Thermal properties of ferromagnetic clusters

The magnetic moments and heat capacities of small iron, cobalt and nickel clusters in a molecular beam have been measured as a function of their size and temperature. The magnetic moment of nickel and cobalt clusters decreases with increasing size and with increasing temperature, slowly converging to the bulk magnetization curve as the cluster size increases. Heat capacity measurement of the clusters in the beam reveals that the magnetic contribution to the heat capacity is well approximated using mean field concepts, combined with measured magnetic properties. In contrast, iron clusters show anomalies, both in the magnetic moments as well as in the heat capacities. We speculate that these effects reflect a sensitivity of the magnetic order on structure as in bulk iron.     

Molecular Spin Clusters: New Synthetic Approaches and Neutron Scattering Studies

We review our recent work in the field of molecular spin clusters and single-molecule magnets, showing how inelastic neutron scattering (INS) can be used to determine magnetic exchange interactions and anisotropy splittings. A general introduction to neutron scattering precedes selected examples, building upon the first determination of exchange coupling in a transition metal complex using INS, through anisotropic exchange in cobalt(II ) spin clusters to the determination of exchange interactions in a dodecanuclear nickel(II ) wheel. The strength of INS for the accurate determination of anisotropy splittings in single-molecule magnets is revealed. Not only can one determine the axial zero-field splitting parameter D, which plays a key role in single-molecule magnet behavior, but also higher-order terms important in understanding the quantum tunneling behavior. Finally, we review two of our synthetic approaches towards new single-molecule magnets based on nickel, manganese, and iron.     

Magnetic properties of small nickel oxide clusters enclosed inY-zeolite

The magnetization of small nickel oxide clusters containing less than four nickel atoms (sample A) and about ten atoms (sample B), respectively, formed inside the supercages ofY-zeolite, was studied in the magnetic field below 50 kOe and in the temperature range of 2 to 600 K. The magnetic susceptibility of sample A obeyed a Curie-Weiss' (C-W) law above about 20 K with a C-W temperature of 12 K. A saturation behavior was observed in the magnetization versus field (M-H) curve below about 20 K. A hysteresis in the M-H curve and a remanent magnetization were found below about 7 K. A similar behavior was observed for sample B. The observed positive C-W temperature indicates a ferromagnetic interaction between nickel ions in each cluster, which is semiquantitatively consistent with nearest neighbor ferromagnetic interactions previously reported for antiferromagnetic NiO single crystals. The hysteresis suggests an enhanced magnetic anisotropy energy in the present clusters.     

Magnetic properties of rare earth clusters

We report results of Stern-Gerlach deflection experiments on terbium clusters, which resemble earlier results for gadolinium clusters. As in gadolinium, we observe two distinct behaviors: clusters that are superparamagnetic and clusters that are described by a locked-moment model. The magnetic behavior is highly size dependent. Certain clusters make a transition from locked-moment to superparamagnetic behavior with increasing temperature and, in this process, exhibit an intermediate behavior. Both superparamagnetic and locked-moment clusters have magnetic moments per atom well below the bulk value. We show that oxygen atoms attached to the clusters have little effect on the clusters' magnetic properties and are not responsible for the two distinct behaviors observed in rare earth clusters. We also present preliminary results from studies on dysprosium clusters.     

PROPERTIES OF Ni-Cu/SiO_2 BIMETALLIC CATALYSTSPREPARED BY SOLVATED METAL ATOM DISPERSION(SMAD) TECHNIQUE

IntroductionAsoneofthemethodsofpreparationofsupportedmetalcatalysts,solvatedmatalatomdispersion(SMAD)onoxidesupportsattractsmuchattentionnowadays.TIstechniquehasbeendevelopedbyKlabunde'SandOzin'sresearchgroups.Itprovidesanumberofadvantages,ascomparedwitht…     

