Electron CO Research Articles

Induced spin polarization in Fe2VAl by substitution of Co at Fe site

The effects on electronic structure properties by Co substitution in Fe2VAl Heusler alloy have been determined by means of first principle calculations using generalized gradient approximation and calculated equilibrium lattice constants and magnetic moments for the series Fe2−xCoxVAl. The value of formation energy is minimized at the Fe (A, C) sites when Co substitutes at Fe, V and Al sites. The semi metallic behavior of Fe2VAl fades, while spin polarization is induced on Fe replaced by Co atom due to the influence of valence electrons of Co at the Fermi level. An enhancement of the total magnetic moment on increase in the Co content is observed due to the ferromagnetic coupling of Fe and Co atoms with V atoms at the B site and hybridization between Co–Co atoms at the A/C sites.

Robust ferromagnetism in the compressed permanent magnetSm2Co17

The compound ${\mathrm{Sm}}_{2}{\mathrm{Co}}_{17}$ displays magnetic properties amenable to permanent magnet applications owing to both the $3d$ electrons of Co and the $4f$ electrons of Sm. The long-standing description of the magnetic interactions between the Sm and Co ions implies a truly ferromagnetic configuration, but some recent calculations challenge this axiom, suggesting at least a propensity for ferrimagnetic behavior. We have used high-pressure synchrotron x-ray techniques to characterize the magnetic and structural properties of ${\mathrm{Sm}}_{2}{\mathrm{Co}}_{17}$ to reveal a robust ferromagnetic state. The local Sm moment is at most weakly affected by compression, and the ordered moments show a surprising resilience to volumetric compressions of nearly 20%. Density functional theory calculations echo the magnetic robustness of ${\mathrm{Sm}}_{2}{\mathrm{Co}}_{17}$.

Identification of the Strongest Bonds in Chemistry

Increasing the effective electronegativity of two atoms forming a triple bond can increase the strength of the latter. The strongest bonds found in chemistry involve protonated species of hydrogen cyanide, carbon monoxide, and dinitrogen. CCSD(T)/CBS (complete basis set) and G4 calculations reveal that bond dissociation energies are misleading strength descriptors. The strength of the bond is assessed via the local stretching force constants, which suggest relative bond strength orders (RBSO) between 2.9 and 3.4 for heavy atom bonding (relative to the CO bond strength in methanol (RBSO = 1) and formaldehyde (RBSO = 2)) in [HCNH](+)((1)Σ(+)), [HCO](+)((1)Σ(+)), [HNN](+)((1)Σ(+)), and [HNNH](2+)((1)Σg(+)). The increase in strength is caused by protonation, which increases the electronegativity of the heavy atom and thereby decreases the energy of the bonding AB orbitals (A, B: C, N, O). A similar effect can be achieved by ionization of a nonbonding or antibonding electron in CO or NO. The strongest bond with a RBSO value of 3.38 is found for [HNNH](2+) using scaled CCSD(T)/CBS frequencies determined for CCSD(T)/CBS geometries. Less strong is the NN bond in [FNNH](2+) and [FNNF](2+).

Giant coercivity in ferromagnetic Co doped ZnO single crystal thin film

The origin of ferromagnetism in ZnO doped with transition metal impurities has been discussed extensively and appeared to be a highly controversial and challenging topic in today's solid state physics. Magnetism observed in this system is generally weak and soft. We have grown Co:ZnO up to 30at% Co in single crystal thin film form on c-plane sapphire. A composition dependent coercivity is observed in this system which reaches peak value at 25at% Co, the values are 860Oe and 1149Oe with applied field along parallel and perpendicular to the film substrate interface respectively. This giant coercivity might pave the way to exploit this material as a magnetic semiconductor with novel logic functionalities. The findings are explained based on defect band itinerant ferromagnetism and its partial interaction with localized d electrons of Co through charge transfer. Besides large coercivity, an increase in the band gap with Co concentration has also been observed along with blue emission peak with long tail confirming the formation of extended point defect levels in the host lattice band gap.

