Spin-polarized Generalized Gradient Approximation Research Articles

DFT-GGA study of NO adsorption on the LaO (001) surface of LaFeO3

In order to demonstrate the adsorption of the nitrogen monoxide molecule (NO) on the LaO (001) surface of LaFeO3, we perform simulations based on density functional theory. The generalized gradient approximation (GGA) for the exchange-correlation energy functional indicates that the electronic state of the LaFeO3 bulk is an anti-ferromagnetic insulator with a local magnetic moment of 4.1 μB at each Fe atom. Using the ultrasoft pseudo-potential method with spin-polarized GGA, fully optimized internal parameters as well as charge and spin density are determined for the NO-adsorbed structure prepared in a slab model. The calculated adsorption energy of NO is around −1.4eV on the LaO (001) surface of LaFeO3. This value decreases down to −4.46eV at an oxygen vacancy site, where the nitrogen atom of NO is embedded in the 1st LaO layer forming a bond with Fe in the 2nd FeO layer.

Geometrical structure and spin order of Gd13 cluster

The spin-polarized generalized gradient approximation to the density-functional theory has been used to determine the lowest energy structure, electronic structure, and magnetic property of Gd(13) cluster. Our results show that the ionic bonding is combined with the covalent characteristics in stabilizing the Gd cluster. The ferrimagnetic icosahedron is found to be the lowest energy configuration, in which the centered Gd atom couples antiferromagnetically with the rest Gd atoms surrounding it. No spin non-collinear evidence has been detected in our calculations. It is identified that the local magnetic moments of Gd atom are about 8 μ(B) regardless of geometrical structure. Finally, the comprehensive electronic structure analyses show that the indirect long-range magnetic coupling between the polarized 4f is mediated by the polarization of 5d, 6s, and 6p conduction electrons, which is the typical Ruderman-Kittel-Kasuya-Yosida interactions.

Ab initio lattice dynamics of Ni 2MnX (X = Sn, Sb) magnetic shape memory alloys

In this study, we present the results of first principles calculations of elastic constants and phonon properties of nickel–manganese based magnetic shape memory compounds Ni 2MnSn and Ni 2MnSb in stoichiometric composition. The plane wave basis sets and pseudopotential method within spin-polarized generalized gradient approximation ( σ-GGA) scheme of the density functional theory is applied. In investigation of the phonon dispersion spectra, linear response technique of the Density Functional Perturbation Theory is used. Phonon softening is observed in dispersion spectra at the transverse acoustic mode (TA 2) in [ ζ ζ 0] direction as an indication of the structural instability of these systems to shear deformation. The vibrational instability of Ni 2MnSb system is larger than that of Ni 2MnSn yielding negative phonon frequencies. This vibrational anomaly is also verified by the low shear modulus and large elastic anisotropy ratio. The minority spin Fermi surfaces of both systems exhibit strong nesting features.

First-principles study of elastic and vibrational properties of Ni2MnIn magnetic shape memory alloys

We present the results of ab initio calculations of lattice dynamics and the second order elastic stiffness constants of nickel-based magnetic shape memory alloy Ni2MnIn in stoichiometric composition. The plane wave basis sets and pseudopotential method within spin-polarized generalized gradient approximation (σ-GGA) scheme of the density functional theory (DFT) is applied. Elastic constants are calculated by tetragonal and monoclinic isochoric strains on cubic L21 structure. The calculated elastic constants agree very well with the recent ultrasonic experimental data. Phonon dispersion spectra are investigated within linear response technique of the density functional perturbation theory (DFPT). A vibrational anomaly is observed in phonon spectra at the transverse acoustic mode (TA2) in [ζζ0] direction at wavevector ζ = 0.3 as an indication of the structural instability of the system to shear deformation. This anomaly is also verified by the low shear modulus and large elastic anisotropy ratio. Phonon dispersion curves are in excellent agreement with the results of recent neutron diffraction experiments.

