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Room temperature ferromagnetic semiconductors not only play an important role in the development of spintronics, but also are interesting from the viewpoint of fundamental physics since they combine two important realms of condensed matter physics: magnetism and semiconduction. This dissertation presents a study on selected magnetic transition metal doped oxides as potential diluted magnetic semiconductors (DMS). The goal is to understand the room temperature ferromagnetism (RTFM) mechanism in TiO2-δ: Co. Several steps have been taken to approach this problem. Polycrystalline and epitaxial TiO2-δ: Co thin films were deposited on (0001) α-Al2O3 substrates by PLD under well controlled growth conditions. Detailed characterization demonstrates that laser ablated TiO2-δ: Co under low oxygen growth pressure (PO2) is a multiphase system with Co nanoparticles precipitated out of the TiO2 lattice. Depending on PO2, the samples show dramatically different magnetic behavior. For example, with PO2 lower than a critical value a transition from a blocked magnetic state at low temperature to a superparamagnetic state at higher temperature is observed, which is a signature of superparamagnetic Co nanoparticles. For samples grown under higher PO2, the blocking temperatures are higher than 350K, as determined from zero field cooled/ field cooled (ZFC/FC) measurements. In agreement with this observation, the hopping transport behavior expected in multiphase granular metal systems, ln(ρ ) ~ T −1/ 2 is observed for most of the samples in the measured temperature range and low bias (linear I-V) regime. Related hopping conduction behaviour in magnetic fields and high electrical fields (non-linear I-V regime) will also be presented. Cross sectional TEM images provide further corroboration of the multiphase structure of these materials. Preliminary work to understand the mechanism dominating the room temperature ferromagnetism in SnO2-δ: Co is also presented. To obtain optimal RTFM, films fabricated over a wide growth parameter space were examined through magnetic, transport, and microstructure characterization. This research suggested a uniform, xviii intrinsic ferromagnetism in SnO2-δ: Co films. Defects, such as oxygen vacancies and crystal lattice defects, are suggested to be the cause of RTFM.
Hopping Conduction, Co Precipitation, Tin Dioxide, Diluted Magnetic Semiconductor, Ferromagnetism, Titanium Dioxide
Date of Defense
February 21, 2007.
Submitted Note
A Dissertation submitted to the Department of Physics in partial fulfillment of the requirements for the degree of Doctor of Philosophy.
Bibliography Note
Includes bibliographical references.
Advisory Committee
Stephan von Molnár, Professor Co-Directing Dissertation; Patricia A. Stampe, Professor Co-Directing Dissertation; Albert E. Stiegman, Outside Committee Member; Jorge Piekarewicz, Committee Member; Peng Xiong, Committee Member; Vladimir Dobrosavljevic, Committee Member.
Publisher
Florida State University
Identifier
FSU_migr_etd-3692
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