Electronic and optical properties of N-doped, Co-doped as well as N,Co-codoped rutile TiO2

We have used the first-principles plane-wave pseudopotential calculations to study the electronic and optical properties of N-doped, Co-doped as well as N,Co-codoped rutile TiO2 crystals. Results show that N-doped rutile TiO2 exhibits a semiconducting characteristic with a stable antiferromagnetic ground state. Co-doped rutile TiO2 shows half-metallic ferromagnetism with a stable ferromagnetic ground state, indicating that this doped material is useful in spin injection applications. N,Co-codoped rutile TiO2 has a nearly half-metallic nature with a stable ferromagnetic ground state. In addition, the optical absorption spectrum shows that N-doped, Co-doped and N,Co-codoped rutile TiO2 all extend the optical absorption in the visible-light region, and especially, Co-doped rutile TiO2 has the best absorption character in the visible-light region. We also discuss the influence of Coulomb interactions of 3d electrons of Co and Ti on the electronic structure of the doped systems studied.

Thermal diffusion of Co into sputtered ZnO:Co thin film for enhancing visible-light-induced photo-catalytic activity

ZnO has received a lot of attention in for the degradation and complete mineralization of environmental pollutants. In this study, efficient ZnO-based visible-light photocatalysts are synthesized via a low-cost method by increasing the Co concentration at the outermost surface of ZnO:Co thin film under an annealing process. The simply adjusted Co concentration gradient (≈1.5% increase) into the surface of ZnO:Co is driven by thermal diffusion. Co ions substitute Zn sites in the ZnO lattice. The band gap narrowed due to sp–d exchange interactions between the conduction band electrons and the d electrons of Co. The annealing process enhances the grain size and the degree of crystallinity of the diffused Co/ZnO:Co, which contributes to the absorption edge red-shift. Photocatalytic activities such as the photo-degradation of methylene blue and the inactivation of bacteria are obviously enhanced. The proposed low-cost method significantly increases the efficiency of ZnO for the generation of the photocatalytic effect under visible-light exposure.

Aqueous alkaline chemical bath deposition of Cd 1 − X Co XS thin films

An aqueous alkaline chemical bath deposition route was used to deposit Cd 1 − X Co XS (0 ≤ X ≤ 0.1) thin films. Present, combinatorial route bring about uniform, tightly adherent and diffusely reflecting thin films with color shifting from orange red to dark chocolate. Cd rich nature of the as-prepared thin film samples was revealed by EDX analysis. Subsequently, hexagonal wurtzite structure of deposited polycrystalline thin films with preferred orientation along (101) was determined by XRD technique. It is perceived that, the assimilation of Co 2+ in CdS lattice pilots the decrease in lattice parameter. Broad absorption edge with high absorption coefficient (α = 10 4–10 5 cm − 1 ) was observed in the as-deposited thin films. Exchange interaction of sp-d kind in between the band electrons in CdS and localized d electrons of Co 2+ decreases optical bandgap gradually from 2.40 eV–2.08 eV with increase in Co 2+ content.

Temperature and field dependent magnetic Compton scattering study of Heusler alloy Co2MnSi

Temperature and field dependent spin polarized electron momentum distributions in ternary Heusler compound Co2MnSi have been measured using the magnetic Compton spectrometer at SPring-8, Japan. The experimental magnetic Compton profile at 10 K reconciles well with the theoretical profile computed using the spin polarized relativistic (SPR) Korringa–Kohn–Rostoker (KKR) method. The temperature and field dependent experimental magnetic profiles have been mainly analyzed to check the role of Mn 3d electrons in the formation of magnetic moments in Co2MnSi. The diffuse contribution from conduction electrons is antiferromagnetically coupled to the spin moment arising from 3d electrons of Co and Mn. The spin moments calculated using SPR-KKR calculations are in good agreement with the present measurements.