Half-metallic magnetism of Co 2CrX (X=As, Sb) Heusler compounds: An ab initio study

In this study, we present the electronic, magnetic, and structural properties of two novel half-metallic full-Heusler compounds, Co 2CrAs and Co 2CrSb, in cubic L2 1 geometry. The calculations are based on the density functional theory within plane-wave pseudopotential method and spin-polarized generalized gradient approximation of the exchange-correlation functional. The electronic band structures and density of states of the systems indicate half-metallic behavior with vanishing electronic density of states of minority spins at Fermi level, which yields perfect spin polarization. The calculated magnetic moments of both systems in L2 1 structure are 5.00 μ B , which are largely localized on the chromium site. The energy gaps in minority spin states are restricted by the 3d-states of cobalt atoms on two different sublattices. The formation enthalpies for both structures are negative indicating stability of these systems against decomposition into stable solid compounds.

Electronic structure and surface properties of cubic perovskite oxide BaMnO3

We present the electronic, magnetic, and structural properties of the cubic perovskite oxide BaMnO3 in both bulk and surface geometry. BaMnO3 is reported as keeping the cubic phase even at low temperatures. The calculations are based on the density functional theory (DFT) within plane-wave pseudopotential method and spin-polarized Generalized Gradient Approximation (GGA) of the exchange–correlation functional. The systems studied are treated in ferromagnetic order. The structures of electronic bands and density of states of the systems show half-metallic behavior in both bulk and BaO- and MnO2-terminated (001) surfaces of E21 structure. The calculated magnetic moment of bulk structure is 3.00μB, which is largely conserved at surface geometries. Average surface and relaxation energies are also calculated. The rumpling of atoms in relaxed surfaces is determined. It is seen that the relaxation of oxygen relative to metal ion is always in outward direction for both terminations indicating a positive rumpling.

Analysis of spin-density wave conductivity spectra of iron pnictides in the framework of density functional theory

The optical conductivity of LaFeAsO, ${\text{BaFe}}_{2}{\text{As}}_{2}$, ${\text{SrFe}}_{2}{\text{As}}_{2}$, and ${\text{EuFe}}_{2}{\text{As}}_{2}$ in the spin-density wave (SDW) state is investigated within density functional theory (DFT) in the framework of spin-polarized generalized gradient approximation (GGA) and $\text{GGA}+U$. We find a strong dependence of the optical features on the Fe magnetic moments. In order to recover the small Fe magnetic moments observed experimentally, $\text{GGA}+{U}_{\text{eff}}$ with a suitable choice of negative on-site interaction ${U}_{\text{eff}}=U\ensuremath{-}J$ was considered. Such an approach may be justified in terms of an overscreening which induces a relatively small $U$ compared to the Hund's rule coupling $J$, as well as a strong Holstein-type electron-phonon interaction. Moreover, reminiscent of the fact that $\text{GGA}+{U}_{\text{eff}}$ with a positive ${U}_{\text{eff}}$ is a simple approximation for reproducing a gap with correct amplitude in correlated insulators, a negative ${U}_{\text{eff}}$ can also be understood as a way to suppress magnetism and mimic the effects of quantum fluctuations ignored in DFT calculations. With these considerations, the resulting optical spectra reproduce the SDW gap and a number of experimentally observed features related to the antiferromagnetic order. We find electronic contributions to excitations that so far have been attributed to purely phononic modes. Also, an orbital-resolved analysis of the optical conductivity reveals significant contributions from all $\text{Fe}\text{ }3d$ orbitals. Finally, we observe that there is an important renormalization of kinetic energy in these SDW metals, implying that the effects of correlations cannot be neglected.

Elastic properties of Ni 2MnGa from first-principles calculations

Elastic properties of Ni 2MnGa in both austenitic and martensitic structures are determined by using ab initio methods based on density functional theory (DFT) within the spin-polarized generalized-gradient approximation. The tetragonal shear elastic constant C′ takes a very small value in the austenitic phase, indicating the elastic instability results in a phase transition to martensitic structure. Isotropic mechanical properties such as bulk modulus, shear modulus, Young's modulus and Poisson's ratio are predicted. The trend of the Debye temperatures calculated for three structures of Ni 2MnGa is comparable with that of the experiment.

