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Abstract
A spintronics device is a new paradigm of a multifunctional device, which allows the interplay of charge carriers and spin in a single system. Compared to conventional microelectronics, spintronics devices have several prominent features, such as non-volatility, fast data processing speed, low power consumption and high integration density. However, the integration of spintronics within the current semiconductor industry has to date been unsuccessful due to the low spin injection from metal to semiconductor. To achieve spin manipulation in semiconductor devices, a new category of materials with both ferromagnetic and semiconductive properties needs to be developed.
Diluted magnetic semiconductor (DMS) is a class of materials in which a fraction of the cations in a non-magnetic semiconductor matrix are substitutionally replaced by magnetic ions. The exchange interaction between the spin of the magnetic dopant and the itinerant carriers results in a collective ferromagnetic ordering. Currently, ZnO and TiO2 are two of the most promising host materials for a DMS for the purpose of room temperature ferromagnetism. In this dissertation, the microstructure and magnetic property of ZnO-based and TiO2-based DMSs will be discussed.
The structure and magnetic properties of Mn-doped and Nd-doped nanoparticles has been investigated in detail. Results indicate that both crystallinity and ferromagnetic ordering of Mn-doped and Nd-doped nanoparticles are decreased with an increase in doping concentration. A high Nd doping level even leads to antiferromagnetic coupling in a ZnO matrix. Additionally, post-annealing in air or argon can largely improve the crystallinity of Mn doped-ZnO, but eliminate ferromagnetic ordering, due probably to the removal of oxygen vacancy or the precipitation of Mn dopants. Oxygen partial pressure also has a significant influence on the microstructure and magnetic property of Co-doped TiO2 films. The films prepared at a low oxygen partial pressure show a homogeneous microstructure with fully magnetic volume, indicating intrinsic ferromagnetism. High oxygen partial pressure leads to the formation of phase segregation and a significantly low magnetic volume fraction. First principle calculations indicate that the origin of ferromagnetism is Co substitution of Ti and the formation of n type carriers enhances the formation of uniform microstructure and intrinsic room temperature ferromagnetism.