Electronic structure and magnetic properties of Gd3Co11B4 compound

The electronic structure of hexagonal Gd₃Co₁₁B₄ compound has been studied by X-ray photoemission spectroscopy (XPS) and ab initio self-consistent tight binding linear muffin tin orbital (TB LMTO) method. We have found a good agreement between the experimental XPS valence band spectra and theoretical LMTO calculations. Results showed that the Gd₃Co₁₁B₄ compound is ferrimagnetic with the calculated total magnetic moment M=14.29 μ_B/f.u. The values of the magnetic moments on Co atoms strongly depend on the local environment. We have also compared the electronic structure and magnetic properties of Gd₃Co₁₁B₄ compound with those of Nd₃Co₁₁B₄ compound.     

Electronic structure and magnetism of disordered bcc Fe alloys

We study the electronic structure and magnetic properties of disordered bcc Co_xFe_1-x, Cr_xFe_1-x and Mn_xFe_1-x alloys in their ferromagnetic phases using the Augmented Space Recursion (ASR) technique coupled with the tight-binding linearized muffin tin orbital (TB-LMTO) method. We calculate the density of states and magnetic moment of these alloys to show the variation upon alloying Fe with the other neighbouring 3d transition metals using arguments based on charge transfer, exchange splitting and hybridization effects. Received 10 April 2001 and Received in final form 15 August 2001     

Electrical resistivity of the Ti\mathsf{_4}O\mathsf{_7} Magneli phase under high pressure

The electronic and positronic properties of the pentanary semiconductor alloys Ga_xIn_1-xP_ySb_zAs_1-y-z lattice matched to GaSb have been studied. The electron wave function is calculated semiempirically using the pseudopotential band model under the virtual crystal approximation. The positron wave function is evaluated under the point core approximation for the ionic potential. Electronic and positronic quantities namely, electronic structure and band gaps, positron band structure, effective mass and affinity, and electron-positron momentum densities have been predicted and their dependence on the phosphorus composition has been discussed. Received 30 August 2002 / Received in final form 12 February 2003 Published online 24 April 2003     

Effect of the main-group elements on the electronic structures and magnetic properties of Heusler alloys Mn2NiZ (Z=In, Sn, Sb)

The magnetic properties and electronic structure of Mn₂NiZ (Z=In, Sn, Sb) have been studied. The magnetic structure of these alloys is mainly determined by the main-group element Z instead of the distance between the Mn atoms. Electronic structure calculations suggest that Mn₂NiIn and Mn₂NiSn are both ferrimagnets with antiparallel alignment between the Mn moments. But this antiferromagnetic coupling is weakened by the increasing number of valence electrons of the Z atoms. When it comes to Mn₂NiSb, a ferromagnetic coupling between the Mn atoms is observed. Mn₂NiSn and Mn₂NiSb have been synthesized successfully. Their M_s at 5 K agree well with the theoretical value.     

Electronic structure of LiMnO : X-ray emission and photoelectron spectra and band structure calculations

The electronic structure of LiMnO₂ and Li₂MnO₃ was studied by means of X-ray photoelectron and soft X-ray emission spectroscopy. For LiMnO₂, LSDA and LSDA+U calculations were carried out. The LSDA+U calculations are in rather good agreement with the measured valence-band structure as well as with the magnetic and electrical properties of LiMnO₂. It is shown that the band gap in LiMnO₂ is determined by the charge-transfer effect. Received 15 March 1999 and Received in final form 14 July 1999     

Observation of superdeformed states in 88Mo

High-spin states in ⁸⁸Mo were studied using the Gammasphere germanium detector array in conjunction with the Microball CsI(Tl) charged-particle detector system. Three γ-ray cascades with dynamic moments of inertia showing similar characteristics to superdeformed rotational bands observed in the neighbouring A= 80 region have been identified and assigned to the nucleus ⁸⁸Mo. The quadrupole moment of the strongest band, deduced by the Residual Doppler Shift Method, corresponds to a quadrupole deformation of β₂≈ 0.6. This confirms the superdeformed nature of this band. The experimental data are interpreted in the framework of total routhian surface calculations. All three bands are assigned to two-quasi-particle proton configurations at superdeformed shape. Received: 20 May 1999 / Revised version: 25 August 1999     

