晶格各向同性应变对闪锌矿结构CrS和CrSe的半金属性和磁性的影响

采用基于密度泛函理论的全势能线性缀加平面波方法,对闪锌矿结构CrS和CrSe的电子结构进行自旋极化计算.闪锌矿相CrS和CrSe处于平衡晶格常数时为半金属性,它们的分子自旋磁矩都为4.00μ_B. 使晶体相对于平衡晶格在±10%的范围内发生各向同性应变,并计算闪锌矿相CrS和CrSe的电子结构.计算结果表明,闪锌矿相CrS和CrSe相对于平衡晶格的各向同性应变分别为-1%–10%和-4%–10%时仍然保持半金属铁磁性,并且分子总磁矩都稳定于4.00μ_B. 关键词： 各向同性应变 电子结构 半金属性 磁性     

Band structure calculations for Heusler phase Co2YBi and half-Heusler phase CoYBi (Y=Mn, Cr)

We have studied the electron structure and magnetic properties of Heusler phase Co₂YBi and half-Heusler phase CoYBi (Y=Mn, Cr) by using the full-potential linearized-augmented plane-wave (FLAPW) method. Co₂MnBi and Co₂CrBi are predicted to be half-metallic magnetism with a total magnetic moment of 6 and 5 μ_B, respectively, well consistent with the Slater-Pauling rule. We also predict CoMnBi to be half-metallic magnetism with a slight compression. The gap origin for Co₂MnBi and Co₂CrBi is due to the 3d electron splitting of Mn (Cr) and Co atoms, and the gap width depends on Co electron splitting. The atom coordination surroundings have a great influence on the electron structure, and consequently the Y site in the X₂YZ structure has a more remarkable electron splitting than the X site due to the more symmetric surroundings. The investigation regarding the lattice constant dependence of magnetic moment shows that the Co magnetic moment exhibits an opposite behavior with the change of the lattice constant for Heusler and half-Heusler alloys, consequently leading to the different variation trends for total magnetic moment. The variation of total and atom magnetic moment versus lattice constant can be explained by the extent of 3d electron splitting and localization of Mn (Cr) and Co atoms for both the series of alloys.     

Spintronic properties of the Ti2CoB Heusler compound by density functional theory

The electronic structure and magnetic properties of the Ti₂CoB Heusler compound with a high-ordered CuHg₂Ti structure were investigated using the self-consistent full potential linearized augmented plane wave (FPLAPW) method within the density functional theory (DFT). Spin-polarized calculations show that the Ti₂CoB compound is half-metallic ferromagnetic with a magnetic moment of 2 μ_B at the equilibrium lattice constant, a=5.74 Å. The Ti₂CoB Heusler compound is ferromagnetic below the equilibrium lattice constant and ferrimagnetic above the equilibrium lattice constant. A large peak in majority-spin DOS and an energy gap in minority-spin DOS are observed at the Fermi level, yielding a spin polarization of 100%. A spin polarization higher than 90% is achieved for a wide range of lattice constants between 5.6 and 6.0 Å.     

Half-metallic ferromagnetism in rocksalt and zinc-blende MS (M=Li, Na and K): A first-principles study

First-principles full-potential linearized augmented plane-wave method is used to investigate the electronic structure and magnetic properties of hypothetical zinc-blende and rocksalt LiS, NaS and KS. We find that all the compounds except rocksalt LiS exhibit half-metallic ferromagnetism with an integer magnetic moment of 1.00 μ_B per formula unit. The ferromagnetism results from the spin-polarization of p states of anion S. Total energies calculations indicate the rocksalt phase is lower in energy than the zinc-blende one. The total energy differences are about 0.38, 0.36 and 0.32 eV per formula unit for LiS, NaS and KS, respectively. Meanwhile, it is shown that rocksalt NaS and KS have the half-metallic gaps of 0.22 and 0.41 eV, respectively, and the half-metallic gaps are 0.03, 0.46 and 0.65 eV for zinc-blende LiS, NaS and KS, respectively. We also find the half-metallicity is robust against the lattice contraction up to 7% and 13% for rocksalt NaS and KS, respectively. Although rocksalt LiS is nonmagnetic and metallic at the equilibrium lattice constant, it shows half-metallic ferromagnetism when the lattice constant is larger than 5.40 Å.     