萃取体系二相界面荷电物质传输的电化学研究 I: 邻菲咯啉体系中Ni(II)、Co(II)相转移的电化学性质

用线性电流扫描极谱、恒电流单阶跃计时电位法及等电流双阶跃计时电位法, 研究了Ni(II)、Co(II)-邻菲咯啉(phen)体系在1,2-二氯乙烷(DCE)/水界面相转移的电化学行为。同时, 研究了相转移过程的机理。结果表明, 电解前, Phen与Ni(II)或Co(II)同时在水相时, Ni(II)、Co(II)行为类同; 若电解前Phen在有机相时, Ni(II)、Co(II)的行为截然不同, 原因在于两者与Phen配位反应的动力学的差异。     

Spatiotemporally controlled electrodeposition of magnetically driven micromachines based on the inverse opal architecture

We describe a double template-assisted electrodeposition of porous metal microstructures. The method combines two-dimensional photolithography and electrophoretic assembly of polystyrene beads, in order to confine the electrochemical growth of a porous magnetic cobalt–nickel alloy within well-defined microscale boundaries. Polystyrene beads are electrophoretically deposited onto a sulfonate derivatized gold substrate where a patterned photoresist layer (first template) is applied. The polystyrene beads trapped in the first template act as the second template, and cobalt–nickel alloy is electrochemically grown through the voids between the beads. After removal of both templates, magnetic microstructures with well-defined shapes and porosity are successfully obtained. Additionally, we demonstrate the capabilities of these magnetic microstructures as wireless cargo microtransporters by loading their pores with a stimulus-responsive hydrogel. Magnetic manipulation experiments are also demonstrated.     

Synthesis and characterization of some Schiff base transition metal complexes derived from vanillin and L(+)alanine

The cobalt(II), nickel(II), copper(II) and zinc(II)-vanillidene-L(+)alanine complexes were synthesized and characterized by elemental analysis, conductivity measurements, magnetic behavior, infrared, electronic spectral measurements, X-ray powder diffraction and biological studies. The conductance measurements indicate that all the complexes are non-electrolytes. The infrared spectra indicate the coordination of imino nitrogen, phenolic oxygen and carboxylato oxygen atoms. The electronic spectral measurements demonstrate that cobalt(II) and nickel(II)-vanillidene-L(+)alanine complexes are tetrahedral, while copper(II)-vanillidene-L(+)alanine complex has square planar geometry. The cobalt(II) complex is found to be ferromagnetic. The powder XRD studies confirm the crystalline nature of the complexes. The ligand and complexes were less active against PN, PA and BC, whereas copper complex shows moderate activity against AN.     

Seven-nanometer hexagonal close packed cobalt nanocrystals for high-temperature magnetic applications through a novel annealing process

Seven-nanometer cobalt nanocrystals are synthesized by colloidal chemistry. Gentle annealing induces a direct structural transition from a low crystalline state to the hexagonal close packed (hcp) phase without changing the size, size distribution, and the lauric acid passivating layer. The hcp structured nanocrystals can be easily redispersed in solvent for further application and processing. We found that the magnetization at saturation and the magnetic anisotropy are strongly modified through the annealing process. Monolayer self-assembly of the hcp cobalt nanocrystals is obtained, and due to the dipolar interaction, ferromagnetic behavior close to room temperature has been observed. This work demonstrates a novel approach for obtaining small size hcp structured cobalt magnetic nanocrystals for many technological applications.     

Metal complexes of bridging neutral radical ligands: pymDTDA and pymDSDA

Metal complexes of the 4-(2'-pyrimidyl)-1,2,3,5-dithiadiazolyl (pymDTDA) neutral radical ligand and its selenium analogue (pymDSDA) are presented. The following series of metal ions has been studied using M(hfac)(2) as the coordination fragment of choice (hfac = 1,1,1,5,5,5-hexafluoroacetylacetonato): Mn(II), Co(II), Ni(II), and Zn(II). The binuclear cobalt and nickel complexes of pymDTDA both exhibit ferromagnetic (FM) coupling between the unpaired electrons on the ligand and the metal ion, while the binuclear zinc complex of pymDTDA is presented as a comparative example incorporating a diamagnetic metal ion. The binuclear manganese complex of pymDTDA, reported in a preliminary communication, is compared to the pymDSDA analogue, and new insight into the magnetic behavior reveals that intermolecular magnetic coupling, mediated by chalcogen-oxygen contacts, gives rise to a significant increase in the χT product at low temperature. Surprisingly, the binuclear nickel complex of pymDSDA forms dimers in the solid state, as do the mononuclear complexes of cobalt and nickel with pymDTDA. In addition, mixed mononuclear/binuclear complexes of Mn- and Zn(pymDTDA) have been identified.     