Effect of Cr on the electronic structure of Co3Al intermetallic compound: A first-principles study

The effect of doping with Cr on the electronic structure and magnetism of Co₃Al has been studied by density functional calculations. It has been found that the Cr atom has a strong site preference for the B-site in Co₃Al. With the substitution of Cr for Co, the total densities of states (DOS) change obviously: A DOS peak appears at E_F in the majority spin states and an energy gap is opened in the minority spin states. The effect of Cr in Co₃Al is mainly to push the antibonding peak of the Co (A,C) atoms high on the energy scale and to form the energy gap around E_F, and also to contribute to the large DOS peak at E_F in the majority spin direction. The calculations indicate a ferromagnetic alignment between the Co and Cr spin moments. The calculated total magnetic moment decreases and becomes closer to the Slater–Pauling curve with increasing Cr content. This is mainly due to the decrease of the Co (A,C) spin moments. At the same time, the moments of Co (B) and Cr (B) only change slightly.     

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.     

Crystal and magnetic structures of the oxyphosphates MFePO 5 (M = Fe, Co, Ni, Cu). Analysis of the magnetic ground state in terms of superexchange interactions

We have studied in detail the crystal and magnetic structures of the oxyphosphates MFePO₅ (M: divalent transition metal) using neutron powder diffraction as a function of temperature. All of them are isomorphic to the mixed valence compound α-Fe₂PO₅ with space-group Pnma. No disorder exists between the two metallic sites. The M²⁺O₆ octahedra share edges between them and faces with Fe³⁺O₆ octahedra building zigzag chains running parallel to the b-axis that are connected by PO₄ tetrahedra. The topology of this structure gives rise to a complex pattern of super-exchange interactions responsible of the observed antiferromagnetic order. The magnetic structures are all collinear with the spin directed along the b-axis except for M = Co. The experimental magnetic moments of Cu⁺² and Ni²⁺ correspond to the expected ionic value, on the contrary the magnetic moment of Fe³⁺ is reduced, probably due to covalence effects, and that of Co²⁺ is greater than the spin-only value indicating a non negligible orbital contribution. Using numerical calculations we have established a magnetic phase diagram adapted for this type of crystal structure and determined the constraints to be satisfied by the values of the exchange interactions in order to obtain the observed magnetic structure as the ground state. Received 15 December 2000 and Received in final form 25 June 2001     

Observation of magnetic splitting in XPS MnL-spectra of Co2MnSn and Pd2MnSn Heusler alloys

The XPS MnL-spectra of Co₂MnSn, a nearly half-metallic ferromagnet (HMF) and Pd₂MnSn were investigated. The most drastical feature of the spectra observed is the well-defined magnetic splitting of the Mn 2p _3/2 , 2p _1/2 lines. This gives direct evidence of the existence of well-defined local magnetic moments in Heusler alloys in comparison with other itinerant-electron ferromagnets. The calculations of Mn2p XPS spectra of these materials were carried out using a fully relativistic generalization of the one-step model of photoemission and show excellent agreement with experiment. Received: 31 October 1997 / Received in final form: 28 November 1997 / Accepted: 18 December 1997     

Electronic structure calculations of Fe-rich ordered and disordered Fe-Al alloys

Tight Binding Linear Muffin-Tin Orbital (TB-LMTO) electronic calculations are presented for the magnetic and structural properties of ordered and disordered FeAl alloys. The total energy, bulk modulus, lattice parameter and magnetic moments of B2, D03 and B32 ordered structures and A2 disordered structure were calculated for different compositions. The different structures are obtained by varying the position of Fe and Al atoms in a BCC superstructure. In this way, we examine the order-disorder transition that takes place in these alloys. Disordered alloys present both larger Fe magnetic moment and lattice parameter than ordered ones. In this work comparison of the calculated quantities with available experimental results is provided and it can be concluded that the results are in quantitative agreement with the experimental trends. Received 7 May 2002 / Received in final form 20 September 2002 Published online 4 February 2003 RID="a" ID="a"e-mail: eaf@we.lc.ehu.es     