Electronic Structure Studies of Nanoferrite CuxCo1–xFe2O4 by X-ray Absorption Spectroscopy

Pure and mixed cobalt copper ferrites are of great interest due to their widespread application in electronics and medicine. We report on the electronic structure of a nanoferrite Cu(x)Co(1-x)Fe2O4 (0.0 < or = x < or = 1.0) system studied by X-ray absorption spectroscopy. These magnetic nanoferrites (average crystallite size approximately 31-43 nm) were synthesized by an auto combustion method and are characterized by high resolution X-ray diffraction and near edge X-ray absorption fine structure measurements at the O K and Co, Cu, and Fe L-edges. The O K-edge spectra suggest that there is a strong hybridization between O 2p and 3d electrons of Co, Cu and Fe cations and Fe L3,2-edge spectra indicate that Fe ions coexist in mixed valence states (Fe3+ and Fe2+) at tetrahedral and octahedral sites of the spinel structure. Copper and cobalt ions are distributed in the divalent state in octahedral sites of the spinel structure. The origin of high saturation magnetization and coercivity in cobalt-copper ferrites are explained in light of these results.

Giant magnetoresistance in semiconductor/granular film heterostructures with cobalt nanoparticles

We have studied the electron transport in SiO${}_2$(Co)/GaAs and SiO${}_2$(Co)/Si heterostructures, where the SiO${}_2$(Co) structure is the granular SiO${}_2$ film with Co nanoparticles. In SiO${}_2$(Co)/GaAs heterostructures giant magnetoresistance effect is observed. The effect has positive values, is expressed, when electrons are injected from the granular film into the GaAs semiconductor, and has the temperature-peak type character. The temperature location of the effect depends on the Co concentration and can be shifted by the applied electrical field. For the SiO${}_2$(Co)/GaAs heterostructure with 71 at.% Co the magnetoresistance reaches 1000 ($10^5$ %) at room temperature. On the contrary, for SiO${}_2$(Co)/Si heterostructures magnetoresistance values are very small (4%) and for SiO${}_2$(Co) films the magnetoresistance has an opposite value. High values of the magnetoresistance effect in SiO${}_2$(Co)/GaAs heterostructures have been explained by magnetic-field-controlled process of impact ionization in the vicinity of the spin-dependent potential barrier formed in the semiconductor near the interface. Kinetic energy of electrons, which pass through the barrier and trigger the avalanche process, is reduced by the applied magnetic field. This electron energy suppression postpones the onset of the impact ionization to higher electric fields and results in the giant magnetoresistance. The spin-dependent potential barrier is due to the exchange interaction between electrons in the accumulation electron layer in the semiconductor and $d$-electrons of Co.

Magnetic properties ofLCoAsO(L=La–Gd)

We synthesized a series of CoAs based weakly itinerant ferromagnetic compound $L\text{CoAsO}$ ($L$: lanthanoids) and systematically studied a $L$ dependence on magnetic properties. Lattice constants $a$ and $c$ decrease monotonically with the decrease in the ion size of ${L}^{3+}$. In the cases of $L=\text{Nd}$, Sm, and Gd, a ferromagnetic-antiferromagnetic transition was observed at ${T}_{\text{N}}=15$, 35, and 75 K, respectively, indicating the existence of unconventional interaction between ferromagnetically ordered itinerant electrons of Co. The Curie temperature ${T}_{\text{C}}$ increases from 55 to 75 K by changing La to Ce while from Ce to Gd the ${T}_{\text{C}}$ does not change so much, being quite similar to the $L$ site dependence of the superconducting transition temperature ${T}_{\text{c}}$ in FeAs-based high-${T}_{\text{c}}$ compound $L\text{FeAsO}$. We discussed the mechanism of ferromagnetic-antiferromagnetic transition and the lanthanoid dependence of ${T}_{\text{C}}$.

Photoelectron-Auger electron angular correlations in photoionization of fixed-in-space molecules

The K-shell Auger decay studies of fixed-in-space diatomic molecules are reviewed. The Auger-electron-photoelectron angular correlations after ejection of the C 1s electron of CO in the laboratory frame are not sensitive to the details of the Auger-electron wave function, as are the Auger-electron angular distributions in the molecular frame. Different arguments in favour of creation of delocalized gerade (g) or ungerade (u), or localized right or left core hole in photoionization process are analyzed. In several high resolution photo- and Auger-electron studies of N2 molecule the g and u states have been resolved, and their results demonstrated delocalized character of the core holes. The recent coincident study of the Auger- and photoelectrons in the molecular frame showed that they form a quantum entangled Bell state, and whether the core hole is better though of as being localized or delocalized depends on the directions in which the photoelectron and the Auger-electron are emitted. This phenomenon exists owing to overlap of g and u lines and interference of the corresponding decay channels.

