Co膜中多次超快瞬态反射突变现象研究

飞秒激光泵浦瞬态热反射技术是研究金属薄膜超快动力学的有效手段,这种技术具有两大突出特点：首先,可以揭示飞秒激光激发的微观电、声子的传输过程是一个非平衡热输运过程。其次,反射率瞬态变化实验中电子运动的超短时间分辨可以用来研究热过程中电子的非平衡相互作用情况。利用磁控溅射真空镀膜技术,在玻璃衬底和硅衬底上蒸镀了不同厚度的Co单层膜,Cr,Co双层膜以及Ag,Co双层膜。利用飞秒激光瞬态反射技术研究了Co膜及其双层膜的瞬态反射率响应。结果表明,在同一厚度的Co膜样品上,施加不同的泵浦光功率时,Co膜内电子的加热时间与泵浦光功率的大小无关,均为0.1344 ps。而对于不同厚度的Co膜,电子的热化时间与薄膜厚度直接相关。此外,发现与以往研究结果不同的是,在泵浦光功率足够大时,玻璃衬底上的Co膜在飞秒激光脉冲泵浦下会出现两次或三次瞬态反射率下降现象,Co膜厚度决定了Co膜内瞬态反射率突变的次数,即Co膜内电子的超快动力学变化次数。     

A non-thermal chemical synthesis of hydrophilic and amorphous cobalt oxide films for supercapacitor application

Present work explored a room temperature, simple and low cost chemical route for the preparation of hydrophilic cobalt oxide films from alkaline cobalt chloride (CoCl₂:6H₂O) and double distilled water precursor solutions. As-deposited cobalt oxide films showed amorphous nature, which is one of the prime requirements for supercapacitor, as confirmed from X-ray diffraction studies. Changes in direct band gap energy and electrical resistivity of as-deposited cobalt oxide films were confirmed after annealing. Spherical grains of about 40-50 nm diameters were uniformly distributed over the substrate surface. Surface wettability studied in contact with liquid interface, showed hydrophilic nature as water contact angle was <90°. Finally, presence of cobalt-oxygen covalent bond was observed from Raman shift experiment.     

Magnetoresistive effect in a rotating magnetic field for nickel and cobalt films

The longitudinal and transverse effects (Thomson effect) have been investigated in nickel and cobalt films from 600 to 1500 Å thick. The change in electrical resistance in a rotating magnetic field has been studied. Certain singularities of the magnetoresistive effect R/R have been observed for cobalt as compared with nickel films. The data for thin films are compared with those for bulk materials.     

Electrochromic and photoluminescence properties of cobalt oxide thin films prepared by spray pyrolysis

Cobalt oxide thin films were prepared by a facile spray pyrolysis technique, using a perfume atomizer with an aqueous solution of hydrated cobalt chloride salt with a molar concentration of 0.025?M as a source of cobalt. The films were deposited onto glass substrates at temperature of 350?°C. The structural, morphological, and electrochromic properties of the obtained films were studied. It was found from X-ray diffraction analysis that the films were polycrystalline in nature with spinel-type cubic structure and preferred orientation along [111] direction. The Scanning Electron Microscopy images revealed a porous structure with the average grain size around 200?nm. The cyclic voltammetry measurements revealed that Cobalt oxide thin film is an anodically coloring electrochromic material with a transmittance variation in the visible range of 31%, and a fast response time (about 2?seconds) and a good cycling stability. These electrochromic performances make cobalt oxide thin film an attractive material for using as an anodic electrochromic material in smart windows devices. The photoluminescence spectra exhibited a strong emission in the visible region confirming the good crystallinity properties of Co₃O₄ thin films.     

Synthesis and magnetic states of cobalt in three-layer Co/Ge/Co films

The magnetic properties of three-layer Co-Ge magnetic films have been studied experimentally as a function of technological conditions of their deposition. It has been found that the films deposited at a high deposition rate have a granular structure, and the films obtained at a low deposition rate have an X-ray amorphous structure. Electron microscopy and nuclear magnetic resonance studies have demonstrated that, at the same cobalt layer thickness, the semiconductor granule sizes depend on the average semiconductor layer thickness and correlate with the formation of different cobalt phases (amorphous, cubic, and hexagonal). The thermomagnetic properties of the films have been investigated.     