The electronic structure and magnetism of GdSi2 by first-principles study

We have investigated electronic and magnetic properties of hexagonal, tetragonal, and orthorhombic GdSi₂, using the full-potential linearized augmented plane-wave method based on general gradient approximation for exchange-correlation potential. Antiferromagnetic (AFM) states of the GdSi₂ are found from total energy calculations to be energetically more stable, compared to ferromagnetic (FM) states in all of the considered present crystal structures. It is in good agreement with an experimental result. The calculated magnetic moments of valence electrons of the Gd atoms are 0.16, 0.14, and 0.14 μ_B for hexagonal, tetragonal, and orthorhombic crystal structures in AFM states, respectively, and the Si atoms are coupled antiferromagnetically to the Gd atoms irrespective of crystal structure even though their magnitudes are negligible.     

Structures and stability of Al<Subscript><Emphasis Type="Bold">7</Emphasis></Subscript>C and Al<Subscript><Emphasis Type="Bold">7</Emphasis></Subscript>N clusters

We report results of the atomic and electronic structures of Al₇C cluster using ab initio molecular dynamics with ultrasoft pseudopotentials and generalized gradient approximation. The lowest energy structure is found to be the one in which carbon atom occupies an interstitial position in Al₇ cluster. The electronic structure shows that the recent observation [Chem. Phys. Lett. 316, 31 (2000)] of magic behavior of Al₇C^- cluster is due to a large highest occupied and lowest unoccupied molecular orbital (HOMO-LUMO) gap which makes Al₇C^- chemically inert. These results have further led us to the finding of a new neutral magic cluster Al₇N which has the same number of valence electrons as in Al₇C^- and a large HOMO-LUMO gap of 1.99 eV. Further, calculations have been carried out on (Al₇N)₂ to study interaction between magic clusters. Received 28 July 2001     

Structural and vibrational properties of C36 and its oligomers (C36) M = 2, 3, 4 by tight-binding molecular dynamics

Non-orthogonal tight-binding molecular-dynamics is employed to calculate structural and vibrational properties of C₃₆ and its oligomers (C₃₆) _{M = 2, 3, 4} . The lowest energy configuration of the C ₃₆ cage is confirmed to have D _6h symmetry. For the dimer, too, the D _2h structure reported in the literature is found. The vibrational spectrum is identified with the power spectrum of the displacement autocorrelation function. Additional vibrational properties are extracted from the dynamical matrix. For the monomer, fair agreement with available ab initio calculations is achieved, with comparatively smaller deviations in the Raman-frequencies than for published semi-empirical calculations. The features of the vibrational modes are correlated with the structural properties of the oligomers. Received 24 November 2000 and Received in final form 24 August 2001     

Electronic structure and lattice properties of zinc-blende InN under high pressure

Dependencies of electronic structure and lattice properties of InN with zinc-blende structure on hydrostatic pressure are presented based on band structures computed using the empirical pseudopotential method. The pressure behavior of the pseudopotential form factors have been analyzed. The effect of pressure on the density of states has been examined. Trends in bonding and ionicity under pressure are also discussed. Our results show as well that the absolute value of the Fourier transform of the valence charge density might be useful in the prediction of the phase transition in zinc-blende materials. Received 25 May 2001 and Received in final form 16 January 2002     

Influence of local environment on electronic properties of Co atoms in the Tm3Co11B4 compound

The electronic structure of the Tm₃Co₁₁B₄ compound has been studied by X-ray photoemission spectroscopy and ab initio self-consistent tight binding linear muffin tin orbital (TB LMTO) method. This compound crystallizes in the hexagonal Ce₃Co₁₁B₄-type structure (P6/mmm). We have found a good agreement between the experimental XPS valence band spectra and theoretical ab initio calculations. The calculated total magnetic moment is equal to 13.635 μ_B/f.u. The magnetic moments on the Co atoms are antiparallel to the moments of the Tm atoms. Their values are depended on the local environment, especially on the number of the Co neighbors. The theoretical results are compared with other calculations, saturation magnetization measurements as well as neutron diffraction data for R₃Co₁₁B₄ (R=Y, Nd, Gd, Tb).     