The transformation of formaldehyde on CoZSM-5 zeolites

The interaction of formaldehyde molecules with Co 2+ ions in CoZSM-5 zeolites of various compositions containing various proportions between Co 2+ in exchange positions and in oxide-like clusters, as well as the transformation of formaldehyde sorbed in zeolites was followed by IR spectroscopy. The interaction of formaldehyde with Co 2+ resulted in a weakening of C O bond, what can be partially due to π-back donation of d electrons of Co 2+ to π * antibonding orbitals of formaldehyde. Only Co 2+ in exchange positions (contrary to Co 2+ in oxide-like clusters) were able to interact with formaldehyde. IR studies illustrated also that formaldehyde molecules are oxidized to formate ions, by either Co 3+ or Co 2+ as oxidants. According to IR results Co 3+ and Co 2+ are reduced mostly to Co + and metallic Co.

Electronic structure and magnetic properties of Y 4Co 3

The electronic structure of Y 4Co 3 has been studied based on the density functional theory within the local-density approximation. The calculation indicates that Y 4Co 3 is very close to ferromagnetic instability. The Fermi surfaces are composed mainly of 3d electrons of Co and 4d electrons of Y.

Electronic structure, magnetic and electric properties of Gd(Ni 1− xCo x) 3 compounds

The results of measurements of the magnetic susceptibility, the electrical resistivity, the crystal and electronic structures of the polycrystalline intermetallic Gd(Ni 1− x Co x ) 3 compounds are reported. The influence of substitution of Co for Ni atoms is reflected in an increase of the lattice parameters and in the change of the Curie temperature T C between 114 K for GdNi 3 and 615 K for GdCo 3. The electronic structure have been studied by using X-ray photoelectron spectroscopy (XPS). The valence band spectra are dominated by the hybridized 3d Co and 3d Ni states. A remarkable decrease of the intensity of states near the Fermi level and a shift of Gd 4f peaks in the energy scale for the Co-rich compounds is observed. These effects are related to different number of 3d electrons of Co and Ni and to different chemical environments in the compounds, respectively.

Binuclear Rhodium(I) Carbonyl Carboxylate Complexes: DFT Study of Structural and Spectral Properties

The geometry of binuclear rhodium(I) complexes, [Rh(μ-Cl)(CO)2]2 and [Rh(μ-RCOO)(CO)2]2 (R = H, CH3, CF3), was optimized by the DFT method; the vibrational spectra and the δ13C chemical shifts were calculated. The calculations were performed using the B3LYP hybrid functional and four basis sets. The calculations with the LanL2DZ basis set reflect all trends in the variation of the characteristics predicted using augmented basis sets and observed in the experiment. The Rh → CO electron density transfer along the π-bond and δ13C increase, while the bond orders, the intrinsic frequencies, and the force constants of the carbonyl groups in the carboxylate complexes decrease following a decrease in the substituent electronegativity in the series CF3 > H > CH3. It was found that the ratio of the ν(CO) intensities in the IR spectra can be used to derive information on the dihedral angle between the Rh(CO)2 planes in the binuclear complexes. Calculations were carried out for a tetranuclear model system [Rh(HCOO)(CO)2]4, which can be treated as an elementary unit of the infinite chains of metal atoms in [Rh(μ-RCOO)(CO)2]2 stack crystals. A local minimum was found on the potential energy surface corresponding to the geometric structure of the crystal fragments.

Total Structural Energy of Top-Site-AdsorbedCO/Nix–Cu1−x Systems

We construct an atomic group model of the disordered binary alloyNix–Cu1−x for the cases that the surface compositionsegregation exists or not. According the model, we calculate theelectronic structure in a top site of Nix–Cu1−x alloysurface by using the recursion method when CO absorbed on theNix–Cu1−x surface under the condition of coverage 0.5.The calculation result indicates that chemical absorption of CO reducesthe density of states of the disordered binary alloyNix–Cu1−x, widens the energy band, and strengthens thecovalent bonds between the d electron of Ni and s or p electron ofCO. Chemisorption of CO inhibits the enrichment of atomCu on the alloy surface especially when bulk Ni concentration xis less than 0.8.