Magnetization-induced third harmonic generation in magnetic nanogranular films: Correlation with giant magnetoresistance

Magnetization-induced third harmonic (TH) generation was observed in magnetic nanostructures—nanogranular Co_xAg_{1? x} films—possessing giant magnetoresistance (GMR). The magnetization-induced contribution to the TH intensity was studied as a function of the concentration of a magnetic component (cobalt) in the films. Magnetic contrast of the TH intensity was found to correlate with the GMR coefficient: both parameters simultaneously reach extremum in the range of cobalt concentrations x～0.3–0.35.     

Enhanced field emission properties of screen-printed doubled-walled carbon nanotubes by polydimethylsiloxane elastomer

Field emission (FE) properties of double-walled carbon nanotubes (DWCNTs) treated by polydimethylsiloxane (PDMS) elastomer with different heating temperature have been systematically studied. The current density of treated DWCNT films decreases with the increase of heating temperature. The screen-printed DWCNTs treated by PDMS elastomer with drying temperature 150 °C for 20 min have the best FE performance with a marvelous field enhancement factor (β = 20194). The optimized FE performance is attributed to the morphological change of DWCNT films after PDMS elastomer treatment and the change of separation energy for the CNT-substrate interface. It is proved that the PDMS treatment is a facile and effective method for field emission display (FED) application, especially for low-temperature FED preparation.     

Pulsed laser deposition and characterization of multilayer metal-carbon thin films

Cobalt-DLC multilayer films were deposited with increasing content of cobalt, keeping carbon content constant by pulsed laser deposition technique. A cobalt free carbon film was also deposited for comparison. Excimer laser was employed to ablate the materials onto silicon substrate, kept at 250 °C, while post-deposition annealing at 400 °C was also performed in situ. The formation of cobalt grains within the carbon matrix in Co-DLC films can be seen through scanning electron and atomic force micrographs while no grains on the surface of the cobalt-free DLC film were observed. Raman spectra of all the films show D- and G-bands, which is a confirmation that the films are DLC in nature. According to Vibrating sample magnetometer (VSM) measurements, the DLC films with cobalt revealed ferromagnetic behaviour whereas the cobalt free DLC film exhibited diamagnetic behaviour. The pure DLC film also shows ferromagnetic nature when diamagnetic background is subtracted. Spectroscopic Ellipsometry (SE) analysis showed that the optical band gaps, refractive indices and extinction coefficients of Co-DLC films can be effectively tuned with increasing content of cobalt.     

Oxygen adsorption on ultrathin Co/Ir(1 1 1) films: Compositional anomaly

Adsorption of oxygen on ultrathin Co/Ir(1 1 1) films thinner than 4 monolayers in an ultrahigh vacuum environment was studied. For oxygen adsorption on cobalt films, the complex adsorption kinetics emerges partly due to the incorporation of oxygen. The amount of oxygen adsorbed at the surfaces is higher than that incorporated into the film as revealed from sputter profiling measurements. At room temperature the CoO layer exhibits paramagnetism and could not contribute to the remanent Kerr intensity. As oxygen exposure increases, the reduction of the Kerr intensity is due to the reduction of the effective layer for the magnetic measurements. Compared with oxygen saturated cobalt films, the concentration of adsorbed oxygen per Co atom shows an oscillatory behavior. A compositional anomaly of a great amount of adsorbed oxygen in submonolayer Co coverage occurs because of the maximized number of adsorption and incorporation sites for oxygen on the surface. A larger charge transfer between Co and oxygen was observed for thinner Co overlayers as revealed from the larger chemical shifts of Auger lines.     

Fast electrochromic response of porous-structured cobalt oxide (Co<Subscript>3</Subscript>O<Subscript>4</Subscript>) thin films by novel nebulizer spray pyrolysis technique

Electrochromic effect of cobalt oxide thin films was studied as a function of substrate temperature (573–673 K). Tricobalt tetraoxide (Co₃O₄) thin films were deposited on glass and fluorine-doped tin oxide (FTO) substrates by nebulized spray technique using cobalt nitrate as precursor material. The XRD patterns indicated (311) plane was dominant for all the films irrespective of the deposition temperature. Williamson-Hall (W-H) analysis was made to understand the strain variation in the prepared Co₃O₄ films under different deposition temperature by employing uniform deformation model (UDM). Scanning electron microscopy images revealed porous morphology for the film prepared at 623 K. The optical parameters such as refractive index (n), extinction coefficient (k), and band gap were derived from UV-visible spectra of Co₃O₄ films. The electrochromic performance of the deposited Co₃O₄ films was analyzed through cyclic voltammetry, chronocoulometry, chronoamperometry, and iono-optical studies.     

