Oxide thin films for spintronics application growth and characterization

• Autor(es): Popovici, Nicoleta, 1973-
• Data: 2009
• Identificador Persistente: http://hdl.handle.net/10451/1635
• Origem: Repositório da Universidade de Lisboa
• Assunto(s): Filmes finos; Física; Teses de doutoramento - 2009
• Tipo de Documento: Tese de doutoramento
• Idioma: Inglês
• Orientador(es): Conde, Olinda Maria Quelhas Fernandes, 1951-
• Contribuidor(es): Repositório da Universidade de Lisboa

Descrição

Tese de doutoramento, Física (Física), Universidade de Lisboa, Faculdade de Ciências, 2009

During my PhD research I have synthesized thin films of a material known as a diluted magnetic semiconductor (DMS) using the pulsed laser deposition (PLD) technique. This material is envisioned to be of importance in the emerging field of spintronics where both the charge and the spin of the carriers can be combined to yield unique functionalities. It was envisaged that if spin polarized charge carriers can be injected into a semiconductor efficiently at practical working temperatures then some novel devices might be fabricated. Few applications include on-chip memories, ultradense non-volatile semiconductor memories, ultra-low power devices, optical emitters with polarized output and far reaching applications in quantum computations. The material of interest in this dissertation is titanium dioxide, an optoelectronic semiconductor with a wide bandgap that can be tuned to higher or lower values by appropriate doping. It is an ideal candidate for spintronics applications because Ti leads to pretty high solubility of transition metals such as Co, in TiO2. X-ray diffraction (XRD) was used for phase analysis and crystallinity studies. High quality, single phase Co-doped TiO2 thin films were synthesized on (0001) sapphire substrates. The chemical composition of the samples was evaluated by Rutherford backscattering measurements, revealing that the distribution of Ti is homogeneous on the surface and along film depth; the Co:Ti ratio was determined for most of the films investigated. The thickness and band gap energy were determined from optical transmittance measurements carried out in a UV-VIS-near IR spectrophotometer (300 1100 nm). The resistivity of the thin films was measured by the standard four-point probe method at room temperature, and the magnetization hysteresis loops were recorded with a vibrating sample magnetometer from room temperature to 10 K. The magnetization vs. temperature curve displayed is not characteristic of a typical ferromagnet but suggest an hybrid behaviour that might be due to the presence of Co nanoclusters. This behaviour was confirmed by magnetic circular dichroism spectroscopy which suggests a spin splitting of the conduction band at the energy band edge of our films.

Fundação para a Ciência e a Tecnologia (FCT), (BD16573/2004) and FCUL Physics Department