Analysis of the reactivity of small cobalt clusters

The electronic structures of small cobalt clusters have been calculated within the local spin density approximation using the LCAO method. The calculations were done for simple geometries with the optimized number of interatomic bonds, and both for the bond length of the cobalt dimer and the bulk metal. The Fermi energy is found to be smaller for Co _N clusters withN=3, 4, 5 andN>10 than for the other ones. The variation of the Fermi energy with the cluster size correlates in a striking way with the observed H₂ tendency for chemisorption as found for cobalt clusters in a supersonic beam. Furthermore, the magnetic moments are somewhat smaller for these active clusters. In addition the lowest unoccupied levels of majority spin appear close to the highest occupied levels of minority spin which is not the case for the inert clusters.     

4-cyanoimidazolate: a new pseudo-cyanide?

Because of its exobidentate nature, pK(a), and electronic properties, 4-cyanoimidazole has been examined as a ligand and as a pseudo-cyanide. The ligand reacts readily as an anion with both cobalt and nickel ions in solution to form coordination polymers. The magnetic susceptibility and thermal stability of these materials are reported. 4-Cyanoimidazolate forms a hexakis complex with nickel, to form the first observed hexakis imidazolate nickelate complex.     

Size and structure dependence of carbon monoxide chemisorption on cobalt clusters

We have carried out a series of ab initio calculations to investigate changes in the structural and magnetic properties of pristine cobalt clusters upon CO chemisorption. Our results show that binding energies of CO to 13-55 atom (0.5-1.5 nm) cobalt nanoparticles and preferred chemisorption sites depend on the cluster structure (whether fcc or icosahedral), size, and surface coverage. In addition, we find a strong influence of CO on the magnetism of the cluster, leading to magnetic moments smaller than in the bulk, at variance with pristine clusters which have magnetic moments larger than the bulk. Our findings suggest important changes in catalytic properties of cobalt at the nanoscale. Our theory suggests that at the nanoscale cluster size and surface coverage might control catalysis.     

Sonochemical synthesis under a magnetic field: fabrication of nickel and cobalt particles and variation of their physical properties

We report on the variation of the physical properties of nickel and cobalt nanoparticles prepared by using ultrasound irradiation as energy source. First, we describe a sonochemical method for preparing aggregated particles. Second, we interpret the results on the basis of Einstein's theory (1905), which deals with a mathematical expression for the diffusivity of particles into solvents. This theory explains the stability of organosols of nickel and cobalt nanoparticles in polyethylene glycol. Finally, the effect of applying an external magnetic field during sonochemical formation of both aggregated particles and their stable colloids is investigated.     

Structure, magnetic, and electric properties of bismuth niobates doped with d elements: II. State of nickel atoms in solid solutions of low-and high-temperature bismuth orthoniobate

The magnetic susceptibility of nickel-doped solid solutions of bismuth orthoniobate of the low-and high-temperature modifications was studied. Parameters of exchange interactions in dimeric clusters of nickel atoms and also the distribution of monomers and dimers in the solid solutions were calculated. The nickel-containing solid solutions of low-and high-temperature bismuth orthoniobate are similar in magnetic behavior.     