High spin polarization in ordered Cr3Co with the DO3 structure: A first-principles study

The electronic structure of the highly ordered alloy Cr₃Co with the DO₃ structure has been studied by FLAPW calculations. It is found that the ferrimagnetic state is stable and that the equilibrium lattice constant of Cr₃Co equals 5.77 Å. A large peak in majority spin density of states (DOS) and an energy gap in minority spin DOS are observed at the Fermi level, which results in a high spin polarization of 90% in the ordered alloy Cr₃Co. The total magnetic moment of Cr₃Co is 3.12μ_B, which is close to the ideal value of 3μ_B derived from the Slater-Pauling curve. An antiparallel alignment between the moments on the Cr (A, C) sites and the Cr (B) sites is observed. Finally, the effect of lattice distortion on the electronic structure and on magnetic properties of Cr₃Co compound is studied. A spin polarization higher than 80% can be obtained between 5.55 and 5.90 Å. With increasing lattice constant, the magnetic moments on the (A, C) sites increase and the moments on the (B, D) sites decrease. They compensate each other and make the total magnetic moment change only slightly.     

Effect of Fe substitution on the magnetic properties of half-Heusler alloy CoCrAl

The effect of Fe substitution for the vacant site in half-Heusler alloy CoCrAl is studied. A series of single phase CoFe_xCrAl (x=0.0, 0.25, 0.5, 0.75 and 1.0) alloys has been successfully synthesized. The lattice constant is found to increase almost linearly with increasing Fe content, indicating Fe atoms enter the lattice of CoCrAl instead of existing as a secondary phase. When Fe entering the vacant site, spin polarization occurs and the alloy turns from a semimetal in CoCrAl to a half-metallic ferromagnet (HMF) in CoFeCrAl. This is due to the reconstruction of the energy band with Fe substitution. The Curie temperature and saturation magnetic moments are enhanced and increase monotonically with increasing Fe content. The variation of the spin moment follows the Slater-Pauling curve and agrees with the theoretical calculation as well.     

Theoretical investigation on structural, magnetic and electronic properties of ferromagnetic GdN under pressure

The tight-binding linear muffin tin orbital (TB-LMTO) method within the local density approximation is used to calculate structural, electronic and magnetic properties of GdN under pressure. Both nonmagnetic (NM) and magnetic calculations are performed. The structural and magnetic stabilities are determined from the total energy calculations. The magnetic to ferromagnetic (FM) transition is not calculated. Magnetically, GdN is stable in the FM state, while its ambient structure is found to be stable in the NaCl-type (B₁) structure. We predict NaCl-type to CsCl-type structure phase transition in GdN at a pressure of 30.4 GPa. In a complete spin of FM GdN the electronic band picture of one spin shows metallic, while the other spin shows its semiconducting behavior, resulting in half-metallic behavior at both ambient and high pressures. We have, therefore, calculated electronic band structures, equilibrium lattice constants, cohesive energies, bulk moduli and magnetic moments for GdN in the B₁ and B₂ phases. The magnetic moment, equilibrium lattice parameter and bulk modulus is calculated to be 6.99 μ_B, 4.935 Å and 192.13 GPa, respectively, which are in good agreement with the experimental results.     

Half-metallic properties of Ti2FeSi full-Heusler compound

In this study, the electronic structure and magnetic properties of novel half-metallic Ti₂FeSi full-Heusler compound with CuHg₂Ti-type structure were examined by density functional theory (DFT) calculations. The electronic band structures and density of states of the Ti₂FeSi compound show the spin-up electrons are metallic, but the spin-down bands are semiconductor with a gap of 0.45 eV, and the spin-flip gap is of 0.43 eV. Fe atom shows only a small magnetic moment and its magnetic moment is antiparallel to that of Ti atoms, which is indicative of ferrimagnetism in Ti₂FeSi compound. The Ti₂FeSi Heusler compound has a magnetic moment of 2 μ_B at the equilibrium lattice constant a=5.997 Å.     

