Showing papers on "Cobalt published in 2001"

Room-Temperature Ferromagnetism in Transparent Transition Metal-Doped Titanium Dioxide

Abstract: Dilute magnetic semiconductors and wide gap oxide semiconductors are appealing materials for magnetooptical devices. From a combinatorial screening approach looking at the solid solubility of transition metals in titanium dioxides and of their magnetic properties, we report on the observation of transparent ferromagnetism in cobalt-doped anatase thin films with the concentration of cobalt between 0 and 8%. Magnetic microscopy images reveal a magnetic domain structure in the films, indicating the existence of ferromagnetic long-range ordering. The materials remain ferromagnetic above room temperature with a magnetic moment of 0.32 Bohr magnetons per cobalt atom. The film is conductive and exhibits a positive magnetoresistance of 60% at 2 kelvin.

Composition and Microstructure of Cobalt Oxide Thin Films Obtained from a Novel Cobalt(II) Precursor by Chemical Vapor Deposition

Abstract: The present work reports the synthesis and the characterization of cobalt oxide thin films obtained by chemical vapor deposition (CVD) on indium tin oxide (ITO) substrates, using a cobalt(II) β-diketonate as precursor. The complex is characterized by electron impact mass spectrometry (EI-MS) and thermal analysis in order to investigate its decomposition pattern. The depositions are carried out in a cold wall reactor in the temperature range 350−500 °C at different oxygen pressures, to tailor film composition from CoO to Co3O4. The crystalline nanostructure is evidenced by X-ray diffraction (XRD), while the surface and in-depth chemical composition is studied by X-ray photoelectron (XPS) and X-ray excited auger electron spectroscopy (XE-AES). Atomic force microscopy (AFM) is employed to analyze the surface morphology of the films and its dependence on the synthesis conditions. Relevant results concerning the control of composition and microstructure of Co−O thin films are presented and discussed.

Chemical effects at metal/oxide interfaces studied by x-ray-absorption spectroscopy

Abstract: A chemical and magnetic characterization of ferromagnet/antiferromagnet interfaces is essential to understand the microscopic origins of exchange anisotropy and other magnetic phenomena. We have used high-resolution L-edge x-ray absorption spectroscopy (XAS), which is element specific and sensitive to chemical environment and spin orientation, to investigate the interface of antiferromagnetic oxides with ferromagnetic metals. Clear quantitative evidence of oxidation/reduction reactions at the as-grown metal/oxide interface is presented. In situ-- and ex situ--grown samples of the form oxide $(5--30 \AA{})/\mathrm{metal}$ $(1--10 \AA{}),$ where oxide is either NiO or CoO and metal is either Fe, Co, or Ni, were studied by high-resolution XAS. For all samples, a metal(oxide) layer adjacent to an oxide(metal) layer was partially oxidized(reduced). Quantitative analysis of the spectra showed that one to two atomic layers on either side of the interface were oxidized/reduced. An elemental series of samples showed that the amount of oxidation/reduction was in accord with the difference in oxidation potentials of the adjacent cations, e.g., oxide layers were more strongly reduced by an iron metal layer than by cobalt or nickel metal layers. Annealing to temperatures, typically used to bias devices, was shown to significantly increase the amount of oxidation/reduction. The oxidation behavior of iron was shown to depend on the amount of oxygen available. Our results are believed to provide important information for the improved understanding of exchange anisotropy.

High temperature resistant part, made of single-crystal or polycrystalline nickel-base superalloy

Abstract: Component made from a nickel-based super alloy contains (in wt.%) 9-11 chromium (Cr),3-5 tungsten (W), 0.5-2.5 molybdenum (Mo), 3-3.5 aluminium (Al), 3-5 titanium (Ti), 3-7 tantalum (Ta), 0-12 cobalt (Co), 0-1 niobium (Nb), 0-2 hafnium (Hf), 0-1 zirconium (Zr), 0-0.05 boron (B), 0-0.2 carbon (C), 1-5 rhenium (Re), 0.1-5 ruthenium (Ru), balance nickel (Ni) and impurities. Preferably the alloy contains at least 2 wt.% Re and a maximum of 3 wt.% Ru. The component is a monocrystalline structure having an isotropic distribution of the orientation of the grain structure.

Activities and stabilities of heterogeneous catalysts in selective liquid phase oxidations: recent developments

Abstract: Different strategies for the heterogenization of redox-active elements in solid matrices are reviewed. These include framework-substituted molecular sieves, amorphous mixed oxides by grafting or sol–gel methods, grafting or tethering to the inner walls of mesoporous molecular sieves, encapsulation by ship-in-a-bottle or other techniques and ion exchange in layered double hydroxides. The different approaches are illustrated by reference to recent developments involving a variety of metal catalysts — titanium, chromium, cobalt, manganese, iron, ruthenium, tungsten, molybdenum, vanadium and tantalum — in oxidations with O 2 , H 2 O 2 and RO 2 H as primary oxidants. Emphasis is placed on an evaluation of the stability of the various catalysts under reaction conditions, a conditio sine qua non for practical utility. Protocols for establishing heterogeneity are discussed. An analysis of experimental results leads to the conclusion that many of the systems described in the literature, particularly those involving oxometal species, are unstable towards leaching or the appropriate rigorous tests for heterogeneity have not been performed.

