Relation between chemical bonding and exchange coupling approaches to the description of ordering in itinerant magnets

Two different approaches to explain and predict the types of magnetic ordering in the 3d metal series and their compounds are reviewed. According to the crossing theorem of Heine and Samson, the effective exchange coupling changes sign from negative (antiferromagnetic ordering) in the middle of 3d band to positive (ferromagnetic ordering) for the nearly empty or nearly filled d band cases. On the other hand, the analytical properties of the Crystal Orbital Hamilton Population, which is a measure of chemical bonding, predict only one crossing at the center of the band in the region of nonbonding states. Thus intermetallic compounds with Fermi energies falling within metal-metal nonbonding states are ordered antiferromagnetically whereas they order ferromagnetically when the Fermi levels fall within antibonding states. The general character of these dependencies is demonstrated for various examples containing the magnetically active 3d metals, examples that include the bcc metals, Heusler alloys, and a series of novel quaternary intermetallic borides.     

Directly probing the hybrid bonding of styrene on Cu111

The interfacial electronic structure of chemisorbed styrene on Cu(111) was successfully investigated with two-photon photoemission spectroscopy. We observed unoccupied states 3.5 eV above the Fermi level and occupied states 2.0 eV below the Fermi level. Polarization results reveal that the occupied and unoccupied states arise from bonding and antibonding orbitals formed by hybridization of copper (surface state and d-band orbitals) and styrene (pi1* and pi2* orbitals).     

Ferromagnetism in Transition Metals: A Chemical Bonding Approach

Band structure calculations from first principles provide the basis of a simple explanation for the appearance of ferromagnetism in iron, cobalt, and nickel that is founded upon a uniquely chemical concept: bonding. It is shown that the onset of ferromagnetism strengthens the metal–metal bonds in these transition metals by reducing the antibonding nearest neighbor interactions that would otherwise appear at the Fermi level.     

Electronic structure and magnetism in transition metals doped 8-hydroxy-quinoline aluminum

We report the room-temperature ferromagnetism in transition metals (Co, Ni)-doped 8-hydroxy-quinoline aluminum (Alq3) by thermal coevaporation of high purity metal and Alq3 powders. For 5% Co-doped Alq3, a maximum magnetization of approximately 0.33 microB/Co at 10 K was obtained and ferromagnetic behavior was observed up to 300 K. The Co atoms interact chemically with O atoms and provide electrons to Alq3, forming new states acting as electron trap sites. From this, it is suggested that ferromagnetism may be associated with the strong chemical interaction of Co atoms and Alq3 molecules.     

Fe原子吸附对单层WS_2结构和性质的影响

本文采用基于密度泛函理论(DFT)的第一性原理方法研究了Fe原子吸附对单层WS_2结构和性质的影响。研究结果表明:Fe原子吸附在W原子的顶位最稳定,相应的原子吸附能为1.84 eV。Fe与衬底间的相互作用削弱了紧邻W―S键,使其键长增大0.011 nm。由于衬底原子的影响,Fe原子d轨道的电子重新分布,形成了2μB左右的局域原子磁矩。在低覆盖度下(0.125和0.25 ML),磁性作用以超交换作用为主,铁磁序不稳定。而在高覆盖度下(0.5和1.0 ML),Fe原子间距减小,磁性作用以RKKY作用为主,铁磁序稳定。电子结构的计算结果显示,在高覆盖度下,Fe/WS_2结构在费米能级处的电子自旋极化率等于100%。自旋向上与向下通道分别为间接带隙的半导体和金属。在1.0 ML覆盖度下,自旋向上的禁带宽度约为0.94 eV。这说明Fe原子吸附可以将直接带隙的WS_2半导体转变成半金属,形成一种潜在的自旋电子器件材料。     

Two-dimensional superdegeneracy and structure-magnetism correlations in strong ferromagnet, Mn2Ga5

The ferromagnetic intermetallic compound Mn2Ga5 has been synthesized and characterized by single-crystal X-ray diffraction study and by magnetic property measurements. The compound, with the Mn2Hg5-type structure, exhibits a saturated magnetic moment of 2.71 muB per formula unit with T(C) approximately = 450 K. The electronic structure of the compound was analyzed by employing FPLO, LMTO, and the extended Hückel tight-binding calculations. The nonmagnetic electronic structure of Mn2Ga5 reveals remarkably flat degenerate (superdegenerate) bands in the vicinity of the Fermi level but only in the a*b*-plane of the Brillouin zone. The resulting high density of the states at the Fermi level, DOS(E(F)), is consistent with the Stoner condition for the observed itinerant electron ferromagnetism. Detailed orbital analysis shows an intriguing structure-magnetism correlation, as the superdegeneracy is found to be a consequence of the unique atomic arrangement and bond angles in the structure.     

