Year

2004

Degree Name

Doctor of Philosophy (PhD)

Department

Faculty of Engineering

Abstract

Metal oxides constitute an interesting class of materials with a wide range of properties. They exhibit a variety of phenomena such as ferromagnetism, ferroelectricity, and superconductivity. In the last few years, a new aspect of the metal oxides has come to the fore. This is colossal magnetoresistance (CMR) exhibited by certain manganese oxides, in particular rare earth manganites of perovskite structure. Due to their CMR, there have been many investigations on perovskite manganese oxides, especially A(subscript 1-x)B(subscript x)MnO(subscript 3) (where A is a lanthanum series element and B is a divalent element such as Sr or Ca) compound. These recent investigations, both theoretical and experimental, contribute to an improved understanding of the fundamental science of these materials. This project was aimed at studying this novel material. Samples were produced by bulk materials synthesis using the solid state reaction method and by thin film growth using the pulsed laser deposition (PLD) technique. We have carried out X-ray diffraction measurements and electrical-magnetic transport measurements to determine the crystal structure and resistivity of CMR samples of different doping. The structural properties, transport properties, and magnetic properties in different samples have been observed and analyzed. In spite of many interesting physical phenomena discovered in recent years, optical properties of ABMnO(subscript 3) compounds have not been investigated in detail. Far-infrared (FIR) spectroscopy is sensitive to lattice vibration modes and so is an appropriate probe of CMR materials, in which the lattice vibration plays an important role. By using FIR techniques, the materials phonon vibration modes have been determined and analyzed for all the samples studied. The major results from the PhD study are: (1) CsI as a carrier (or matrix) is for the first time used for CMR materials in the FIR transmission experiment. The CsI transmission range is wilder than the traditional carrier KBr, and the FIR transmission data area similar to reflection data while only needing little material. The FIR results on different materials samples prepared by using CsI have been published in five papers. (2) La(subscript 1-x)Ca(subscript x)Mn(subscript 09)Li(subscript 0.1)O(subscript 3) compounds, when 10% Li ions are doped on the Mn site, are of orthorhombic structure with lattice parameters that decrease with increasing Ca concentration. The FIR spectra indicate the Li ions occupy the Mn site - while the Li ion is too light to be detected by EDS. (3) The FIR transmission spectra of A(Co(subscript 05)Mn(subscript 05)O(subscript 3) [or (A(subscript 2)CoMnO(subscript 6)] (A = La, Nd, Dy, Ho, Yb) revealed the phonon modes systematic shift and split and the appearance of new modes as the mass of the lanthanide elements increases. The data suggests a strong correlation between the phonon modes and the magnetic properties of these compounds, most likely through the electron-phonon interaction. Also the spin glass state was observed in these materials. (4) The surface and interface of La(subscript 0.7)Ca(subscript 0.3)MnO(subscript 3) thin film on MgO substrate were studied by atomic force microscopy (AFM) and secondary ions mass spectroscopy (SIMS). By comparing study thin film and the bulk target, it was found: (i) The transition temperature (T(subscript MI)) is around 150 K for thin film, about 100 K lower than the target. We explain that from theory of weak localization and additional carrier scattering caused from possible diffusion. (ii) The resistivity of the thin film decreases much faster in the insulating regime because the activation energy is smaller than in the target, assuming the carrier transport in the insulating regime is of the activation type. (iii) From the FIR refection spectra of thin film, a new surface mode was observed. [Note: this abstract contained scientific formulae that would not come across on this form. Please see the 01Front files abstract for the full details.]

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