Lattice dynamics and electronic structure of cobalt–titanium spinel Co2TiO4

Physics of the Solid State - The results are presented on phonon excitations and the electronic structure of Co2TiO4 inverse spinel in which magnetically ordered cobalt ions Co2+ (3d 7) are in...     

Current-driven excitations in symmetric magnetic nanopillars

We study experimentally the current-driven magnetic excitations in symmetric Co/Cu/Co nanopillars. In contrast with all the previous observations where the current of only one polarity is capable of exciting a multilayer system saturated by an externally applied magnetic field, we observe that both polarities of the applied current trigger excitations in a symmetric multilayer. This may indicate that in symmetric structures the current propels high-frequency magnetic oscillations in all magnetic layers. We argue, however, that only one layer is excited in our multilayers but, interestingly, currents of opposite polarities excite different layers. This hypothesis is supported by modeling the spin accumulation in symmetric magnetic multilayers.     

Normal-state hourglass dispersion of the spin excitations in FeSexTe(1-x)

We use cold neutron spectroscopy to study the low-energy spin excitations of superconducting (SC) FeSe0.4Te0.6 and essentially nonsuperconducting (NSC) FeSe0.45Te0.55. In contrast with BaFe2-x(Co,Ni)xAs2, where the low-energy spin excitations are commensurate both in the SC and normal state, the normal-state spin excitations in SC FeSe0.4Te0.6 are incommensurate and show an hourglass dispersion near the resonance energy. Since similar hourglass dispersion is also found in the NSC FeSe0.45Te0.55, we argue that the observed incommensurate spin excitations in FeSe(1-x)Tex are not directly associated with superconductivity. Instead, the results can be understood within a picture of Fermi surface nesting assuming extremely low Fermi velocities and spin-orbital coupling.     

Current-induced excitations in single cobalt ferromagnetic layer nanopillars

Current-induced excitations in Cu/Co/Cu single ferromagnetic layer nanopillars ( approximately 50 nm in diameter) have been studied experimentally as a function of Co layer thickness at low temperatures for large applied fields perpendicular to the layers. For asymmetric junctions current-induced excitations are observed at high current densities for only one polarity of the current and are absent at the same current densities in symmetric junctions. These observations confirm recent predictions of spin-transfer torque induced spin-wave excitations in single layer junctions with a strong asymmetry in the spin accumulation in the leads.     

催化剂对碳纳米管产率及质量的影响

本文研究了添加钴/二茂铁、镍/二茂铁、钴、镍/钴不同催化剂对高温热解法制备碳纳米管质量、产率等的影响。高分辨率透射电镜图象显示在800℃左右,镍/二茂铁、钴/二茂铁和钴催化条件下,有多壁碳纳米管生成,而用镍/钴作催化剂时,只有直径在0 5μm左右,长度十几个微米的非晶态棒状物生成。通过对生成碳纳米管的质量和产量进行比较,催化剂的催化活性满足二茂铁>钴>镍。简单分析了在碳源高温热解环境下不同金属催化剂的性能差异,并对不同催化条件下生成物的拉曼光谱进行了分析。     

Influence of V and Mo overlayer on magneto-optical Kerr effect in ultrathin Co films

A method for characterization of sub-nanometer thick Co/V and Co/Mo interfaces is proposed that uses magneto-optical ellipsometry. Both the polar Kerr rotation and ellipticity are fitted simultaneously to different models of interface layer. The magneto-optical data are measured for varying thicknesses of the cobalt layer and overlayer by scanning of a laser beam over the samples with two orthogonal wedges. Decrease of magneto-optical effect at both interfaces Co/V and Co/Mo were observed, which corresponds to interface layers of thicknesses ranging from one to two monoatomic layers. In the case of vanadium, the interface layer is sharper and can be explained either by reduced magnetic moment of cobalt, or by anti-parallel magnetic moment of vanadium near the Co/V interface.     

Evidence of epitaxial growth of bcc Co on Cr(100)

We report a LEED/Auger study of the growth of cobalt ultra-thin films on Cr(100) surfaces. We demonstrate that Co can be grown epitaxially at room temperature, probably in a metastable bcc phase on this chromium surface. A 1 × 1 LEED pattern is observed at least up to 20 cobalt monolayers. The Auger data are consistent with an abrupt interface and a layer-by-layer growth mode.     

Correlation effects in 3d transition metals: Presence of a two-hole core-satellite in cobalt

Photoemission spectra of the 2p-level have been recorded from evaporated films of the 3d transition metals vanadium, chromium and cobalt. Structures on the high binding energy side of the spectra are observed; cobalt shows a weak satellite structure that resembles the well-known shakeup satellite in the nickel core level spectrum, only less intense, whereas the core level spectra of V and Cr only show satellites due to collective electron excitations.     