First principles study of half-metallic ferromagnetism in Cr-doped CdTe

We report a systematic study of the structural, electronic and magnetic properties of Cr-doped CdTe for various Cr concentrations x (=0.25, 0.5, 0.75 and 1.0) using first principles calculations based on the density functional theory (DFT). The electronic band structure of the alloy has been calculated using the Wu-Cohen (WC) as well as the Angel-Vosko (EV) generalized gradient approximation (GGA) for the exchange-correlation potential. The analysis of the density of states (DOS) curves shows the half-metallic ferromagnetic character with half-metallic gap more than 0.52 eV. While the origin of half-metallic ferromagnetism is explained, the band structure calculations are used to determine s (p)-d exchange constants N₀α (conduction band) and N₀β (valence band) that agree with typical magneto-optical experiment. It is found that the p-d hybridization reduces the magnetic moment of Cr from its free space charge value and produces small magnetic moments on the Cd and Te sites. Lastly, we discuss the robustness of half-metallicity with respect to the variation of lattice constants of the Cr_xCd_1−xTe alloys.     

A first-principles study on the lower-valence coexisting Cr2TiX (X=Al, Ga, Si, Ge, Sn, Sb) Heusler alloys

The electronic, magnetic, and bonding properties of the Cr₂TiX (X=Al, Ga, Si, Ge, Sn, Sb) Heusler alloys have been investigated using first-principles calculations. The results show that Cr₂TiSb exhibits a half-metallic nature and Cr₂TiGa and Cr₂TiSn exhibit a nearly half-metallic nature. From analysis of the density of states and the electron density difference along the Ga→Sn→Sb series for sp atoms, we found that the Cr-Ti bond demonstrates covalent character with more or less the ionic and metallic nature. In addition, the Cr-Ti bonding strength increases along this series. All the compounds have a negative total magnetic moment, most of which are confined to the Cr atoms. There exists a 1.0μ_B increasing trend of the total moment along the III→IV→V main group for sp atoms, and only the total moment of Cr₂TiSb coincides well with the Slater-Pauling behavior.     

Ab initio prediction of half-metallic ferromagnetism in Zn(TM)O2 (TM=Cr,Mn, Fe,Co, Ni) compounds

From the results of first principles tight-binding linear muffin-tin orbital (TB-LMTO) calculations, half-metallic ferromagnetism is proposed in Zn(TM)O₂ with a chalcopyrite structure. The calculated electronic and magnetic property shows that consistent with the integer value for the total magnetic moment, half metallicity is obtained for ZnCrO₂, ZnMnO₂, ZnFeO₂, ZnCoO₂ and ZnNiO₂. A careful analysis of the spin density reveals the ferromagnetic coupling between the p–d states and the cation dangling-bond p states, which is believed to be responsible for the stabilization of the ferromagnetic phase. The calculated heat of formation, bulk modulus and cohesive energy are reported.     

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.     

Ab-initio investigation of electronic properties and magnetism of half-Heusler alloys XCrAl (X=Fe, Co, Ni) and NiCrZ (Z=Al, Ga, In)

The electronic structures and magnetism of the half-Heusler alloys XCrAl (X=Fe, Co, Ni) and NiCrZ (Z=Al, Ga, In) have been investigated to search for new candidate half-metallic materials. Here, we predict that NiCrAl, and NiCrGa and NiCrIn are possible half-metals with an energy gap in the minority spin and a completely spin polarization at the Fermi level. The energy gap can be attributed to the covalent hybridization between the d states of the Ni and Cr atoms, which leads to the formation of bonding and antibonding peaks with a gap in between them. Their total magnetic moments are 1μ_B per unit cell; agree with the Slater-Pauling rule. The partial moment of Cr is largest in NiCrZ alloys and moments of Ni and Al are in antiferromagnetic alignment with Cr. Meanwhile, it is also found that FeCrAl is a normal ferromagnetic metal with a magnetic moment of 0.25μ_B per unit cell and CoCrAl is a semi-metal and non-magnetic.     