氧空位缺陷对PbTiO_3铁电薄膜漏电流的调控

基于非平衡格林函数及密度泛函理论第一性原理计算方法,计算了Fe, Al, V和Cu四种阳离子掺杂对氧空位缺陷引起的PbTiO_3铁电薄膜漏电流的调控.研究表明:Fe和Al离子掺杂将会增大由其中氧空位缺陷导致的铁电薄膜的漏电流,而Cu和V离子掺杂对该漏电流的大小具有明显抑制作用.这是因为Cu和V掺杂对氧空位缺陷有明显的钉扎作用.相比于半径更大的Cu离子,由于V的离子半径更小,且更接近于PbTiO_3铁电薄膜中Ti的离子半径,可以预言V离子更可能被掺杂进入薄膜,从而抑制氧空位缺陷引起的漏电流.研究结果对铁电薄膜器件的电学性能控制和优化有一定的理论指导意义.     

Structure and the magnetic and magneto-optical properties of Co-Sm-O nanogranular films

The magnetic properties and magneto-optical effects in nanocomposites based on Co-Sm-O films prepared through pulsed plasma sputtering of a SmCo₅ target are investigated. It is shown that, depending on the technological conditions and regimes of subsequent annealing, the films can have different structures from cobalt nanoparticles distributed in the dielectric samarium oxide matrix with a magnetic phase volume of more than 60% to a continuous polycrystalline cobalt film with embedded samarium oxide nanoparticles. The evolution of the spectra of the magneto-optical Kerr effect and the field dependences of the magnetization is studied as a function of the film structure.     

Enhanced field emission characteristics of thin-Au-coated nano-sheet carbon films

This paper reports that the nano-sheet carbon films (NSCFs) were fabricated on Si wafer chips with hydrogen--methane gas mixture by means of quartz-tube-type microwave plasma chemical vapour deposition (MWPCVD). In order to further improve the field emission (FE) characteristics, a 5-nm Au film was prepared on the samples by using electron beam evaporation. The FE properties were obviously improved due to depositing Au thin film on NSCFs. The FE current density at a macroscopic electric field, E, of 9~V/μ m was increased from 12.4~mA/cm² to 27.2~mA/cm² and the threshold field was decreased from 2.6~V/μ m to 2.0~V/μ m for Au-coated carbon films. A modified F-N model considering statistic effects of FE tip structures in the low E region and a space-charge-limited-current effect in the high E region were applied successfully to explain the FE data of the Au-coated NSCF.     

Thickness-dependent of conduction mechanism and bias electric field modulation of transport properties for SrRuO3/PMN-PT heterostructures

Ultra-thin SrRuO₃ (SRO) films have been grown on ferroelectric and piezoelectric PMN-PT substrates. The structural properties of these films have been characterized by atomic force microscopy, x-ray diffraction and cross-sectional transmission electron microscopy. The nature of electric transport was analyzed in detail and the conduction mechanism of SRO films evolves through three regimes: from a three-dimensional (3D) metallic through a weakly localized to a strongly localized behavior as film thickness is reduced. The bias electric field modulations of transport properties and magnetic properties were explored for these films. We also demonstrate that ferroelectric (FE) domain switching induces a reversible tuning of the magnetic and electric properties in SRO/PMN-PT heterostructure. The FE domain switching in the substrate contributes to an in-plane strain that changes the spin exchange coupling in the SRO layer, and therefore results in a reversible resistance difference of up to 16%. This modulation effect on the electric properties by an electric field demonstrates great potential for the applications of all-oxides spintronics devices.     

Electrical conductivity of Zn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Co<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O ferromagnetic films at low temperatures

The results of experiments on electrical conductivity and magnetic properties of thin cobalt-doped zinc oxide films are reported. The results indicate the predominance of the hoping conduction mechanism at low temperatures and the band mechanism at high temperatures. An increase in the cobalt concentration from 1.5 to 6.3 at % leads to the reduction of the electrical conductivity of the films. The contribution of hopping conduction to the conductivity increases due to a decrease in the crystallinity of the films and localization of a part of electron states upon an increase in the cobalt concentration. For cobalt-containing films, a hysteresis of the magnetic moment as a function of the magnetic field is observed. The dependence of the shape of the magnetization curves on the cobalt concentration is irregular. The paramagnetic contribution to the magnetic susceptibility increases with the cobalt concentration.     