Electronic structure of half-metallic ferromagnet Co2MnSi at high-pressure

In this study, first principles calculation results of the half-metallic ferromagnetic Heusler compound Co2MnSi are presented. All calculations are based on the spin-polarized generalized gradient approximation (σ-GGA) of the density functional theory and ultrasoft pseudopotentials with plane wave basis. Electronic structure of related compound in cubic L21 structure is investigated up to 95 GPa uniform hydrostatic pressure. The half-metal to metal transition was observed around ~70 GPa together with downward shift of the conduction band minimum (CBM) and a linear increase of direct band gap of minority spins at Γ-point with increasing pressure. The electronic density of states of minority spins at Fermi level, which are mainly due to the cobalt atoms, become remarkable with increasing pressure resulting a sharp decrease in spin polarization ratio. It can be stated that the pressure affects minority spin states rather than that of majority spins and lead to a slight reconstruction of minority spin states which lie below the Fermi level. In particular, energy band gap of minority spin states in equilibrium structure is obviously not destroyed, but the Fermi level is shifted outside the gap.

GGA and GGA+ U calculations of the ground state of SrMnO 3

A very recent experimental report suggests the ground state of SrMnO 3 is orthorhombic with space group C222 1. I perform spin-polarized generalized gradient approximation (GGA) calculations with/without on-site Coulomb interaction included (GGA+ U) for this ground state of SrMnO 3. In contrast to the latest theoretical report of cubic and hexagonal SrMnO 3 where GGA method is considered to be able to reproduce the insulating behavior of SrMnO 3, I find that for the orthorhombic SrMnO 3, GGA method is not enough to reproduce its insulating behavior. When the on-site Coulomb interaction is included by means of GGA+ U, the insulating character is recovered. The results show that correlation interaction plays an important role in the ground state of SrMnO 3.

Electronic structure and magnetic coupling in CaV2O5: spin dimer versus spin ladder

The electronic structure and magnetic exchange interactions of the ladder vanadate CaV2O5 have been studied by ab initio electronic structure calculations based on density functional theory (DFT). Geometry optimization and electronic structure calculations are performed using spin-polarized generalized gradient approximation (GGA) exchange-correlation functionals for four possible spin-ordered states. The experimentally observed insulating behavior has been reproduced successfully in the framework of the band theory by considering the magnetic ordering. Calculated results reveal that the true magnetic ground state of CaV2O5 is the antiferromagnetic (AFM) state with AFM exchange interactions both inside the rungs and along the ladder legs. Calculated exchange parameters indicate that the ladder structural vanadate CaV2O5 should be described as weakly coupled dimer system rather than as spin ladder compound. The AFM interactions inside the dimer are crucial to the insulating ground state and magnetic characteristics of CaV2O5.

First principles electronic structure calculations of Co2CrBi Heusler system

First principles calculation results of a new full Heusler system Co2CrBi in stoichiometric composition were presented. The calculations are based on the density functional theory (DFT) within the spin-polarized generalized gradient approximation (σ-GGA) and plane wave pseudopotential method. The system shows nearly half-metallic behavior with very low electronic density of states of minority spins at Fermi level yielding high spin polarization ratio R=0.96. The total magnetic moment of the system was calculated as 5.05μB, which is largely localized at chromium site with μCr=3.35μB. The electronic character of the compound is determined by the 3d electronic states of cobalt and chromium atoms. f-Electronic states of bismuth atom were also included in calculations although it has no distinct effect on electronic structure. The system can be labelled as a new half-metallic material for technological applications.

Adjusting magnetic moments of Sc13and Y13clusters by doping different X atom (X = Na, Mg, Al, Si, P)

