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Type of Document Dissertation Author Hu, Erhong URN etd-02232007-182932 Title Investigation of Oxygen Growth Pressure Effects on TiO2-ä: Co Degree Doctor of Philosophy Department Physics, Department of Advisory Committee
Advisor Name Title Stephan von Molnár Committee Chair Patricia A. Stampe Committee Co-Chair Albert E. Stiegman Committee Member Jorge Piekarewicz Committee Member Peng Xiong Committee Member Vladimir Dobrosavljevic Committee Member Keywords
- Hopping Conduction
- Co Precipitation
- Tin Dioxide
- Diluted Magnetic Semiconductor
- Ferromagnetism
- Titanium Dioxide
Date of Defense 2007-02-21 Availability unrestricted Abstract ABSTRACTRoom 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-ä: Co. Several steps have been taken to approach this problem. Polycrystalline and
epitaxial TiO2-ä: Co thin films were deposited on (0001) á-Al2O3 substrates by PLD
under well controlled growth conditions. Detailed characterization demonstrates that
laser ablated TiO2-ä: 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(ñ ) ~ 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-ä: 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,
intrinsic ferromagnetism in SnO2-ä: Co films. Defects, such as oxygen vacancies and
crystal lattice defects, are suggested to be the cause of RTFM.
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