How deeply are core electrons perturbed when valence electrons are spin polarized? The case study of transition metal compounds

The ferromagnetic and antiferromagnetic wave functions of the KMnF₃ perovskite have been evaluated quantum-mechanically by using an all electron approach and, for comparison, pseudopotentials on the transition metal and the fluorine ions. It is shown that the different number of α and β electrons in the d shell of Mn perturbs the inner shells, with shifts between the α and β eigenvalues that can be as large as 6 eV for the 3s level, and is far from negligible also for the 2s and 2p states. The valence electrons of F are polarized by the majority spin electrons of Mn, and in turn, spin polarize their 1s electrons. When a pseudopotential is used, such a spin polarization of the core functions of Mn and F can obviously not take place. The importance of such a spin polarization can be appreciated by comparing (i) the spin density at the Mn and F nuclear position, and then the Fermi contact constant, a crucial quantity for the hyperfine coupling, and (ii) the ferromagnetic–antiferromagnetic energy difference, when obtained with an all electron or a pseudopotential scheme, and exploring how the latter varies with pressure. This difference is as large as 50% of the all electron datum, and is mainly due to the rigid treatment of the F ion core. The effect of five different functionals on the core spin polarization is documented.     

Unusual onset of p-element magnetization in a two dimensional structure

Based on density functional theory electronic and magnetic structure characterizations an unusual onset of spin polarization of p states is demonstrated leading to a stable ferromagnetic order within a carbon layered honeycomb-like compound. Specifically structural relaxation of formerly studied C₂N in 3D network and devised here in 2D layered AlB₂-type derived structure shows that the resulting ordered compound maintains the hexagonal crystal symmetry with an exceptionally large c/a ratio leading to strong localization of N states along c and letting magnetization develop within N-p_z orbitals with 1.1 μ_B per formula unit. Anisotropic antibonding interactions between C and N layers allow interpreting the results. The compound is energetically characterized in ferromagnetic ground state versus less stable anti-ferromagnetic order.     

Comparative electronic band structure study of the intrachain ferromagnetic versus antiferromagnetic coupling in the magnetic oxides Ca3Co2O6 and Ca3FeRhO6

In the (MM'O6)infinity chains of the transition-metal magnetic oxides Ca3MM'O6 the MO6 trigonal prisms alternate with the M'O6 octahedra by sharing their triangular faces. In the (Co(2O6)infinity chains of Ca3Co2O6 (M = M' = Co) the spins are coupled ferromagnetically, but in the (FeRhO6)infinity chains of Ca3FeRhO6 (M = Fe, M' = Rh) they are coupled antiferromagnetically. The origin of this difference was probed by carrying out spin-polarized density functional theory electronic band structure calculations for ordered spin states of Ca3Co2O6 and Ca3FeRhO6. The spin state of a (MM'O6)infinity chain determines the occurrence of direct metal-metal bonding between the adjacent trigonal prism and octahedral site transition-metal atoms. The extent of direct metal-metal bonding in the (Co2O6)infinity chains of Ca3Co2O6 is stronger in the intrachain ferromagnetic state than in the intrachain antiferromagnetic state, so that the intrachain ferromagnetic state becomes more stable than the intrachain antiferromagnetic state. Such a metal-metal-bonding-induced ferromagnetism is expected to occur in magnetic insulators and magnetic metals of transition-metal elements in which direct metal-metal bonding can be enhanced by ferromagnetic ordering. In the (FeRhO6)infinity chains of Ca3FeRhO6 the ferromagnetic coupling does not lead to a strong metal-metal bonding and the adjacent spins interact by the Fe-O...O-Fe super-superexchange, hence leading to an antiferromagnetic coupling.     

Density-Functional Theory of Vibrations in Ni<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>V<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Clusters

We have performed the calculation of the vibrational frequencies, Fermi energy and binding energy for several clusters of Ni and vanadium atoms by using the first principles. The calculations are performed by using the density-functional theory in the local-density approximation with spin polarized orbitals. The calculation of vibrational frequencies shows that some of the clusters have positive vibrational frequencies which describe the oscillations of the stable clusters. The negative vibrational frequencies indicate that these clusters are instable with respect to these vibrations when no energy of this frequency is supplied. We find that for vanadium concentration less than 11.1% the clusters of Ni and V atoms are not stable. Hence ferromagnetism in Ni is predicted below 11.1% vanadium. We find the vibrational frequencies of several clusters for which the vanadium concentration is more than 11.1%. We are able to find a phase transition by use of quantum mechanics alone without the use of classical mechanical variables or thermodynamic variables such as temperature.     

