Electronic and magnetic properties of Fe<Subscript>2</Subscript>SiC

The electronic structure and magnetic properties of Fe₂SiC compound have been studiedusing the framework of an all-electron full-potential linearized augmented-plane wave(FP-LAPW) method within the local density (LSDA) and + U corrected(LSDA + U)approximations. An antiferromagnetic spin ordering of Fe atoms is shown to be the groundstate for this compound. From the electronic band structures and density of states (DOS),Fe₂SiC has ametallic character and from the analysis of the site and momentum projected densities, itis deduced that the bonding is achieved through hybridization of Fe-3d with C-2p states andFe-3d withSi-3pstates. It is also pointed out that the Fe-C bonding is more covalent than Fe-Si. In theFM phase, the spin polarized calculations indicate that the total magnetic moment ofFe₂SiC increasesfrom 0.41 to 4.33μ _B when the Hubbard U parameter for iron isconsidered.     

The influence of imperfection of the crystal lattice on the electrokinetic and magnetic properties of disordered titanium monoxide

The conductivity and magnetic susceptibility of disordered titanium monoxide TiO_y (0.920≤y≤1.262) containing vacancies in titanium and oxygen sublattices are investigated. For TiO_y monoxides with an oxygen content y≤1.069, the temperature dependences of the conductivity are described by the Bloch-Grüneisen function at a Debye temperature ranging from 400 to 480 K and the temperature dependences of the magnetic susceptibility are characterized by the contribution from the Pauli paramagnetism due to conduction electrons. The behavior of the conductivity and magnetic susceptibility of TiO_y monoxides with an oxygen content y≥1.087 is characteristic of narrow-gap semiconductors with nondegenerate charge carriers governed by the Boltzmann statistics. The band gap ΔE between the valence and conduction bands of TiO_y monoxides with y≥1.087 falls in the range 0.06–0.17 eV.     

Nonbonding oxygen holes and spinless scenario of magnetic response in doped cuprates

Both theoretical considerations and experimental data point to a more complicated nature of the valence hole states in doped cuprates than is predicted by the Zhang-Rice model. Actually, we deal with a competition of a conventional hybrid \({\text{Cu}} {\text{3}}d-{\text{O}} {\text{2}}p b{1g} \propto d{x^2-y^2} \) state and purely oxygen nonbonding state with e _u x, y∝p _{x, y} symmetry. The latter reveals a nonquenched Ising-like orbital moment that gives rise to a novel spinless purely oxygen scenario of the magnetic response in doped cuprates with the oxygen localized orbital magnetic moments of the order of tenths of Bohr magneton. We consider the mechanism of ^{63, 65}Cu-O 2p transferred orbital hyperfine interactions due to the mixing of the oxygen O2p orbitals with Cu3p semicore orbitals. Quantitative estimates point to a large magnitude of the respective contributions to both the local field and electric field gradient, and their correlated character.     

Electronic structure and correlations of vitamin B<Subscript>12</Subscript> studied within the Haldane-Anderson impurity model

We study the electronic structure and correlations of vitamin B₁₂ (cyanocobalamine) by using theframework of the multi-orbital single-impurity Haldane-Anderson model of atransition-metal impurity in a semiconductor host. The parameters of the effectiveHaldane-Anderson model are obtained within the Hartree-Fock (HF) approximation. Thequantum Monte Carlo (QMC) technique is then used to calculate the one-electron andmagnetic correlation functions of this effective model. We observe that new states forminside the semiconductor gap found by HF due to the intra-orbital Coulomb interaction atthe impurity 3d orbitals. In particular, the lowest unoccupiedstates correspond to an impurity bound state, which consists of states from mainly the CNaxial ligand and the corrin ring as well as the Co e_g-like orbitals. We alsoobserve that the Co?(3d) orbitals can develop antiferromagneticcorrelations with the surrounding atoms depending on the filling of the impurity boundstates. In addition, we make comparisons of the HF+QMC data with the density functionaltheory calculations. We also discuss the photoabsorption spectrum of cyanocobalamine.     

Features of Electronic Structure of Intermetallic Compounds CeNi<Subscript>4</Subscript>M (M = Fe,Co, Ni,Cu)

The evolution of the electronic structure of CeNi₄M (M = Fe, Co, Ni, Cu) intermetallics depending on the type of nickel substitutional impurity is explored. We have calculated band structures of these compounds and considered options of substituting one atom in nickel 3d sublattice in both types of crystallographic positions: 2c and 3g. The analysis of total energy self-consistent calculations has shown that positions of 2c type are more energetically advantageous for single iron and cobalt impurities, whereas a position of 3g type is better for a copper impurity. The Cu substitutional impurity does not change either the nonmagnetic state of ions or the total density at the Fermi level states. Fe and Co impurities, on the contrary, due to their considerable magnetic moments, induce magnetization of 3d states of nickel and cause significant changes in the electronic state density at the Fermi level.     