We have investigated the structural and magnetic properties of the doped XM12 and charged M13 (X = Na, Mg, Al, Si, P; M = Sc, Y) clusters using the density-functional theory with spin-polarized generalized gradient approximation. It was found that doped atoms can induce significant change of the magnetic moments of Sc13 and Y13 clusters. The total magnetic moments of the NaM12, MgM12, AlM12, SiM12, and PM12 clusters are regular 5, 6 (12), 7, 8, and 9 μb, respectively (but 19 μb for Sc13 and Y13, 12 μb for Y, 18 μb for Sc, Sc, and Y). The doped atom substituting the surface atom of the plausible icosahedral configuration is viewed as the ground-state structure of the XM12 (X = Na, P; M = Sc, Y) and MgSc12 clusters. While for XM12 (X = Al, Si; M = Sc, Y) and MgY12 clusters, the doped atom occupying the central position of the icosahedral configuration is viewed as the ground-state structure. The doping and the charging both enhance the stability of the Sc13 and Y13 clusters. These findings should have an important impact on the design of the adjustable magnetic moments systems. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

First-principles study on the electronic structures of the ladder compound NaV2O4F

The electronic structures of ladder structural compound NaV2O4F are studied by first-principles calculations with pseudo-potential plane-wave method and spin-polarized generalized gradient approximation (GGA) based on density functional theory (DFT). Four possible spin-ordered states are simulated and the calculated results reveal that the magnetic ground state of NaV2O4F is the antiferromagnetic (AFM) state with AFM interactions both inside the rungs and along the ladder legs. The insulating behavior is successfully simulated with a band gap of about 1.0eV. According to crystal-field theory, the dxy orbitals of V atoms located in the VO4F pyramids have the lowest energy and are split from other d orbitals. The covalent interaction on the rungs becomes weak with the presence of F-ions. Using the calculated total energies for the various spin-ordered states of NaV2O4F, the spin exchange coupling constants are fit out with Noodleman's broken symmetry method. The calculated results indicate that there are ferromagnetic(FM) interactions between the ladders with competitive strength to that on the rungs and thereby NaV2O4F may not be a spin-ladder material.

First-principles investigation of the electronic properties of multiferroic BaCoF4

The electronic properties of multiferroic BaCoF4 with ferroelectric antiferromagnetic phase and paraelectric phase are calculated using density functional theory with spin-polarized generalized-gradient approximation and plane wave pseudopotentials. It is found that anti-ferromagneticism probably favors to the ferroelectric stability at low temperature and the ion bond interaction is the main interation in the BaCoF4 system due to the strong electronegativity of F atoms. As to the CoF6 octahedron, there is entirely ionic bond between Co ion and F（2） or F（3） ions （bc plane）, but weak covalent bond between Co ion and F（1） ion and still weaker for F（4） ion. The ferroelectric distortion is only induced by relative displacement of Ba ion and F ion along c axis, and F（4） displacement contributes least to ferroelectric phase transformation . In addition, the energy of s and p orbitals of F（2） or F（3） ion is lower in the ferroelectric phase than in the centor-symmetric phase and the covalence character of F（1） ion, whose contribution to the displacement is the largest, is almost lost, which stabilizes the structure of the ferroelectric system.

First-principles investigation on the role of ions in ferroelectric transition of BiFeO3

The electronic properties of multiferroic BiFeO3 with paraelectric paramagnetic phase at high-temperature and ferroelectric antiferromagnetic phase are calculated using density functional theory with spin-polarized generalized-gradient approximation (GGS) and plane wave pseudopotentials. study of Born effective charges shows that the greatest contributor to the ferroelectric distortion is Bi atoms' displacement. We obtained very large theoretical electric polarization, which agrees very well with the results of thin-film test. The calculation shows that BiFeO3 ground state has G type anti-ferromagnetic order and the theoretical magnetic moments of Fe ion accord with experimental values. After ferroelectric phase transformation, the chemical bond changes much due to Bi-6s and Bi-6p state charge transfer, and the effect of Bi-6s electrons becomes more pronounced. Partial density of states calculation indicates that the energy split between bonding orbit of Bi-6p state and anti-bonding orbit is bigger than that between other electronic states, by which covalent bond between Bi-6p state and O-2p state is strengthened, which is the origin of off-center displacement. Thus the discussion about the BiMnO3 is testified. Bi-6s state is polarized due to the static electric repulsion, but does not take part in the hybridization of Bi-6p and O-2p electrons. The charge transfer originates from its weak covalent interaction with the O-2s, 2p orbits, by which the Bi-6s state electronic polarization is reinforced. This kind of covalent interaction helps the relative replacement of Bi-O, which is the reason for the strong systemic electric polarization.