Co元素对硬质合金基底金刚石涂层膜基界面结合强度的影响

采用基于密度泛函理论的第一性原理平面波赝势方法, 研究了硬质合金刀具基底黏结相Co元素对金刚石涂层膜基界面结合强度的影响机理. 借助Materials Studio软件建立了WC/Diamond膜基界面模型和WC-Co/Diamond膜基界面模型, 采用CASTEP仿真软件计算了WC/Diamond膜基界面模型和WC-Co/Diamond膜基界面模型的最优稳定结构. 通过仿真计算, 获得了WC/Diamond膜基界面模型和WC-Co/Diamond膜基界面模型的界面结合能、电荷密度图及Mulliken重叠布居数. 经对比分析后发现, 硬质合金基底中磁性元素Co的存在能转移金刚石涂层膜基界面处W元素及C元素的电荷, 从而使膜基界面处的原子因失电荷而相斥, 这直接导致了金刚石涂层膜基界面间距变大, 使得金刚石涂层膜基界面结合能降低.     

Electron spin resonance and optical spectroscopic studies of Co-ZSM-5 with nitric oxide

The reaction of nitric oxide with ZSM-5 zeolite ion-exchanged by Co(II) (Co-ZSM-5) has been studied by electron spin resonance (ESR), Fourier transform infrared (FTIR) and diffuse reflectance ultraviolet-visible spectroscopy. When dehydrated Co-ZSM-5 reacts at room temperature with nitric oxide, the ESR spectrum of Co(II) atg = 5.5, which is only observable below 45 K, is greatly reduced and a new⁵⁹Co hyperfine octet forms at g_avg = 2.11. The overall Co(II) ESR intensity decreases by about 50% which suggests formation of some diamagnetic cobalt complex. Mononitrosyl cobalt complexes such as Co(I)-NO⁺ or less probably Co(III)-(NO)⁻ are suggested as possible precursors of a dintrosyl cobalt complex. The octet indicates hyperfine interaction with⁵⁹Co and is associated with a cobalt dinitrosyl complex. FTIR bands at 1813 and 1896 cm⁻¹ confirm a dinitrosyl species and a broad band from 1600–1800 cm⁻¹ is tentatively interpreted as a mononitrosyl species. The visible spectrum for dehydrated Co-ZSM-5 shows a tetrahedral Co(II) band from 500–700 nm with three components which disappears after NO adsorption at room temperature. We suggest that Co(0)-(NO)₂²⁺ forms after NO adsorption onto Co(II)-ZSM-5 zeolite on the basis of ESR and ultraviolet-visible spectroscopy.     

Effect of platinum promoters on the removal of O from the surface of cobalt catalysts: A DFT study

Cobalt is one of the commonly used catalysts in Fischer–Tropsch Synthesis (FTS). Small amounts of Pt are often added to cobalt to prevent deactivation and improve activity [1]. Removal of oxygen from the cobalt surface is one of the final steps in FTS mechanism. We investigate the role of the surface platinum promoter in the removal of oxygen from the cobalt surface using density functional theory (DFT) calculations. The activation barriers and transition states on both flat and stepped Co(0001) surfaces for the removal of oxygen from the cobalt surface with and without the presence of platinum were calculated using the Climbing Image Nudged Elastic Band (CI-NEB) method [2], [3], [4]. When Pt atoms are present on the surface, the activation barrier for the removal of O is reduced when compared to that on an un-promoted cobalt catalyst. The reduced barrier is attributed to the change in the electronic structure of the catalyst surface as evidenced from a shift in the d-band center and also a transfer of electrons from Co to Pt in agreement with an experimental study [5]. A micro-kinetic analysis shows that the turnover frequency (TOF) of oxygen removal for promoted Co surface increases when compared to that on un-promoted Co surface, with potential for further improvement. The results support the conjecture that surface Pt atoms helps reduce deactivation of cobalt catalyst by promoting oxygen removal from the surface.     

Current-driven resonances in magnetic multilayers

We describe complex variations in resistance of a Co/Cu multilayer, generated by injection of an adjustable dc current density ( approximately 10(9) A/cm(2)) via a point contact. We attribute these variations to coupling of current-induced spin waves to lattice vibrations, leading especially to current-driven resonant excitations of phonons. We propose a simple model to explain the observed structured behavior of the variations as a function of the applied current and magnetic field.     

Spectra study and size control of cobalt nanoparticles passivated with oleic acid and triphenylphosphine

This paper compares the performance of two surfactants—triphenylphosphine (TPP) and oleic acid (OA) as a pair of capping agents in the synthesis of magnetic Co nanoparticles (NPs). Magnetic colloids of cobalt NPs are prepared by reducing solute cobalt chloride in the presence of stabilizing agents at a high temperature and characterized by TEM. Infrared spectra reveal that a chemical bond can be formed between O of CO band and Co atoms while a coordinate bond forms between P and Co atoms around the NPs on the surface. OA binds strongly to the particle surface during synthesis that hinders the particle from growing; the TPP reversibly coordinates neutral metal surface sites that favor rapid growth. We studied the influence of changing the TPP/OA concentration ratio on the particle size distribution and crystallinity of Co NPs. Our results indicate the presence of TPP/OA is able to control particle growth, stabilize the colloidal suspension and prevent the final product from oxidation by air.     