Hierarchy of percolation thresholds and the mechanism for reduction of magnetic moments of transition metals intercalated into TiSe<Subscript>2</Subscript>

The concentration dependences of the effective magnetic moment of transition metal atoms intercalated into TiSe₂ are analyzed in the framework of the percolation theory. It is shown that, depending on the degree of localization of impurity states, the effective magnetic moment is determined by the overlap of 3d orbitals of transition metals or orbitals of titanium atoms coordinated by impurity atoms.     

Electrical and Magnetic Properties of Hafnium Disulfide Intercalated with Iron Atoms

Simultaneous study of the dependences of the structural parameters, electrical, and magnetic properties of hafnium disulfide intercalated iron atoms in the dependence on the intercalate concentration and temperature has been performed for the first time. The temperature dependences of the electrical resistance are shown to exhibit the activation character with the activation energies characteristic of impurity conduction. The effective magnetic moments of iron ions in Fe _x HfS₂ is found to be significantly lesser than the values of free iron ions and to decrease as the iron content increases. The character of the temperature dependences of the effective magnetic moments and negative values of the paramagnetic Curie temperatures indicate possible interactions of the antiferromagnetic type between intercalated atoms. However, the dependences of the magnetization on field for Fe_0.33HfS₂ and Fe_0.5HfS₂ obtained at T = 2 K demonstrate the hysteresis phenomenon characteristic of the ferromagnetic state. The results are discussed assuming the existence of hybridization 3d electron states of intercalated iron atoms with the electronic states of HfS₂ matrices and the competition of various exchange interaction.     

Electronic Structure and Magnetic Phase Transition in Helicoidal Fe<Subscript>1 - <Emphasis Type="Italic">x</Emphasis></Subscript>Co<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Si Ferromagnets

LSDA + U + SO calculations of the electronic structure of helicoidal Fe_{1 - x}Co _x Si ferromagnets within the virtual crystal approximation have been supplemented with the consideration of the Dzyaloshinski-Moriya interaction and ferromagnetic fluctuations of the spin density of collective d electrons with the Hubbard interactions at Fe and Co atoms randomly distributed over sites. The magnetic-state equation in the developed model describes helicoidal ferromagnetism and its disappearance accompanied by the occurrence of a maximum of uniform magnetic susceptibility at temperature T _C and chiral fluctuations of the local magnetization at T > T _C . The reasons why the magnetic contribution to the specific heat at the magnetic phase transition changes monotonically and the volume coefficient of thermal expansion (VCTE) at low temperatures is negative and has a wide minimum near T _C have been investigated. It is shown that the VCTE changes sign when passing to the paramagnetic state (at temperature T _S ).     

Hyperfine interactions in Sc<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Y<Subscript>x</Subscript>Fe<Subscript>2</Subscript> cubic Laves alloys

Effects of hybridization of 3d bands of iron with 3d bands of scandium and 4d bands of yttrium in Sc_1?xY_xFe₂ cubic Laves alloys (0≤_x≤1) are studied by the nuclear magnetic resonance method. The concentration dependences of the lattice parameters a, saturation magnetization σ, and hyperfine fields at the ⁵⁷Fe, ⁴⁵Sc, and ⁸⁹Y nuclei—as well as the ²⁷Al impurity nuclei, whose atoms substitute iron atoms in the lattices of these alloys—are measured. The “local” and “induced” contributions to hyperfine fields at the ⁵⁷Fe nuclei are separated and the magnetic moments at iron atoms are estimated. It is found that the hybridization effect leads to the formation of magnetic moments at Sc and Y atoms (whose direction is opposite to the direction of the magnetic moment at iron atoms) and is responsible for the ferrimagnetic structure in Sc_1?xY_xFe₂ alloys.     

Electronic structure and spectra of CuO

We report the electronic structure of monoclinic CuO as obtained from first principles calculations utilizing density functional theory plus effective Coulomb interaction (DFT + U) method. In contrast to standard DFT calculations taking into account electronic correlations in DFT + U gave antiferromagnetic insulator with energy gap and magnetic moment values in good agreement with experimental data. The electronic states around the Fermi level are formed by partially filled Cu 3d _x²?y² orbitals with significant admixture of O 2p states. Theoretical spectra are calculated using DFT + U electronic structure method and their comparison with experimental photoemission and optical spectra show very good agreement.     