Magnetic coupling and intermetallic electron transfer in the heterodinuclear bioctahedral complexes MW(III)Cl(9)(n-) (M = V(II), Cr(III), Mn(IV)): tweaking the balance between ferromagnetism and antiferromagnetism

Density functional theory (DFT) calculations have been used to investigate the effect of intermetallic electron transfer on the mode of magnetic coupling in the face-shared bimetallic complexes MWCl(9)(n-) (M = V, Cr, Mn; all with a nominal d(3) valence electronic configuration on each metal atom). These calculations illustrate a simple rule: when the oxidation state of M is lower than that of W, antiferromagnetic coupling is preferred, while ferromagnetism (via crossed exchange pathways) is favored when M has the higher oxidation state. This underlying trend in intermetallic interactions is seen to depend on the interplay among ligand field splitting, spin polarization splitting of alpha- and beta-spin orbitals, and the relative energies of the M and W valence d orbitals, and is mirrored in the results seen in a wider survey of mixed-metal, face-shared complexes.     

Reaction-induced magnetic transition in Mn2 dimers

The magnetic coupling between Mn atoms in Mn(2) dimers embedded in a rare gas matrix is antiferromagnetic but undergoes ferromagnetic transition at a higher temperature or when ionized to the Mn(2)(+) state. By use of density functional theory and hybrid functional for exchange-correlation potentials, we show that ferromagnetic transition can also be induced when Mn(2) reacts with Cl and/or BO(2). Because of their highly electronegative character, both Cl and BO(2) draw electrons from the Mn(2) dimer leaving it in a positively charged state. The resulting shrinkage in the Mn-Mn bond brought about by the removal of an antibonding electron causes the magnetic transition. We further show that the coupling between Mn atoms remains ferromagnetic when two Mn(2)Cl units are allowed to interact with each other. The ability to induce a magnetic transition through a chemical reaction provides a way to synthesize new magnetic materials.     

A SCF Xα MO study of the optical and Ti2p core photoemission spectra of TiCl4

SCF Xα MO calculations on the ground state and optical excitation transition states of TiCl₄ accurately predict the energies of its UV absorption peaks. Calculations on the Ti2p core ion state and associated transition states indicate that the recently observed low energy (4.0 eV) Ti2p satellite arises from ligand to metal charge transfer excitations while the satellite at high energy (9.4 eV), similar to those previously observed in Ti(IV) compounds, can be attributed to transitions from the highest filled orbitals to empty orbitals with Cl3pTi4s. 4p antibonding character.     

Electronic structure and magnetism at the active site in ferredoxin: Ab initio approach to (Fe2S2)2+ complex with the 1st peptide shell

We have studied the iron–sulfur cluster systems which model an active site of ferredoxin proteins by using the first-principles electronic structure calculation. The modeled molecule is a complex between the (Fe₂S₂)²⁺ core and the amino acid residues which surround the core. The electronic structure of oxidized state for the molecules is presented. The antiferromagentic arrangement for Fe atomic magnetizations was obtained as the ground state. The spin polarized state of the half-filled Fe 3d orbitals is consistent with the formal valence of Fe³⁺. The induced spin density on the cysteine S atoms was found to be parallel to the direction of magnetization on the nearest Fe atom. The hybridized states consisting of N 2p and C 2p orbitals at the side chain of Arg residue appeared just above the highest occupied molecular orbital level for the free standing peptide.     

A re-evaluation of the two-step spin crossover in the trinuclear cation [Co3(dipyridylamido)4Cl2]+

Density functional theory is used to explore the electronic states involved in the remarkable two-step spin crossover (S = 0 --> S = 1 --> S = 2) in the cationic extended metal atom chain [Co(3)(dpa)(4)Cl(2)](+) (dpa = the anion of 2-dipyridylamine) (R. Clérac, F. A. Cotton, K. R. Dunbar, T. Lu, C. A. Murillo and X. Wang, J. Am. Chem. Soc., 2000, 122, 2272). The calculations are consistent with a model in which all three spin states share one common feature-a vacancy in the d(xy) orbital on the central cobalt atom which is stabilised by pi donation from four amide groups. As a result, all three can be considered to contain a Co(2+)-Co(3+)-Co(2+) chain. The singlet and triplet states arise from antiferromagnetic and ferromagnetic coupling, respectively, between the unpaired electron in this d(xy) orbital and another localised entirely on the terminal cobalt centres (the antisymmetric combination of Co d(z(2))). The singlet-triplet transition does not, therefore, populate any additional antibonding orbitals, and as a result the structure is almost invariant around the characteristic temperature of the singlet-triplet transition. In the most stable quintet, in contrast, the symmetry of the Co-Co-Co chain is broken, giving rise to a localised high-spin Co(II) centre (S = 3/2), ferromagnetically coupled to a Co(III)-Co(II) dimer (S = 1/2). The structural changes associated with this transition are apparent in the X-ray data in subtle changes in both Co-N and Co-Cl bond lengths, although their magnitude is damped by the relatively low population (18%) of the quintet even at 300 K.     