Magnetic Anisotropy of a Single Cobalt Nanocluster

Abstract: Using a new micro-SQUID setup, we investigate magnetic anisotropy in a single 1000-atom cobalt cluster. This system opens new fields in the characterization and understanding of the origin of magnetic anisotropy in such nanoparticles. For this purpose, we report three-dimensional switching field measurements performed on a 3 nm cobalt cluster embedded in a niobium matrix. We are able to separate the different magnetic anisotropy contributions and evidence the dominating role of the cluster surface.

Synthesis of “Solid Solution” and “Core-Shell” Type Cobalt−Platinum Magnetic Nanoparticles via Transmetalation Reactions

Abstract: In this article, we report the synthesis of “solid solution” and “core-shell” types of well-defined Co−Pt nanoalloys smaller than 10 nm. The formation of these alloys is driven by redox transmetalation reactions between the reagents without the need for any additional reductants. Also the reaction proceeds selectively as long as the redox potential between the two metals is favorable. The reaction between Co2(CO)8 and Pt(hfac)2 (hfac = hexafluoroacetylacetonate) results in the formation of “solid solution” type alloys such as CoPt3 nanoparticles. On the other hand, the reaction of Co nanoparticles with Pt(hfac)2 in solution results in “CocorePtshell” type nanoalloys. Nanoparticles synthesized by both reactions are moderately monodispersed (σ < 10%) without any further size selection processes. The composition of the alloys can also be tuned by adjusting the ratio of reactants. The magnetic and structural properties of the obtained nanoparticles and reaction byproducts are characterized by TEM, SQUID, UV/v...

Magnetic properties of ultrafine cobalt ferrite particles synthesized by hydrolysis in a polyol medium

Abstract: Fine CoFe2O4 powders with monodisperse, almost equi-axial nanometer-sized particles were synthesised in a polyol medium by forced hydrolysis of ionic Co(II) and Fe(III) salts at 160 °C. K(Co) XANES and 57Fe Mossbauer spectroscopy show that the structure of this ferrite is slightly deviated from an inverse spinel structure: 16% of cobalt atoms are in tetrahedral sites. The particles are superparamagnetic above 300 K and ferrimagnetic below this blocking temperature with, at low temperature, strong coercivity, a saturation magnetisation value close to the bulk value and high reduced remanence. The saturation magnetisation measured at 5 K is clearly enhanced with respect to CoFe2O4 nanometer-sized particles previously prepared by other methods. These magnetic characteristics suggest that these particles have a high crystallinity which may result from this novel synthesis route.

ASM Specialty Handbook: Nickel, Cobalt, and Their Alloys

Abstract: ASM specialty handbook: nickel, cobalt, and their alloys , ASM specialty handbook: nickel, cobalt, and their alloys , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی

Flux Closure Structures in Cobalt Rings

Abstract: Measurements are reported on the magnetization reversal in submicron magnetic rings fabricated by high-resolution electron beam lithography and lift-off from cobalt thin films. For all dimensions investigated, with diameters of 300-800 nm and a thickness of 10-50 nm, the flux closure state is the stable magnetization configuration. However, with increasing diameter and decreasing film thickness a metastable near single domain state can be obtained during the reversal process in an in-plane applied field.

Semiconducting system and production method

Abstract: The object of the present invention is to prevent rise of resistance due to oxidation of the copper wiring and diffusion of copper. The above object can be attained by the present invention providing a semiconductor device which contains a wire protective film 1 covering the top of the copper wiring 2 formed in the insulation film and a barrier film surrounding the side and bottom of the copper wiring; wherein the wire protective film and/or barrier film is formed with cobalt alloy film containing (1) cobalt, (2) at least one of chromium, molybdenum, tungsten, rhenium, thallium and phosphorus, and (3) boron, as in the case of other embodiments.

