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Title: Ferromagnetism in carbon-doped ZnO thin films and nanostructures
Authors: Wei, Changsong
Degree: Ph.D.
Issue Date: 2012
Abstract: The ZnO-based diluted magnetic semiconductors (DMS) have attracted scientific interest due to their unique properties and multifunctionality in ferromagnetic and ferroelectric. Transition metals have been chosen as dopants for ZnO-based DMS. However, the origin of the ferromagnetism is controversial as it is suspected that the ferromagnetism is induced by transition metal dopant clusters or secondary phases. In order to avoid the ambiguous ferromagnetism in ZnO-based DMS, carbon, as a non-magnetic element, has been suggested to be an appropriate dopant for ZnO-based DMS. Hence, in this thesis, the possibility of using carbon as a dopant to produce ZnO-based DMS is explored. The ZnO thin films were prepared firstly by the filtered cathodic vacuum arc technique. Optimization of the growth parameters, such as substrates temperature and oxygen pressure, were achieved in relation to the structural, electrical and optical properties of the ZnO thin films. The fabrication of the carbon-doped ZnO (ZnO:C) thin films and nanostructures were performed by the ion beam irradiation (IBI) technique. The two vital parameters of IBI, ion beam energy and irradiation time, were investigated systematically to determine the effective carbon doping parameters for the formation of ferromagnetic ZnO:C thin films and nanostructures.
The structural, electrical and magnetic properties of the ZnO:C thin films and nanoneedles were characterized. The ZnO:C thin films and nanoneedles all showed the hexagonal wurtzite structure with high crystallinity. The ferromagnetism in the ZnO:C system with Curie temperature (TC) higher than 330 K was found. The saturated magnetization (Ms) was enhanced with the increase of the carbon concentration. The anomalous Hall effect and negative magnetoresistance with p-type conduction were detected in the ZnO:C system. These unusual transport behaviors in the ZnO:C system are regarded as the effect correlating to the magnetic inhomogeneous and magnetic phase transition. The ferromagnetism in the ZnO:C system is stable in ambient air for more than one year. In addition, the magnetic anisotropy was detected as a signature of intrinsic ferromagnetism in the ZnO:C system. As for the ZnO:C nanoneedles, there is another dominated effect due to the strong dipoledipole magnetic interaction among the nanoneedles resulted in the magnetic anisotropy. Several characterization methods were employed to investigate the origin of ferromagnetism in the ZnO:C system. Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) analysis suggest that the carbon doping introduced some defects in the ZnO samples leading to the decrease of grain size of the ZnO:C with increasing the carbon concentration. X-ray diffraction (XRD) spectra show the reduction of lattice constant c with carbon doping which is expected as smaller C ions substituted into the O sites of ZnO lattices. The X-ray absorption near-edge structure (XANES) spectra indicate a strong hybridization of O 2p orbitals with C 2p states in ZnO:C system. This p-p interaction leads to an indirect ferromagnetic coupling of C atoms. It is speculated that the substitution of C ions into O sites is related to the origin of the ferromagnetism in the ZnO:C system.
Subjects: Ferromagnetism.
Zinc oxide thin films -- Magnetic properties.
Hong Kong Polytechnic University -- Dissertations
Pages: xvii, 121 leaves : ill. (some col.) ; 30 cm.
Appears in Collections:Thesis

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