Equilibrium magnetic and orbital states of the manganites with four manganese atoms in the unit cell

The carrier energy spectrum and the total energy of various magnetic and orbital crystal-structure configurations of the manganites R _1?x A _xMnO₃ (R=La, Pr, Nd, Sm, etc.; A=Ca, Sr, Ba) with four manganese atoms in the unit cell have been calculated for the electron doping region x>0.5. The equilibrium magnetic and orbital configurations of the model are determined by minimizing the total energy of the system with respect to the angles θ _i ^s , φ_i, and θ _i ^o , which define the directions of the local manganese magnetic moments and the type of orbital mixing of the e _g electrons in the manganites. Assuming the parameters of the Heisenberg exchange interaction to be 0.018t<J _AFM<0.022t, the Hund exchange interaction to be J _H=2.5t, and the Jahn-Teller splitting to be Δ=1.5ty, the model with four manganese atoms in the unit cell predicts the experimentally observed magnetic phase alternation sequence G-C-A with increasing doping level y=1?x. For the values J _AFM<0.018t and y<0.28, this model allows the existence of a collinear phase H not observed earlier.     

Influence of particle size,stoichiometry, and degree of long-range order on magnetic susceptibility of titanium monoxide

In situ measurements of the magnetic susceptibility of ordered and disordered titanium monoxides TiO_y in the temperature range from 300 to 1200 K have revealed that it depends on the size of crystals, their stoichiometry, and long-range order parameters. Analysis of the data for both the ordered and disordered TiO_y has demonstrated that the dependence of the Van Vleck paramagnetism on the nanocrystal size is inversely proportional due to the breaking of symmetry of the local environment of titanium and oxygen atoms near the surface of nanocrystals. It has been found that the Van Vleck contribution from the atomic vacancy disorder in monoxide nanocrystals of superstoichiometric composition, as well as in the crystalline stoichiometric monoxide, is proportional to the deviation of the degree of long-range order from the maximum value.     

Electronic structure of Gd-doped MgO

The electronic structure of Gd-doped MgO is investigated using the LSDA+U (local spin density approximation with U-correction) method and compared with the MgO structure. The total density of states obtained accounting for the correlation effects in the 4f shell of gadolinium is found to be formed by the oxygen 2p states at the valence band and the 4f gadolinium occupied states, while the conduction band is represented by a mixture of empty electronic states. Magnetic properties of the calculated Gd-doped MgO are found to be formed solely by the Gd-4f-magnetic moment of about 7μ_B, in good agreement with recent experimental results suggesting a ferromagnetic coupling of the local magnetic moments induced by Gd.     

Ab initio calculations of B<Subscript>2</Subscript> type RHg (R = Ce,Pr, Eu and Gd) intermetallic compounds

Spin polarized ab initio calculations have been carried out to study the structural, electronic, elastic and thermal properties of RHg (R = Ce, Pr, Eu and Gd) intermetallic compounds in B₂ structure. The calculations have been performed by using both generalized gradient approximation (GGA) and local spin density approximation (LSDA). The calculated value of lattice constant (a ₀) for these compounds with GGA is in better agreement with the experimental data than those with LSDA. Bulk modulus (B), first-order pressure derivative of bulk modulus and magnetic moment (μ _B ) are also presented. The energy band structure and electron density of states show the occupancy of 4f states for light as well as heavy rare earth atom. The elastic constants are predicted from which all the related mechanical properties like Poisson’s ratio (σ), Young’s modulus (E), shear modulus (G _H ) and anisotropy factor (A) are calculated. The ductility or brittleness of these compounds is predicted from Pugh’s rule (B/G _H ) and Cauchy pressure (C ₁₂ ? C ₄₄). The Debye temperature (θ _D ) is estimated from the average sound velocity, which have not been calculated and measured yet.     

Band structure and the magnetic and elastic properties of SrFeO<Subscript>3</Subscript> and LaFeO<Subscript>3</Subscript> perovskites

The band structure and the magnetic and elastic characteristics of SrFeO₃ and LaFeO₃ perovskites with ferromagnetic and antiferromagnetic collinear spin configurations (of the A, C, and G types) are investigated using the ab initio pseudopotential method (the VASP program package) with the inclusion of the single-site Coulomb correlations (the LSDA + U formalism). It is shown that, in the pressure range 0–50 GPa, the most stable states are the ferromagnetic metal state for the SrFeO₃ compound and the antiferromagnetic insulator state of the G type for the LaFeO₃ compound.     