Synthesis of Heterogeneous ZnO/Co3O4 Nanostructures by Chemical Deposition from Solutions

Technical Physics - Nanocrystalline cobalt oxide Co3O4 and composite ZnO/Co3O4 are obtained by chemical deposition from solutions followed by thermal annealing. It is shown that the presence of...     

Structure and composition of secondary phase particles in cobalt-doped TiO2 films

The microstructural properties of secondary phase particles formed in epitaxial Co_xTi_1−xO₂ anatase thin films grown on (0 0 1)LaAlO₃ by a reactive RF magnetron co-sputter deposition are examined. These films exhibit ferromagnetic behavior in magnetization measurements, showing a M–H loop at room temperature with a saturation magnetization on the order of 0.7 μ_B /Co. X-ray photoemission spectrometry indicates that the Co cations are in the Co²⁺ valence state. Cross-section electron microscopy reveals that a significant fraction of the cobalt segregates into Co–Ti–O secondary phase particles. Selected area electron diffraction shows that the secondary phase particles are cobalt-rich anatase. While the cobalt is concentrated in the segregated particles, local energy dispersive spectrometry indicates some Co throughout the film.     

X-ray absorption analysis of sputter-grown Co/Pt stackings before and after helium irradiation

We have studied the local order around cobalt atoms in Pt/Co/Pt layered systems gradually modified by irradiation using 30 keV helium ions. Using X-ray diffraction and X-ray Absorption Fine Structure, we have determined the crystallographic order, the number of Co-Co and Co-Pt bonds and the corresponding bond lengths both in-plane and in the perpendicular direction. The comparison with an interdiffusion model highlights the unexpected complexity of the initial Pt/Co/Pt nanostructure. We use the Néel/Bruno model of magnetic anisotropy to correlate the structure and the magnetic hysteresis properties. We then identify the structural consequences of irradiation onto the short range order. The irradiation induces a substitutional mixing maintaining the initial crystallographic structure. We confirm that the mixing rate is in agreement with a ballistic mechanism of mixing. In addition, we show that the previously reported irradiation-induced controlled decrease of the magnetic anisotropy can not be solely attributed to Co-Pt intermixing: irradiation also significantly releases the 3.4% tensile strain of the cobalt-rich dense planes. We finally speculate on the class of magnetic materials the concept of light ion irradiation could be extended to.     

Co(1,10-phen·)32+-KIO4-鲁米诺化学发光体系与应用

本文研究了Co(1,10-phen·)32+配合物对KIO4氧化鲁米诺所产生的强化学发光反应.以1,10-phen·为配位体时,Co(Ⅱ)的检出限为8×10-8g/L,工作曲线响应浓度范围在1×10-6~1×10-4g/L,测定1×10-4g/LCo(Ⅱ)离子的相对标准偏差为2.5%.配合物化学发光法检测啤酒、维生素B12针剂中微量Co(Ⅱ)可获得满意结果.     

Anisotropic interface magnetoresistance in Pt/Co/Pt sandwiches

We report on an effect of reduced dimensionality on the magnetotransport in cobalt layers sandwiched by platinum. In a current in-plane geometry it is found that the resistivity depends on the magnetization orientation within the plane perpendicular to the current direction. The resistivity shows a symmetry adapted cos(2) dependence on the angle to the surface normal, with the maximum along the surface normal. The Co thickness dependence of the effect in Pt/Co/Pt sandwiches clearly points out that the mechanism behind this effect originates at the Co/Pt interfaces and is disparate to the texture induced geometrical size effect.     

Structural and magnetic studies of Co thin films

The trend in reducing device dimension induces new physical properties and requires the development of measurement tools at the nanometer scale. This paper deals with the relation between magnetism and structure of thin films. We have chosen cobalt as a ferromagnetic layer and chromium as a bcc buffer. Magnetic and structural investigations have been led on epitaxial Co/Cr layers grown on MgO (001) substrates. The thickness of the cobalt layer varies from 0.75 to 20 nm. Investigations on the cobalt layer by EXAFS and HRTEM give evidence for a bcc or a hcp structure depending on the cobalt thickness. Magnetic measurements using SQUID indicate that the saturation magnetisation per volume unit is constant for the layers. EELS experiments have been carried out to measure any evolution in the I(L3)/I(L2) ratio for ferromagnetic layers of different thickness. We discuss the influence of structural and magnetic contributions on the evolution of the ratio with the cobalt thickness.