H2 adsorption on 3d transition metal clusters: a combined infrared spectroscopy and density functional study

The adsorption of H2 on a series of gas-phase transition metal (scandium, vanadium, iron, cobalt, and nickel) clusters containing up to 20 metal atoms is studied using IR-multiple photon dissociation spectroscopy complemented with density functional theory based calculations. Comparison of the experimental and calculated spectra gives information on hydrogen-bonding geometries. The adsorption of H2 is found to be exclusively dissociative on Sc(n)O+, V(n)+, Fe(n)+, and Co(n)+, and both atomic and molecularly chemisorbed hydrogen is present in Ni(n)H(m)+ complexes. It is shown that hydrogen adsorption geometries depend on the elemental composition as well as on the cluster size and that the adsorption sites are different for clusters and extended surfaces. In contrast to what is observed for extended metal surfaces, where hydrogen has a preference for high coordination sites, hydrogen can be both 2- or 3-fold coordinated to cationic metal clusters.     

过渡金属原子簇离子结合能的光离解测量

本实验以激光离解的方式, 较系统地测量了钴、铌及其他一些过渡金属原子簇阳离子的结合能. 实验结果表明: 金属原子簇离子的结合能与其成族原子数关系不大, 而明显地受其组分的影响. 本文还探讨了金属原子簇离子的光离解机理及其几何结构模型。     

Microsolvation of Co2+ and Ni2+ by acetonitrile and water: photodissociation dynamics of M(2+)(CH3CN)n(H2O)m

The microsolvation of cobalt and nickel dications by acetonitrile and water is studied by measuring photofragment spectra at 355, 532 and 560-660 nm. Ions are produced by electrospray, thermalized in an ion trap and mass selected by time of flight. The photodissociation yield, products and their branching ratios depend on the metal, cluster size and composition. Proton transfer is only observed in water-containing clusters and is enhanced with increasing water content. Also, nickel-containing clusters are more likely to undergo charge reduction than those with cobalt. The homogeneous clusters with acetonitrile M(2+)(CH(3)CN)(n) (n = 3 and 4) dissociate by simple solvent loss; n = 2 clusters dissociate by electron transfer. Mixed acetonitrile/water clusters display more interesting dissociation dynamics. Again, larger clusters (n = 3 and 4) show simple solvent loss. Water loss is substantially favored over acetonitrile loss, which is understandable because acetonitrile is a stronger ligand due to its higher dipole moment and polarizability. Proton transfer, forming H(+)(CH(3)CN), is observed as a minor channel for M(2+)(CH(3)CN)(2)(H(2)O)(2) and M(2+)(CH(3)CN)(2)(H(2)O) but is not seen in M(2+)(CH(3)CN)(3)(H(2)O). Studies of deuterated clusters confirm that water acts as the proton donor. We previously observed proton loss as the major channel for photolysis of M(2+)(H(2)O)(4). Measurements of the photodissociation yield reveal that four-coordinate Co(2+) clusters dissociate more readily than Ni(2+) clusters whereas for the three-coordinate clusters, dissociation is more efficient for Ni(2+) clusters. For the two-coordinate clusters, dissociation is via electron transfer and the yield is low for both metals. Calculations of reaction energetics, dissociation barriers, and the positions of excited electronic states complement the experimental work. Proton transfer in photolysis of Co(2+)(CH(3)CN)(2)(H(2)O) is calculated to occur via a (CH(3)CN)Co(2+)-OH(-)-H(+)(NCCH(3)) salt-bridge transition state, reducing kinetic energy release in the dissociation.     

Electrodeposition of mesoporous semimetal and magnetic metal films from lyotropic liquid crystalline phases

Mesoporous semimetal bismuth film and magnetic metal nickel and cobalt thin films have been electrodeposited from hexagonal or lamellar structured lyotropic liquid crystalline phases with polyoxyethylene surfactant. The liquid crystalline templates are characterized by low-angle X-ray diffraction (XRD) and polarized-light optical microscopy (POM). The metal films are characterized by low-angle and wide-angle XRD, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The magnetic measurements on the mesoporous nickel and cobalt films are shown to have higher coercivity (Hc) than the nonporous polycrystalline films.     

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.