On the origin of a band gap in compounds of diamond-like structures

Electronic structure calculations were performed to examine the origin of a band gap present in most 18-electron half-Heusler compounds and its absence in NaTl. In these compounds of diamond-like structures, the presence or absence of a band gap is controlled by the sigma antibonding between the valence s orbitals, and the bonding characteristics of the late-main-group elements depend on the extent of their ns/np hybridization. Implications of these observations on the formal oxidation state and the covalent bonding of the transition-metal atoms in 18-electron half-Heusler and related compounds were discussed.     

The spin-unrestricted molecular Kohn-Sham solution and the analogue of Koopmans's theorem for open-shell molecules

Spin-unrestricted Kohn-Sham (KS) solutions are constructed from accurate ab initio spin densities for the prototype doublet molecules NO(2), ClO(2), and NF(2) with the iterative local updating procedure of van Leeuwen and Baerends (LB). A qualitative justification of the LB procedure is given with a "strong" form of the Hohenberg-Kohn theorem. The calculated energies epsilon(isigma) of the occupied KS spin orbitals provide numerical support to the analogue of Koopmans' theorem in spin-density functional theory. In particular, the energies -epsilon(ibeta) of the minor spin (beta) valence orbitals of the considered doublet molecules correspond fairly well to the experimental vertical ionization potentials (VIPs) I(i) (1) to the triplet cationic states. The energy -epsilon(Halpha) of the highest occupied (spin-unpaired) alpha orbital is equal to the first VIP I(H) (0) to the singlet cationic state. In turn, the energies -epsilon(ialpha) of the major spin (alpha) valence orbitals of the closed subshells correspond to a fifty-fifty average of the experimental VIPs I(i) (1) and I(i) (0) to the triplet and singlet states. For the Li atom we find that the exact spin densities are represented by a spin-polarized Kohn-Sham system which is not in its ground state, i.e., the orbital energy of the lowest unoccupied beta spin orbital is lower than that of the highest occupied alpha spin orbital ("a hole below the Fermi level"). The addition of a magnetic field in the -z direction will shift the beta levels up so as to restore the Aufbau principle. This is an example of the nonuniqueness of the mapping of the spin density on the KS spin-dependent potentials discussed recently in the literature. The KS potentials may no longer go to zero at infinity, and it is in general the differences nu(ssigma)( infinity )-epsilon(isigma) that can be interpreted as (averages of) ionization energies. In total, the present results suggest the spin-unrestricted KS theory as a natural one-electron independent-particle model for interpretation and assignment of the experimental photoelectron spectra of open-shell molecules.     

Effect of hydrogen chemisorption on the magnetism of nickel clusters

Results of self-consistent field, local spin density, scattered wave calculations are reported for nickel clusters of 10, 13, and 14 atoms and these clusters interacting with one or two chemisorbed hydrogen atoms. The pure nickel clusters all have a reasonable average atomic magnetic moment (the average over all the clusters is 0.66µ_B) and the addition of hydrogen reduces this moment in each case. The reduction of magnetic moment is clearly larger on the nickel atoms that are nearest to hydrogen but there is also a noticeable change in the moments of the other atoms of the clusters. Three factors, of varying importance for the different clusters, contribute to the changes in the overall and local magnetic moments: (i) The extra electron brought in with the hydrogen goes into a down-spin Ni d level, reducing the moment. (ii) The reelectron duced moment is accompained by a reduced exchange splitting and consequently some up-spin d electrons, not directly involved in the bonding to hydrogen, are transferred to lower lying down-spin d orbitals. (iii) For atoms close to the adsorbate, d character in the local density of states is pushed above the Fermi level through antibonding interactions with the hydrogen, further reducing the moments of these atoms.     

Gapped ferromagnetic graphene nanoribbons

We theoretically design a graphene-based all-organic ferromagnetic semiconductor by terminating zigzag graphene nanoribbons (ZGNRs) with organic magnets. A large spin-split gap with a 100% spin polarized density of states near the Fermi energy is obtained, which is of potential application in spin transistors. The interactions among electron, spin and lattice degrees of freedom are studied using the first-principles calculations including non-collinear spin orientations. All of the calculations consistently demonstrate that although no d electrons existing, the antiferromagnetic π-π exchange together with the strong electron-lattice interactions between organic magnets and ZGNRs make the ground state ferromagnetic.