Magnetism and phase transitions of iron under pressure

The spin-polarized generalized gradient approximation within the plane-wavepseudopotential density functional theory is employed to investigate the magnetism andphase transition of iron under pressure. It is found that iron has a ferromagneticbody-centered-cubic (bcc) ground state, while at high pressure (such as at theEarth’s lower mantle and core pressure), the most stable phase is the nonmagnetichexagonal-close-packed (hcp) phase. For the face-centered-cubic (fcc) iron, we find thatthere is an intermediate-spin state (IS) during the transformation from the high-spin state(HS) to the low-spin (LS) state under pressure. The transition pressures of the HS IS and the IS LS are about 15 GPa and 50 GPa, respectively. The magnetism can affect the properties ofiron up to 72.9 GPa. From the enthalpy difference between every two phases, we find thephase transition pressures of FM-bcc FM-hcp, FM-bcc NM-hcp and NM-bcc NM-hcp are 14.4 GPa, 29.5 GPa and 42.7 GPa, respectively.

Electronic structure of the weakly coupled edge-sharingMnO4 spin- chain compound LiMnVO4: an ab initio study

Based on fully self-consistent first-principles density functional theory(DFT) calculations as well as classical spin analysis, we study the electronicstructure and magnetic interactions of the spinel-related compoundLiMnVO4. Four possible ordered spin states have been considered by spin-polarized generalizedgradient approximation (GGA) calculations. The antiferromagnetic (AFM) configurationwith both intra-chain and inter-chain AFM coupling interactions is energetically favorableamong these magnetic ordered states. The calculated AFM solution agrees well witha series of experimental measurements. The intra-atomic exchange splitting ofthe Mn 3d spin-up and spin-down states results in the insulating behavior ofLiMnVO4. The Heisenberg Hamiltonian is used to deduce the magnetic coupling parametersby adopting Noodleman’s broken symmetry method. The intra-chain AFMinteractions are much stronger than the inter-chain AFM interactions and thusLiMnVO4 can be described as a weakly coupled edge-sharing spin chain system. We propose that the presence of the side groups of edge-sharingLiO4 and VO4 tetrahedra are contributing to the intra-chain AFM interactions in the nearly90° Mn–O–Mn bond configuration of the edge-sharingMnO4 chains.

First-principles investigation on extraction of lithium ion from monoclinic LiMnO 2

Changes with Li extraction in the crystal structure and electronic structure of monoclinic LiMnO 2 have been investigated by first-principle calculations using spin-polarized generalized gradient approximation method. It is found that the Li extraction has changed the oxidation state of Mn ion from 3+ to 4+ and thereby induced a phase transition from the monoclinic structure to rhombohedral symmetry, which opens channels for the migration of Mn ions from their own octahedral sites to the interlayer Li vacancies. It is the migration of Mn ions that responds for the phase transition to a spinel-like structure and the severe capacity loss during the electrochemical cycles of monoclinic LiMnO 2.

Electronic properties of bilayered manganiteCa2.5Sr0.5GaMn2O8from first-principles calculations

By means of first-principles calculations using spin-polarized generalized gradient approximation method and ultrasoft pseudopotentials, the electronic and magnetic structure of the recently synthesized orthorhombic phase of bilayer manganite ${\text{Ca}}_{2.5}{\text{Sr}}_{0.5}{\text{GaMn}}_{2}{\text{O}}_{8}$ compound is obtained. Our calculations have shown that the specific antiferromagnetic ordering, in accordance with experimental findings, is the most stable one. The total energies calculated for the several possible magnetic states of the title compound in the orthorhombic phase enable us to estimate spin exchange interactions for nearest ${J}_{nn}=\ensuremath{-}1.60\text{ }\text{meV}$ and next\penalty1000\-\allowhyphens{} nearest neighbors ${J}_{nnn}=\ensuremath{-}0.21\text{ }\text{meV}$. Calculated Curie--Weiss temperature in the mean-field approximation is in excellent agreement with the measured magnetic phase transition temperature.