Metal-Oxide Interfaces in Magnetic Tunnel Junctions

Metal-oxide interfaces play an important role in spintronics—a new area of microelectronics that exploits spin of electrons in addition to the traditional charge degree of freedom to enhance the performance of existing semiconductor devices. Magnetic tunnel junctions (MTJs) consisting of spin-polarized ferromagnetic electrodes sandwiching an insulating barrier are such promising candidates of spintronic devices. The paper reviews recent results of first-principle density-functional studies of the atomic and electronic structure of metal-oxide interfaces in Co/Al2O3/Co and Co/SrTiO3/Co MTJs. The most stable interface structures, O-terminated for fcc Co (111)/α-alumina(0001) and TiO2-terminated with oxygens on top of Co atoms for fcc Co (001)/SrTiO3(001) were identified based on energetics of metal-oxide cohesion at the interface. The covalent character of bonding for both the Co/alumina and Co/SrTiO3 interface structures has been determined based on the pattern of electron distribution across the interface. The Al-terminated Co/alumina interface that corresponds to an under-oxidized MTJ exhibits a metallic character of bonding. The unusual charge transfer process coupled with exchange interactions of electrons in Co results in quenching of surface magnetism at the interface and substantial reduction of work of separation. The electronic structure of the O-terminated Co/Al2O3/Co MTJ exhibits negative spin polarization at the Fermi energy within the first few monolayers of alumina but it eventually becomes positive for distances beyond 10 A. The Co/SrTiO3/Co MTJ shows an exchange coupling between the interface Co and Ti atoms mediated by oxygen, which results in an antiparallely aligned induced magnetic moment on Ti atoms. This may lead to a negative spin polarization of tunneling across the SrTiO3 barrier from the Co electrode. The results illustrate the important fact that spin-polarized tunneling in magnetic tunnel junctions is not determined entirely by bulk density of states of ferromagnet electrodes, but is also very sensitive to the nature of the insulating tunneling barrier, as well as the atomic structure and bonding at the ferromagnet/insulator interface.

CO adsorption on MgO( [formula omitted]) surface with oxygen vacancy and its low-coordinated surface sites: embedded cluster model density functional study employing charge self-consistent technique

The adsorption of CO on regular and defect sites of MgO(0 0 1) surface has been studied theoretically using embedded cluster models by DFT/B3LYP method. The value of embedded point charges is determined by the charge self-consistent technique. The calculated results indicate that CO adsorption energy on the regular site of MgO(0 0 1) surface can agree well with the recent experimental data. The frequency shifts of CO for regular five-coordinated terrace, low four-coordinated edge and three-coordinated corner sites, via C bound down on cationic centers of MgO(0 0 1) surface are also satisfactorily close to the values of experiment. At the same time, the adsorption of CO on MgO(0 0 1) surface with neutral and charged oxygen vacancies, F s, F s + and F s 2+ centers, has been investigated whose results show that MgO(0 0 1) surface with neutral oxygen vacancy has probably the good catalysis structure for CO adsorptive-decomposition. This is consistent with our previous study using a different method. By analyzing the bond component of Mg–C, it is found that the essential reasons why C–O bond strength is weaken or strengthen are the competitive results of 4σ* lone pair electrons and 5σ electrons of CO transferring from the adsorbate to substrate simultaneously.

Electronic and Magnetic Structures in Slab Models Including Al/Co and O/Co Interfaces

On the basis of the density-functional theory, we numerically calculated the interfacial magnetism of slab models including Al/Co and O/Co interfaces. We obtained the local magnetic moments and spin polarizations for various types of models. We found that the interfacial Al layers exhibit positive spin polarizaition in specific atomic arrangements, such as O/Al/Co. The sign is consistent with the experimental result of the ferromagnet/insulator/superconductor junction. This result suggests that in the junction Co/Al2O3/Co, the tunneling of s-character electrons in Al is favored to that of d-character electrons in Co.