Tight-binding study of structural and electronic properties of silver clusters

Tight-binding model is developed to study the structural and electronic properties of silver clusters. The ground state structures of Ag clusters up to 21 atoms are optimized by molecular dynamics-based genetic algorithm. The results on small Ag_n clusters (n = 3-9) are comparable to ab initio calculations. The size dependence of electronic properties such as density of states, s-d band separation, HOMO-LUMO gap, and ionization potentials are discussed. Magic number behavior at Ag₂, Ag₈, Ag₁₄, Ag₁₈, Ag₂₀ is obtained, in agreement with the prediction of electronic ellipsoid shell model. We suggest that both the electronic and geometrical effect play significant role in the coinage metal clusters. Received 7 August 2000     

Magnetic properties of ErFe6Fa6 studied by magnetization and neutron diffraction

Neutron-diffraction experiments reveal that ErFe₆Ga₆ forms in the tetragonal ThMn₁₂-type of structure (space group I₄/mmm). The Fe sublattice orders ferromagnetically below K. The Er moments order antiparallel to the Fe moments which, below about 250 K, leads to a decrease of the total magnetization. The easy magnetization direction of ErFe₆Ga₆ is perpendicular to the c-axis in the whole temperature range. Refinement at 2 K shows that ErFe₆Ga₆ orders ferrimagnetically with Er moments of 8.5 (2) and Fe moments at the 8(j) site of 1.9 (1) and at the 8(f) site of 1.7 (1) , respectively. At room temperature, ErFe₆Ga₆ exhibits the same type of magnetic order, however with substantially lower Er moments of 1.0 (4) and Fe moments at the 8(f) site of 1.2 (2) . The Fe moments at the 8(j) site amount to 1.9 (5) /Fe. Received 24 November 1999 and Received in final form 27 March 2000     

Electronic structure of LaNi and its hydride LaNi H

The electronic structure of LaNi₅ and its hydride LaNi₅H₇ are obtained using the self-consistent cluster-embedding calculation method. The Fermi level of LaNi₅H₇ is 5.172 eV higher than that of LaNi₅. In both materials, the La 5d electrons locate nearby the Fermi levels, and make only a small contribution to the density of states (DOS) of the valence bands. There is no significant charge transfer from La to Ni in LaNi₅. But for LaNi₅H₇, there is a charge transfer of 1.16 electrons from La to H, and H atoms are combined mainly with Ni to form hybridized orbitals in the energy regions far below the Fermi level. An explanation of hydrogenation of LaNi₅ is proposed: It is easy for hydrogens to take off some localized La 5 d electrons near the Fermi level, and combine with Ni to form hybridized orbitals in lower energy regions. This process is therefore in favor of energy, and forces a lattice expansion until the Fermi level rises to zero. Received 13 July 2001 / Received in final form 10 December 2001 Published online 17 September 2002     

The influence of the dynamics of ionic multiplets onto electronic transport properties of heavy-fermion systems: a semi-phenomenological approach

We present calculations of the electronic transport properties of heavy-fermion systems within a semi-phenomenological approach to the dynamical mean field theory. In this approach the dynamics of the Hund's rules 4f (5f )-ionic multiplet split in a crystalline environment is taken into account. Within the scope of this calculation we use the linear response theory to reproduce qualitative features of the temperature-dependent resistivity and hall conductivity, the magneto-resistivity and the thermoelectric power typical for heavy-fermion systems. The model calculations are directly compared with experimental results on CeCu ₂ Si ₂. Received 30 June 2000 and Received in final form 15 December 2000