Metal-support interactions in cobalt-aluminum co-precipitated catalysts: XPS and CO adsorption studies

Abstract: Cobalt-aluminum catalysts were prepared using either the precipitation of Co2+ in the presence of freshly prepared Zn-Al hydrotalcite (the promoted sample) or the co-precipitation of Co2+ and Al3+ (the unpromoted samples). The evolution of the initial hydrotalcite-like structure was monitored during its calcination and the reductive treatment by means of XPS. It was shown that at 480°C the reduction of the calcined samples results in the formation of Co0 species, the further reduction at 650°C results in an increase of the amount of the Co0 species. The samples reduced at 650°C chemisorb readily carbon monoxide at 77 K, while the sample reduced at 480°C does not chemisorb CO at 77 K. At elevated temperatures, all reduced samples are found to be able to chemisorb CO. Terminal CO moieties as well as monodentate carbonates, formates and carboxyl species were detected at the surface of the reduced samples at their exposure to the CO medium at the elevated temperature. The intensity of the IR absorption bands of chemisorbed CO are found proportional to the surface fraction of the Co0 species, measured by XPS. The apparent red shift of the IR absorption bands is observed for CO adsorbed on the samples reduced at 480°C. The obtained data correlate with the catalytic properties of the Co-Al samples in hydrogenation reactions. The conclusion on the existence of a strong metal–support interaction in the samples under the study is made.

The First Cobalt Single-Molecule Magnet

Abstract: The first cobalt molecule to function as a single-molecule magnet, [Co4(hmp)4(MeOH)4Cl4], where hmp- is the anion of hydroxymethylpyridine, is reported. The core of the molecule consists of four Co(II) cations and four hmp- oxygen atoms ions at the corners of a cube. Variable-field and variable-temperature magnetization data have been analyzed to establish that the molecule has a S=6 ground state with considerable negative magnetoanisotropy. Single-ion zero-field interactions (DSz2) at each cobalt ion are the origin of the negative magnetoanisotropy. A single-crystal of the compound was studied by means of a micro-SQUID magnetometer in the range of 0.040-1.0K. Hysteresis was found in the magnetization versus magnetic field response of this single crystal. It is concluded that this is the first cobalt molecule to function as a single-molecule magnet.

Surface properties and catalytic activity of ferrospinels of nickel, cobalt and copper, prepared by soft chemical methods

Abstract: Ferrospinels of nickel, cobalt and copper and their sulphated analogues were prepared by the room temperature coprecipitation route to yield samples with high surface areas. The intrinsic acidity among the ferrites was found to decrease in the order: cobalt>nickel>copper. Sulphation caused an increase in the number of weak and medium strong acid sites, whereas the strong acid sites were left unaffected. Electron donor studies revealed that copper ferrite has both the highest proportion of strong sites and the lowest proportion of weak basic sites. All the ferrite samples proved to be good catalysts for the benzoylation of toluene with benzoyl chloride, copper and cobalt ferrites being much more active than nickel ferrite. The catalytic activity for benzoylation was not much influenced by sulphation, but it increased remarkably with calcination temperature of the catalyst. Surface Lewis acid sites, provided by the octahedral cations on the spinel surface, are suggested to be responsible for the catalytic activity for the benzoylation reaction.

The continuous hydrothermal synthesis of nano-particulate ferrites in near critical and supercritical water

Abstract: Magnetic spinel type oxides such as magnetite, Fe3O4, cobalt, nickel, and zinc ferrites, MFe2O4 (M = Co, Ni, Zn) and the mixed nickel and cobalt ferrite, NixCo1 − xFe2O4 have been synthesised continuously by the hydrolysis and simultaneous oxidation of mixtures of Fe(II) acetate and different M(II) acetates in near-critical and supercritical water using a flow reactor. The materials have been characterised by powder X-ray diffraction (PXD) and, in selected cases, by transmission electron microscopy (TEM). The bulk composition of the samples was determined by Atomic Absorption analysis (AA). Additionally, Energy-dispersive Detection X-ray analysis (EDX) was carried out on some of the samples. TEM pictures showed a “bimodal” particle size distribution: small particles of ca. 10 nm and larger particles of up to 100 nm, both of which are highly crystalline. Possible reaction mechanisms are discussed, which may be responsible for the observed morphology. The effects of temperature and residence time on the reaction have been studied.

Pore-Size Control of Cobalt Dispersion and Reducibility in Mesoporous Silicas

Abstract: The effect of porosity in a wide range of mesopore diameters (dp = 20−330 A) on the dispersion and reducibility of cobalt species in mesoporous silicas is examined using nitrogen adsorption, X-ray diffraction, thermogravimetric analysis, and in situ X-ray absorption. It is shown that modification of mesoporous silicas by cobalt via aqueous impregnation results in small Co3O4 crystallites located in the pores of silicas. The sizes of these crystallites increase with increasing mesopore diameters. In situ X-ray absorption and thermogravimetric analyses show that the reduction of Co3O4 crystallites in hydrogen leads to CoO and Co metal particles. The porous structure of the supports strongly affects the extent of cobalt reduction. It is found that smaller particles in the narrow pores (20−50 A) are much more difficult to reduce to metal species than larger ones situated in the broad pores (>50 A) of mesoporous silicas.