Tiehan H. Shen

Monocrystalline hexagonal-close-packed and polycrystalline face-centered-cubic Co nanowire arrays fabricated by pulse dc electrodeposition

Cobalt nanowire arrays have been grown onto anodic aluminum oxide templates by using a pulse dc electrodeposition technique. We demonstrate that the crystal structure, grain size, and related magnetic properties can be controlled by applying different pulse frequencies. For a pulse frequency of 25Hz, monocrystalline hexagonal-close-packed Co nanowires are formed with a well defined [101¯0] growth axis along the wire length. At a higher pulse frequency of 1000Hz, face-centered-cubic Co nanowires are deposited with a small grain size and a preferred [111] texture directed along the length of the wire. Possible mechanisms for the observed growth characterization are discussed. An investigation of the corresponding magnetic properties indicates that the fcc Co nanowire arrays have the larger coercivity and squareness values: reasons for this behavior are explored.

Room-temperature single-electron tunnelling in surfactant stabilised iron oxide nanoparticles

Abstract Iron oxide nanoparticles have been synthesised by the reduction of FeCl 3 ·6H 2 O and FeCl 2 ·4H 2 O (mass ratio 2:1) in propan-2-ol. The particles were stabilised by the addition of lauric acid and the resultant particles could be readily dispersed in chloroform. Room-temperature single-electron tunnelling through these iron oxide particles has been achieved (to our knowledge) for the first time, using an STM tip–particle–substrate double junction. Characterisation of the particles by XPS and FTIR show that they are Fe 2 O 3 , or possibly Fe 3 O 4 with an outer layer of Fe 2 O 3 , coated with a carboxylic acid monolayer. The particle diameter given by TEM is of the order of 5.0±0.9 nm. From the Coulomb staircase behaviour observed in the I – V curves, we estimate a nanoparticle capacitance of 6.7×10 −19 F which is in agreement with that expected for particles of this size.

Co-rich cobalt platinum nanowire arrays: Effects of annealing

The effects of annealing on the crystal structure and magnetic properties of Co-rich cobalt platinum nanowire arrays embedded in anodic aluminium oxide membranes have been investigated. For this purpose, a rapid thermal annealing to temperatures of 300°Cto800°C has been used. Transmission electron microscopy and scanning electron microscopy show that the nanowires have a mean diameter of 14nm and an estimated wire density of 7.8×1010cm−2. From x-ray diffraction patterns, we find that the nanowires are hcp and possess a preferred texture in which the c axis of the grains tends to lie along the major axis of the wire. Vibrating sample magnetometry measurements indicate that the easy axis is along the nanowire axis direction. Hysteresis loops, saturation magnetization, squareness ratio (Mr∕Ms), and coercivity (perpendicular and parallel to the nanowire axis) have all been investigated as a function of the annealing temperature (TA). Coercivity parallel to the wire axis first increases with TA, attains a maxi...

Magnetic nanowire arrays: A study of magneto-optical properties

Nanocomposite films of highly ordered magnetic nanowires embedded in anodic aluminum oxide templates were fabricated and magneto-optical properties were studied. Following our previous work, studies on Co nanowire arrays were carried out, where the MO properties in relationship with nanowire lengths and diameters were investigated. The results were found to be considerably different from corresponding bulk metal. We demonstrated that the Faraday effect was a convenient and useful probe for the study of the magnetic properties of these semitransparent nanocomposite films. A preliminary study on the photon energy dependence of the magneto-optical properties of Fe nanowire arrays was also conducted in the visible spectrum regime.

Magneto-optical characteristics of magnetic nanowire arrays in anodic aluminum oxide templates

Nanocomposite films consisting of regularly ordered iron nanowires embedded in anodic aluminum oxide templates have been fabricated and their magneto-optical properties studied by determining the four Stokes parameters of the transmitted laser beam (λ=670 nm), originally linearly polarized and at normal incidence to the film surfaces. The results of the nanowire arrays are found to be considerably different from that of bulk iron. While an increase in diameter of the nanowire leads to a substantial increase in the values of the Faraday rotation angles per unit length at a fixed value of the magnetic fields, they are substantially less than that of bulk iron, indicating that the effective media theory may not be directly applicable.

