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    A comparison of the magnetic properties of Ni and Co nanowires deposited in different templates and on different substrates
    (ELSEVIER SCIENCE BV, 2015) Yalcin, O.; Kartopu, G.; Cetin, H.; Demiray, A. S.; Kazan, S.
    Nickel (Ni) and cobalt (Co) nanowire arrays (NWs) grown by electrode position in porous nano-templates are studied by the ferromagnetic resonance (FMR) technique at room temperature (RT) by comparing the effects of template type (alumina and polycarbonate) and the deposition substrate (i.e., metallic back contact). The line-width and resonance field of the FMR spectra strongly depends on the orientation of the applied field direction. A model is developed to analyze the spectra in order to extract the magnetic parameters such as g-values, spin-spin relaxation times (T-2) and uniaxial an isotropy parameters. The experimental FMR spectra and their resonance field values were fitted using the imaginary part of magnetic susceptibility and a dispersion relation of magnetization, including the Bloch-Bloembergen type damping term. The easy axes of magnetization for all Ni and Co NWs were found to be perpendicular to the wire-axis. Surface spin modes have been observed only when pure Au was used as substrate. A discussion will be provided to explain the observed differences in terms of the anisotropic behavior and magnetic parameters of the NWs for different substrates and growth templates. (C) 2014 Elsevier B.V. All rights reserved.
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    Perpendicular silver nanowire arrays on an ITO/glass substrate for plasmonic applications
    (IOP PUBLISHING LTD, 2014) Kartopu, G.; Choy, K-L; Yalcin, O.
    Ordered metal nanoparticle arrays are currently considered for novel optoelectronic and magnetic applications due to their unique properties. Template-grown ordered nanorod or nanowire arrays in particular are very interesting since they can be produced inexpensively over large scales and have tailorable properties. In this study, synthesis and properties of silver (Ag) nanowire arrays grown on transparent conducting indium tin oxide/glass substrate using thin-film anodic alumina nano-templates are investigated. The optical (plasmonic) properties of freestanding Ag nanowire arrays characterized by absorption spectroscopy were explained by modelling via the finite element method. The plasmonic activity of such nanowire arrays was then demonstrated by the well-known surface-enhanced Raman scattering effect through Raman mapping versus optical microscopy. Simulations confirmed the dependence of plasmonic properties to sample geometry and suggested their tunability through parameters such as the wire diameter and packing density. Such plasmonic nanowire arrays can find use in novel applications, e.g. the third generation solar cells, nano-sensors, etc since they can be addressed both optically and electrically through the conducting, transparent substrate.
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    Size effects and origin of easy-axis in nickel nanowire arrays
    (AMER INST PHYSICS, 2011) Kartopu, G.; Yalcin, O.; Choy, K. -L.; Topkaya, R.; Kazan, S.; Aktas, B.
    High quality compact Ni nanowire (NW) arrays with aspect ratios (wire length/diameter) varying between 70-171 for a wire length of similar to 6 mu m, and between 3-400 for a constant wire diameter of 60 nm were successfully grown by direct current electrodeposition into free standing porous alumina templates having a lattice constant, i.e., interpore distance, of 105 nm. The NWs have been investigated using a combination of scanning-and transmission-electron microscopies, selected-area electron diffraction, x-ray diffraction analysis, ferromagnetic resonance (FMR), and vibration sample magnetometer techniques at room temperature. Microscopic and diffraction results show that the wires are uniform and mostly single-crystalline, being 220-oriented along the growth direction. Magnetic properties of samples are heavily dependent on the wire length as well as the diameter or packing factor, P (the volume fraction of wires/template). The FMR spectra and the field orientation dependence of the resonance field values were fitted using the imaginary part of magnetic susceptibility and a dispersion relation of magnetization, including the Bloch-Bloembergen type damping term. Combined with hysteresis (M-H) results, these indicate that the preferred (easy) axis of magnetization is parallel to the NW-axis for P < 33%. On the other hand, strong interwire exchange interactions are determined to supply a magnetization state with preferred axis perpendicular to the wires for samples with P > 33%. Comparison with relatively thick (110 nm diameter) and loosely packed (P similar to 10%) Ni NW arrays suggest that the magnetization reversal mechanism could be that of curling type. (C) 2011 American Institute of Physics. [doi:10.1063/1.3531565]