors the antiferromagnetic ground state by about 0.3 eV.     

Ab initio study of pressure-induced magnetic transition in manganese pnictides

We report a density functional calculation on the NiAs-type Mn-based pnictides. The total energy as a function of volume is obtained by means of self-consistent tight-binding linear muffin–tin orbital method by performing spin and non-spin polarized calculation. From the present study, we predict a magnetic-phase transition from ferromagnetic (FM) to non-magnetic (NM) around 49 and 35.7 GPa for MnAs and MnSb, respectively. The pressure-induced transition is found to be a second-order transition. The band structure and density of states (DOS) are plotted for FM and NM states. Apart from this the ground-state properties like magnetic moment, lattice parameter and bulk modulus are calculated and are compared with the available results. Under large volume expansion these compounds exist in zinc-blende (ZB) structure, which shows half metallicity. The magnetic moment and equilibrium lattice constants for ZB structure are obtained as well as band structure and DOS are presented.     

Optical and magneto-optical spectroscopy of thin composite GaAs-MnAs layers

Ellipsometric and magneto-optical investigations of MnAs layers and GaAs-MnAs composites obtained by laser deposition have been performed in the energy range 1.0–4.5 eV. The spectra of the equatorial Kerr effect (EKE) and the refractive and absorption indices have been studied. Diagonal and off-diagonal components of the dielectric tensor have been calculated. It is established that the shape of the EKE spectra of MnAs layers depends on the crystallographic orientation of these layers. Features near 1.9, 3, and 4 eV are distinguished in the energy dependences of the interband density of states of the layers; these features correspond to the energies of interband d-p transitions, obtained from calculations of the MnAs band structure.     

Magnetic structures of the rare earth orthotitanites RTiO3; R = Tb,Dy, Tm and Yb

The magnetic structures of the rare earth orthotitanites, RTiO₃, R = Tb, Dy, Tm and Yb, have been solved using neutron powder diffraction techniques.Two different types of magnetic structure have been found. One has the titanium and rare earth moments antiparallel along the c axis. The other structure has the rare earth moments in the ab plane with both ferromagnetic and antiferromagnetic components. In TbTiO₃, the terbium moment of (8.1 ± 0.4)μ_β has ferromagnetic and antiferromagnetic components along the [100] and [010] directions, respectively, with the moments lying at an angle of (36 ± 3)° to the [100] direction. In DyTiO₃, the dysprosium moment of (9.7 ± 0.7)μ_β has ferromagnetic and antiferromagnetic components along the [010] and [100] directions, respectively, with the moments making an angle of (31 ± 5)° to the [010] direction. TmTiO₃ has a thulium moment of (6.0 ± 0.4)μ_β in a ferromagnetic array along the [001] direction. The average titanium moment in the orthotitanites is (0.7 ± 0.3)μ_β in a direction antiparallel to the ferromagnetic component of the rare earth moment. The ytterbium moment in YbTiO₃ is quenched. It is found to be (1.7 ± 0.2)μ_β assuming a moment direction along [001]. The rare earth moment directions are found to be remarkably consistent in the series RMO₃, M = Ti, Cr, Fe and Al.     

Electronic structure and magnetic states of crystalline and fullerene-like forms of nickel dichloride NiCl<Subscript>2</Subscript>

The electronic structure and magnetic properties of the crystalline and fullerene-like forms of nickel dichloride NiCl₂ are investigated in the framework of the local spin density functional theory. It is demonstrated that the band gap can be reproduced in the energy band spectrum of the NiCl₂ compound with inclusion of the magnetic ordering in the calculation of the band structure. The metamagnetic nature of the NiCl₂ dichloride (i.e., the transition from an antiferromagnetic phase to a ferromagnetic phase in a weak magnetic field) is explained in terms of a small difference (0.025 eV/cell) between the total energies of the ferromagnetic and antiferromagnetic phases. Polyhedral three-shell nanoparticles of the NiCl₂ compound exhibit magnetic properties (the magnetic moment of nickel lies in the range 2.0–2.3 μ_B). For isostructural nanoparticles of the FeCl₂ dichloride, the magnetic moment of iron is larger and falls in the range 4.2–4.5 μ_B, whereas nanoparticles of the CdCl₂ dichloride are found to be nonmagnetic. The results of analyzing the interatomic interactions indicate that the composition of fullerene-like nanoparticles of the dichlorides under investigation can deviate from the 1: 2 stoichiometric composition.     

First principles study on the electronic structure and effect of vanadium doping of BN nanowires

The electronic structure and effect of vanadium doping of BN nanowires are studied by first principles calculations. For the pure nanowires, it can be found that B atoms move inwards whereas N atoms move outwards, and BN nanowires have a constant band gap about 4.08 eV with larger diameter. The above-mentioned features are in agreement with those of BN nanotubes. We also find that the pure nanowires become more and more stable with increasing diameter. For V-doped BN nanowires, the V atoms move outwards, and the total energies of pair V-doped BN nanowires indicate that the ferromagnetic ground state, and the electronic structures show half-metallicity. The majority of total spin magnetic moments originate from V atoms, and B atoms which near dopant have a little contribution, while N atoms provide a little reverse magnetic moment. This study may be useful in spintronics and nanomagnets.     