Generalized theory and application of Stokes parameter measurements made with a single photoelastic modulator

We report in this paper a generalized theory that describes the interaction between a monochromatic light beam and an optical system that includes one photoelastic modulator, one analyzer, and one photodetector. Based on the theory, a detailed four-step procedure is presented, which allows a precise measurement of the four Stokes parameters. An analysis of the systematic and random errors arising from the four-step measurements is also given as well as a calibration procedure that involves the use of a general retardation plate. As a practical application the procedure is used to analyze the magneto-optical properties of magnetic thin films grown on GaAs(001) substrates.

Composition-dependent structural and magnetic properties of Co1−xPtx (0.09⩽x⩽0.86) nanowire arrays

Highly ordered 14nm diam cobalt platinum alloy nanowire arrays with Pt content ranging from 9to86at.% have been electrodeposited into nanoporous anodic aluminum oxide templates. The crystal structure of the as-deposited Co1−xPtx nanowire arrays changes progressively from hcp to a mixture of hcp and fcc phases and finally to pure fcc with increasing Pt content. The coercivity, squareness, and saturation magnetization of the as-deposited nanowire arrays have been investigated as a function of nanowire composition.

Ion-beam-induced bending of freestanding amorphous nanowires: The importance of the substrate material and charging

Ion-beam irradiation offers great flexibility and controllability in the construction of freestanding nanostructures with multiple advanced functionalities. Here, we present and discuss the bending of free-standing nanowires, against, towards, and ultimately parallel to a flux of directional ion irradiation. Bending components both along and perpendicular to the incident ion beam were observed, and the bending behavior was found to depend both on the ion beam scanning strategy and on the conductivity of the supporting substrate. This behavior is explained by an ion-irradiation-related electrostatic interaction. Our findings suggest the prospect of exploiting this technique to engineer 3D nanostructures for advanced applications.

Composition and annealing temperature dependent properties of Co1−xPtx(0<x⩽0.2) alloy nanowire arrays

Arrays of Co1−xPtx(0<x⩽0.2) alloy nanowires have been fabricated by electrochemical deposition of metals into anodic aluminium oxide templates. Compositional and annealing dependent properties of these samples have been investigated by a range of techniques. X-ray diffraction experiments show that the crystal structure of alloy nanowire varies with the chemical composition, ranging from predominantly c axis oriented for pure Co to textured, e.g., Co87Pt13 and to nontextured pure Pt wires. Hysteresis loops, saturation magnetization, squareness ratio (Mr∕Ms), and coercivity (perpendicular and parallel to the wire axis) have all been investigated as a function of the composition. Rapid thermal annealing treatment demonstrates that nanowires with different composition shows different response to the annealing temperatures. The annealing temperature related changes in coercivity, squareness, crystal structure, and ultraviolet-visible spectra are also presented.

Ion irradiation of Co/Pt multilayer films

This article describes a microstructural and magnetic investigation of the modification of Co/Pt multilayer films by Ar ion irradiation which results in a change of the orientation of the magnetization from perpendicular to the film plane to the in-plane direction. Transmission electron microscopy observation confirms the general structure of the as-deposited samples and correlates with the x-ray reflectivity measurements in showing the eventual mixing of the layers and a large increase in grain size caused by the irradiation. TRIM (transport of ions in matter) calculations show that large numbers of recoils are placed outside the individual layers and in the adjacent layers on irradiation. This signifies considerable roughening of the interfaces and mixing in the multilayer structure. Magnetic measurements and magnetic force microscopy observations confirm the decrease of perpendicular anisotropy and the changes of the magnetization direction from the perpendicular direction to an in-plane direction afte...