Half-metallic ferrimagnetism in the Ti2CoAl Heusler compound

Density functional calculations performed on the Ti₂CoAl Heusler compound confirm it to be a half-metallic ferrimagnet with the spin-down energy gap of 0.49 eV. The Ti₂CoAl Heusler compound has a magnetic moment of 2 μ_B at the equilibrium lattice constant a=6.14 Å. The Ti₂CoAl Heusler compound is ferrimagnetic and maintains the half-metallic character having 100% polarization for lattice constants ranging between 5.85 and 6.44 Å.     

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.     

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.     

Half-metallic ferromagnetism in I2-VI compounds with non-magnetic dopants

First-principles calculations based on the tight-binding linear muffin-tin orbital (TB-LMTO) method were performed to investigate the occurrence of spin polarization in the alkali metal oxides (M₂O) [M: Li, Na, K, Rb] in antifluorite (anti- CaF₂-type) structure with non-magnetic (N, P, As, Sb and Bi) dopants. The calculations reveal that non-magnetic substitutional doping at anion site can induce stable half-metallic ferromagnetic ground state in I₂-VI compounds. Total energy calculations show that the antifluorite ferromagnetic state is energetically more stable than the antifluorite non-magnetic state at equilibrium volume. Ground state properties such as equilibrium lattice constant and bulk modulus were calculated. The calculated magnetic moment is found to be 1.00 μ_B per dopant atom. The magnetic moment is mainly contributed by p orbitals of dopant atom.     

Electronic and magnetic structure studies of double perovskite Sr2CrReO6 by first-principles calculations

First-principles calculations have been performed to study the electronic band structure and ferromagnetic properties of the double perovskite Sr₂CrReO₆. The density of states (DOS), the total energy, and the spin magnetic moment were calculated. The calculations reveal that the Sr₂CrReO₆ has a stable ferromagnetic ground state and the spin magnetic moment per molecule is 1.0 μ_B, in good agreement with the experimental value. By analysis of the band structure, we propose that the ordered double perovskite Sr₂CrReO₆ is a strong candidate for half-metallic ferromagnet.     

第一性原理研究闪锌矿结构MnSb和MnBi半金属性和磁性的稳定性

采用基于第一性原理的全势能线性缀加平面波方法对闪锌矿结构MnSb和MnBi的电子结构进行自旋极化计算。闪锌矿结构MnSb和MnBi处于晶格平衡时都是半金属性的,并且它们自旋向下电子能带带隙分别是1.32eV 和1.27eV。闪锌矿结构MnSb和MnBi的自旋总磁矩都为4.00μB/formula,总磁矩主要来源于Mn的原子磁矩,Sb和Bi的原子磁矩对总磁矩的贡献很小而且为负值,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MnSb和MnBi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-1 % ~ 10 %和-4 % ~10 %时,闪锌矿结构MnSb和MnBi仍然保持半金属铁磁性,并且总磁矩都稳定于4.00μB/formula.     

Half-metallic ferromagnetism in (C, Si, Ge, Sn and Pb)-doped I2-VI compounds: An ab initio study

First-principles calculations were performed to investigate the stability, electronic structure and magnetism in Group IV elements-doped alkali-metal oxides (M₂O) [M: Li, Na, K, Rb] in antifluorite structure using the linear muffin-tin orbital method in its tight-binding representation (TB-LMTO). The calculations reveal that non-magnetic dopants can induce stable half-metallic ferromagnetic ground state in I₂-VI compounds. Total energy calculations show that the ferromagnetic state is energetically more stable than the non-magnetic state at equilibrium volume. Ground state properties such as equilibrium lattice constant and bulk modulus were calculated. The magnetic moment is found to be 2.00 μ_B per dopant atom.