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Preparation of TiO_2 based diluted magnetic semiconductors and magnetic studies

Author: MengHaiJuan
Tutor: HouDengLu
School: Hebei Normal
Course: Condensed Matter Physics
Keywords: Diluted magnetic semiconductors TiO2 Room temperature ferromagnetism Structure defects Oxygen vacancies
CLC: O472
Type: Master's thesis
Year: 2007
Downloads: 164
Quote: 0
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Abstract


Diluted magnetic semiconductors （DMSs） consist of nonmagnetic semiconducting materials doped with a few at% of impurity magnetic cations of 3d transition metal （Fe、Co、Ni） or 4f rare earth （Eu, Gd）. Due to their potential application in the rapidly developing field of spintronics, DMSs have attracted widespread attention. For device application, however, increase in Curie temperature up to at least room temperature is required.The discovery of room temperature ferromagnetism in Co-doped TiO2 film has prompted searches in this wide gap oxide semiconductor.The origin of ferromagnetism in oxide DMSs remains a very controversial topic. There are many different even contrary experimental results. Some studies indicate that the ferromagnetism in transition metal doped TiO2 may come from precipitation of magnetic clusters or from secondary magnetic phases. On the other hand, some reports suggest the absence of magnetic clusters or secondary phases and support intrinsic ferromagnetic origin.In order to verify these issues, we have grown TM（ Fe, Co, Ni, Cu） doped TiO2 thin films using reactive magnetron sputtering on SiO2 substrates under different treatments and fabricated Co-doped TiO2 compounds by the sol-gel technique. The structure and magnetic properties of samples have been thoroughly investigated by X ray diffraction, Scanning Probe Microscopy, X-ray Photoelectron Spectroscopy, Vibrating Sample Magnetometer, Physical Property Measurement System, and so on. The structure of Co: TiO2 compounds transit from amorphous phase to anatase then change to rutile with the increase of annealing temperature. The XRD results also indicate a monotonic decrease in the lattice constant with increasing Co concentration. The decrease may be due to the substitution of Co ion into the Ti site. But magnetic measurements show that all of the powders are paramagnetic.Ni-doped TiO2 and Fe-doped thin films were fabricated by reactive magnetron sputtering on SiO2 substrates. The doping and annealing pressure effects on structure and magnetism for the films have been systematically investigated. The results show that ferromagnetism originated from the doped matrix and enhancement （suppression） of ferromagnetism is strongly correlated with the increase （decrease） of oxygen vacancies in TiO2.?Further, we report here the observation of significant room temperature ferromagnetism in a semiconductor doped with nonmagnetic impurities, Cu-doped TiO2 thin films grown by reactive magnetron sputtering. Films annealed in air were nonmagnetic while those annealed in vacuum were ferromagnetic with a Curie temperature about 350 K. The magnetic moment per copper atom decreased as the copper concentration increased. These results show that both the oxygen vacancies and the distance between nearest neighbor copper atoms play a crucial role for the appearance of magnetism.

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