Doctor of Philosophy
Institute for Superconducting and Electronic Materials - Faculty of Engineering
Farhoudi, Mohammad Mehdi, Studies of structures, transport and magnetic properties of doped novel three-dimensional perovskite compounds, Doctor of Philosophy thesis, Institute for Superconducting and Electronic Materials - Faculty of Engineering, University of Wollongong, 2009. http://ro.uow.edu.au/theses/3047
Perovskite oxide compounds have generated a great amount of interest over the past twenty years, as they exhibit exotic magnetic, electric, and magnetoelectric properties, which, apart from their intrinsic interest, would have a wide range of applications in industry, with special utilization in the engineering of data storage devices.
Starting with a general definition of the perovskite structure of ABO3 materials, the magnetic and transport properties of cobalt and manganese based compounds are reviewed. The objective of this thesis is focused on the investigation of the structures, and the magnetic and transport properties of some typical cobalt or manganese based three-dimensional perovskite compounds doped with various elements.
Polycrystalline samples of RE1-xSrxCoO3 (RE = Gd, Nd, Ho, Y, and Dy), REMnxCo1-xO3 (RE = Gd and Ho), and La0.33Sr0.66Fe1-xCrxO3 were fabricated and characterized in detail using X-ray diffraction, Rietveld structural refinement, magnetic and transport measurements, X-ray absorption near-edge (XANES) spectroscopy, and far-infrared (FIR) spectroscopy.
Perovskite GdCo1-xMnxO3 (x = 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8) compounds were prepared by solid-state reaction. Structures were characterized using X-ray diffraction and the Rietveld refinement method. The compounds crystallized in the orthorhombic structure with Plmn space group. Crystal lattice parameters constants decreased with increasing Mn doping level. DC magnetization and ac susceptibility were studied over a wide temperature range and different frequencies. A typical spin glass state was observed in all samples around the same temperature of 122 K, where the compounds changed from the paramagnetic to the ferromagnetic state. For x = 0.5, secondary transitions were observed both in dc and in ac measurements, which were independent of the frequency, but depended on the doping level. The valences of Co and Mn were estimated with possible spin states.
The structures, magnetism, giant magnetoresistance, and spin and valence states of Co ions were investigated in perovskite Gd1-xSrxCoO3 (x = 0-0.5) compounds synthesized by a solid state reaction method. Results showed that all the samples are ferromagnetic around 150 K, and the magnetization and coercive field increase with the Sr doping level. A spin glass state was observed. Furthermore, the resistivity drops systematically with x. Giant magnetoresistance of over 60% was observed for the Gd0.5Sr0.5CoO3 sample. Spin state assessment indicates that the Co3+ and Co4+ ions are present in intermediate and high spin states.
Polycrystalline perovskites compounds HoMn1-xCoxO3 (x = 0-0.8) were prepared by conventional solid-state reaction. Rietveld refinement indicated that, instead of a hexagonal structure with P63cm symmetry for HoMnO3, the compounds doped with Co crystallized in the GdFeO3-type orthorhombic structure containing distorted (Co/Mn)O6 octahedral. The dc magnetization was measured in fields up to 5 T over a wide temperature range from 350 K down to 5 K. It was found that the system gradually changed from paramagnetic for x = 0 to ferromagnetic with a Curie temperature, TC, of 90 K for x = 0.5, then to paramagnetic again for x = 0.8. X-ray absorption near-edge spectra (XANES) were collected to determine the valences of both Co and Mn ions. The results showed that Co and Mn were present mainly as mixed valences of Co2+ + Co3+, Mn4+ + Mn3+, Co3+ +Co4+, and Mn4+, depending on the value of x. When the XANES results were combined with Curie-Weiss fitting, it was found that both Mn4+ and Co3+ were present as high spin states for x > 0.2, whereas, Co4+ appeared in either intermediate or high spin states, closely corresponding to the lattice variations along the (010) direction.
HoMn1-xCoxO3 (x = 0-0.8) compounds were also studied by far-infrared (FIR) spectroscopy in order to gain information on the infrared active phonon modes. The data indicated that phonon modes significantly changed with increasing cobalt doping level. Four main bands were assigned as external, torsional, bending, and stretching bands. The external vibration energy remained the same at 1190 cm-1 for Co doping with x < 0.5 and shifted to higher energy for x > 0 5. The torsional and bending bands exhibited splitting. The stretching band is at 600 cm-1 for all samples, but the bandwidth is reduced as Co doping is increased. The transmission spectrum of HoMn4/5Co1/5O3 was analyzed to obtain the spectrum of the optical density. The minimum number of oscillators to obtain a reliable fit is 5, using a sum of non-interacting harmonic oscillators.
Cr doping effects on the crystal structures and magnetic properties for La1/3Sr2/3Fe1-xCrxO3 were studied in this work. It was observed that single phase samples with the cubic crystal structure can only form within a narrow solubility range. Magnetic measurements showed that the resulting compounds are magnetic insulators.
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