Electronic Structure and Exchange Interactions in RNi<Subscript>4</Subscript>Co (R = Eu,Yb) Compounds

The electronic structure and the exchange interactions in EuNi₄Co and YbNi₄Co compounds have been calculated in terms of a theoretical approach with the inclusion of electronic correlations (LSDA + U method); the variants of substitution of cobalt ion for nickel in the 3d lattice in both types of crystallographic positions 2c and 3g are considered. The total energies obtained in self-consistent calculations show that individual cobalt impurities are more preferably arranged in position of the 3g type. A Co ion in RNi₄Co (R = Eu, Yb) is characterized by a significant magnetic moment, which leads to significant increase in the exchange interaction of Co and Ni ions in the 3d metal sublattice.     

Structural,optical, electronic,and magnetic properties of Ag-Cu bimetallic clusters: a density functional theory study

In this study, the structural, optical, electronic, and magnetic properties of Ag_mCu_n (m?+?n?=?3 to 6) bimetallic clusters were systematically investigated by density functional theory in the theoretical framework of the generalized gradient approximation exchange-correlation functional. The results show that the ground state structures of these clusters are planar structures, with triangular geometries for three-atom Ag-Cu clusters, rhombic geometries for four-atom Ag-Cu clusters, trapezoids for five-atom Ag-Cu clusters, and triangular geometries for six-atom Ag-Cu clusters. The Ag₂Cu₂, Ag₂Cu₃, and Ag₃Cu₃ clusters are the geometric magic clusters for four-, five-, and six-atom Ag-Cu clusters, respectively. As the number of Cu atoms increases, the vertical ionization potential values of the four- to six-atom Ag-Cu clusters increase, while the vertical electron affinity values of the three- to five-atom Ag-Cu clusters decrease. Compared to pure Ag clusters, the main absorption peaks of the Ag-Cu clusters of the same number of atoms appear to blueshift. The even-numbered clusters exhibit no magnetic moments, while the odd-numbered clusters exhibit large magnetic moments of 1.00 μ_B. The magnetic moments of these Ag-Cu clusters are believed to be related to the atom sites.     

Dependence of Van-Vleck paramagnetism on the size of nanocrystals in superstoichiometric TiO<Subscript> <Emphasis Type="Italic">y</Emphasis> </Subscript>

In situ measurements of the magnetic susceptibility of titanium monoxide nanocrystals with superstoichiometric composition TiO_y (y > 1) in the 300–1200 K temperature range showed that this value depends not only on the structural state of a sample, but also on the size of crystals. Analysis of data obtained for both ordered and disordered TiO_y showed that the Van-Vleck paramagnetism is inversely proportional to the nanocrystal size because of breakage of the symmetry of local environment of the near-surface atoms of titanium and oxygen. The Van-Vleck paramagnetism contribution due to atomic-vacancy disorder in superstoichiometric titanium monoxide nanocrystals, as well as in the stoichiometric composition, is proportional to a deviation of the degree of long-range order from its maximum value.     

Synthesis,structure, and properties of BIFENBVOX

Solid solutions Bi₄V_2-x Fe _x/2Nb _x/2O_11-δ (х?=?0.05–1.0) and Bi₄V_2-х-y Fe _x Nb _y O_11–δ (with fixed x or y?=?0.2 and variable х or y?=?0.2–0.5 with step 0.1) were synthesized by the standard ceramic technology in the temperature range 773–1113 K and by mechanochemical activation method using Bi₂O₃, V₂O₅ Fe₂O₃, and Nb₂O₅ oxides as initial compounds. The formation of solid solutions was studied. Ranges of stability and temperature values of phase transitions for different polymorphous modifications were defined using dylatometric and thermo gravimetric studies. The morphology and the local chemical composition of the ceramic samples were studied. Samples with concentration of dopants x?>?0.3 contain two phases; both major and impurity phases are solid solutions of the BIFENBVOX type although the dopants atoms distribution between them is random. The thermal expansion coefficients (TEC) were measured. The electrical conductivity of ceramic samples was investigated in a wide range of temperatures. The highest conductivity values among the studied solid solutions are observed for the sample with a small amount of dopants x?=?0.25.     

The Influence of Copper Impurity on the Electronic Structure and Optical Properties of TmNi<Subscript>5</Subscript> Compound

Investigations of the electronic structure and optical properties of TmNi_5–хСu_x (х = 0, 1, 2, 3) compounds were performed. Self-consistent calculations of the band spectrum have been performed in the local electron-spin-density approximation with a correction on strong electron interactions within the LSDA + U method. Total and partial densities of electron states associated with thulium, nickel, and copper atoms have been calculated. Optical conductivity, the behavior of which is interpreted for each compound taking into account the performed calculations, has been measured by the ellipsometry method in a wide wavelength range of 0.22–16 μm. Concentration dependences of plasma and relaxation frequencies of conduction electrons have been determined.