Structural and magnetic properties of high magnetic moment electroplated CoNiFe thin films

CoNiFe alloy thin films deposited at various cobalt concentrations were galvanostatically electrodeposited on the pre-cleaned copper substrates. The effects of cobalt concentration on the structural, compositional, morphological, and magnetic properties of the films were investigated. X-ray diffraction patterns revealed that the deposited films possess polycrystalline in nature with mixed (fcc–bcc) cubic structure at optimized cobalt concentration. Microstructural properties of the films were calculated from predominant diffraction lines. The surface morphology and surface roughness were characterized using scanning electron microscopy and atomic force microscopy, respectively. EDAX results were revealed that the cobalt content increases as nickel content decreases whereas ferrous content initially increases and then eventually decreases in the CoNiFe alloy. VSM results show a higher value of saturation magnetization (4πM _s) above 2 T with coercivity 154 A/m for films deposited in the optimized deposition condition.     

MFM and AFM study of thin cobalt films modified by fluorosilane

Polycrystalline cobalt films 100 nm thick were thermally evaporated on oxidized Si(100) substrates. Then 1H, 1H, 2H, 2H perfluorodecyltrichlorosilane (FDTS) films of various thicknesses, in the range of about 2 nm to 30 nm, were grown on cobalt surfaces by vapor phase deposition (VPD). The cobalt films modified by FDTS were investigated using magnetic force microscopy (MFM) and atomic force microscopy (AFM). MFM observation showed that the magnetic structure of the cobalt films modified by FDTS is composed of domains with a considerable component of magnetization perpendicular to the film surface. This in turn indicates that the cobalt films on oxidized Si(100) substrates crystallize in the hexagonal close-packed (HCP) phase and exhibit a texture with the hexagonal axis perpendicular to the film surface. The magnetic domains formed a maze structure. The domain width increased from typically 80–120 nm to 400–500 nm with increasing the thickness of FDTS films from about 2 nm to 30 nm. AFM imaging of the surfaces of FDTS films revealed the presence of an agglomerate morphology. The agglomerates varied in size from typically 30–70 nm to 150–300 nm as the film thickness was increased from about 2 nm to 30 nm.     

Field emission characteristics of nano-sheet carbon films deposited by quartz-tube microwave plasma chemical vapour deposition

Nano-sheet carbon films are prepared on Si wafers by means of quartz-tube microwave plasma chemical vapour deposition （MPCVD） in a gas mixture of hydrogen and methane. The structure of the fabricated films is investigated by using field emission scanning electron microscope （FESEM） and Raman spectroscopy. These nano~carbon films are possessed of good field emission （FE） characteristics with a low threshold field of 2.6 V/μm and a high current density of 12.6 mA/cm^2 at an electric field of 9 V/μm. As the FE currents tend to be saturated in a high E region, no simple Fowler-Nordheim （F-N） model is applicable. A modified F N model considering statistic effects of FE tip structures and a space-charge-limited-current （SCLC） effect is applied successfully to explaining the FE data observed at low and high electric fields, respectively.     

Formation of ultrathin magnetic cobalt films on the Si(111)7 × 7 surface

The phase composition, electronic structure, and magnetic properties of ultrathin cobalt films (no thicker than 20 ?) applied on a Si(111)7 × 7 surface at room temperature are studied by high-resolution photoelectron spectroscopy using synchrotron radiation and magnetic linear dichroism. It is shown that, as the cobalt thickness increases, first interface cobalt silicide and then an island (discontinuous) film of silicon-in-cobalt solid solution form on the silicon surface. A metal cobalt film starts growing after the deposition of a ∼7-?-thick Co layer. It is found that the ferromagnetic ordering of the system, which is characterized by surface magnetization, sets in after the deposition of a ∼6-?-Co layer at the stage of Co-Si solid solution formation.     

Mass-filtered cobalt clusters in contact with epitaxially ordered metal surfaces

Mass-filtered cobalt clusters with a size between 5 nm and 12 nm have been deposited in situ under soft-landing conditions onto epitaxially ordered iron and nickel films. The spin and orbital moments of both the clusters as well as the substrate films have been investigated using the element-specific method of X-ray magnetic circular dichroism in photoabsorption. Here, the ferromagnetic films with in-plane magnetic anisotropies have been used to magnetize the clusters remanently without applying external magnetic fields during the measurements. Experimental results from the cobalt clusters are discussed with respect to the different substrates. Furthermore, the influence of oxygen exposure on the spin and orbital moments of cobalt clusters has been investigated in in situ oxidation experiments. PACS 73.22.-f; 75.70.-i; 75.75.+a; 81.07.-b