Room-temperature Ferromagnetism in Nanostructured Co-doped ZnO
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Room-temperature Ferromagnetism in Nanostructured Co-doped ZnO M. Wei, N. Khare, K. A. Yates, D. Zhi, and J. L. MacManus-Driscoll Department of Materials Science and Metallurgy, University of Cambridge, Cambridge, UK ABSTRACT Nanosized Co-doped ZnO samples were synthesized using an ultrasonic spray assisted chemical vapour deposition method. Microstructural and magnetic properties of these samples were studied. The room-temperature ferromagnetism was observed in the Co-doped ZnO. Also, x-ray analysis revealed a wurtzite ZnO structure with a small change of the lattice constants due to the doping of Co in ZnO. Raman spectroscopy of the Co-doped ZnO films indicated direct substitution of Co. Scanning electron microscopy showed nanostructured Co-doped ZnO with a ring or cup shape. Transmission electron microscopy analysis revealed nano grains within the rings of an average diameter of around 10 nm. Both energy dispersive spectroscopy and energyfiltered transmission electron microscopy indicated a uniform distribution of Co.
INTRODUCTION Dilute magnetic semiconductors (DMSs) have attracted great interests in recent years because of the potential use of both the charge and spin of electrons for spintronic devices [1,2]. DMSs with practical ordering temperatures could lead to new classes of device and circuits, including spin transistors, ultra-dense non-volatile semiconductor memory and optical emitters with polarized output [3]. Among the DMS materials studied so far, oxide semiconductors, are of particular interest and a number of transition element doped semiconducting oxides have been either theoretically predicted or experimentally demonstrated, to show ferromagnetism above room temperature [4-13]. Most of the reported studies on the DMS oxides have focused on bulk and thin films. In this paper, we synthesize Co-doped ZnO nanowire spheres and nanocrystalline rings and cups exhibiting room-temperature ferromagnetism using a catalyst-free and self-seeded chemical vapour deposition method that we recently developed for ZnO nanowire growth [14].
EXPERIMENTAL DETAILS Solutions of zinc acetate dihydrate (Zn(CH3COO)2·2H2O) and cobalt acetate hydrate in a methanol/ethanol in deionized water with different concentrations and Co/Zn ratios were prepared as precursor solutions. Before deposition of the nano-features, 50 ml of a 0.005M precursor solution was made with the purpose of forming a seed layer on the substrate. Once the Si substrate reached the desired temperature, the precursor aerosol was generated from the vibrations of the transducers and was delivered towards the heated substrate by an Ar carrier gas. The gas flow rate used was between 0.5 and 5 l/min. A thin seed layer was deposited at a substrate temperature of 400ºC. The deposition time for the seed layer was 5-10 minutes. Immediately after growth of the seed layer, the substrate temperature was increased to 500ºC for
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the subsequent growth of nanowires, during which the precursor solution concentration was changed to 0.001 M. The subsequent
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