Calculated electronic and magnetic structure of UNi2Al3 and UPd2Al3

The electronic band structure of UNi₂Al₃ and UPd₂Al₃ is calculated in the local spin-density functional approximation using the self-consistent scalar relativistic ASW-method including spin-orbit coupling. The chemical bond is discussed qualitatively and an explanation is given for the small observed magnetic moments ofU in terms of nearly compensating spin and orbital moments. Calculated total energies are used to discuss a number of possible ground state magnetic-moment configurations.     

Electronic structure of disordered Pd3Fe

The electronic structure of disordered Pd₃Fe is studied within an almost self-consistent KKR-CPA procedures. Our starting point are the self-consistent potentials for the ordered ferromagnetic Pd₃Fe obtained by the LMTO method. We perform the ferromagnetic calculation and examine the influence of disorder on the electronic structure of this alloy through the analysis of the Bloch spectral functions and densities of states and compare our results with experiment. We also propose a mechanism for the formation of magnetic moments in ferromagnetic alloys.     

Optical investigations of the clathrate α-Eu<Subscript>8</Subscript>Ga<Subscript>16</Subscript>Ge<Subscript>30</Subscript>

We performed measurements of the optical reflectivity in the energy range 0.007–30 eV on the clathrate-VIII type compound α-Eu₈Ga_{16- x}Ge_{30 x} in order to investigate its electronic band structure. The very low charge carrier concentration as well as ferromagnetic ordering of the divalent Eu ions below 10.5 K characterize the spectra at photon energies below ≃0.4 eV in accordance with the results of band structure calculations. Disorder induced bound states have been identified to affect the optical conductivity at energies between 10 and 100 meV.     

Spin-polarized calculations of electronic structures in ferromagnetic and antiferromagnetic Zn0.75TM0.25Se (TM=Cr,Fe, Co and Ni)

In this work, we aim to examine the spin-polarized electronic band structures, the local densities of states as well as the magnetism of Zn_1−xTM_xSe (TM=Cr, Fe, Co and Ni) diluted magnetic semiconductors in the ferromagnetic (FM) and antiferromagnetic (AFM) phases, and with 25% of TM. The calculations are performed by the developed full-potential augmented plane wave plus local orbitals method within the spin density functional theory. As exchange-correlation potential we used the generalized gradient approximation (GGA) form. We treated the ferromagnetic and antiferromagnetic phases and we found that all compounds are stable in the ferromagnetic structure. Structural properties are computed after total energy minimization. Our results show that the cohesive energies of Zn_0.75TM_0.25Se are greater than that of zinc blende ZnSe. We discuss the electronic structures, total and partial densities of states, local moments and the p–d exchange splitting. Furthermore, we found that p–d hybridization reduces the local magnetic moment of TM and produces small local magnetic moments on the nonmagnetic Zn and Se sites. We found also that in the AFM phase the TM local magnetic moments are smaller than in the FM phase; this is due to the greater interaction of the TM d-up and d-down orbitals.     

Electronic structures and local magnetic moments in ferromagnetic and antiferromagnetic FexRh1−x alloys

Ground-state magnetic properties of ordered FeRh alloys are discussed by using the realistic canonical d-band model within the Hartree-Fock and coherent potential approximations. Local magnetic moments of Fe and Rh atoms in ferromagnetic and antiferromagnetic Fe_xRh_1-x alloys are computed as a function of x, the Fe concentration. Calculated results are in good agreement with the neutron-diffraction data. The band-energy calculation indicates that in Fe-rich alloys excess Fe atoms substituted on Rh sites having larger magnetic moments than Rh atoms, play an important role in the antiferromagnetic to ferromagnetic transition observed at low temperatures.     

Effect of interfacial defects on the electronic and magnetic properties of epitaxial CrAs/InAs and CrAs/CdSe half-metallic multilayers

We present an extended study of single impurity atoms at the interface between the half-metallic ferromagnetic zinc-blende CrAs compound and the zinc-blende binary InAs and CdSe semiconductors in the form of very thin multilayers. Contrary to the case of impurities in the perfect bulk CrAs studied in Galanakis and Pouliasis [J. Magn. Magn. Mater. 321 (2009) 1084] defects at the interfaces do not alter in general the half-metallic character of the perfect systems. The only exception are Void impurities at Cr or In(Cd) sites which lead, due to the lower-dimensionality of the interfaces with respect to the bulk CrAs, to a shift of the p bands of the nearest neighboring As(Se) atom to higher energies and thus to the loss of the half-metallicity. But Void impurities are Schottky-type and should exhibit high formation energies and thus we expect the interfaces in the case of thin multilayers to exhibit a robust half-metallic character.