Physical Properties of Magnetic As-Grown and Oxygen Annealed SnO₂: Co Thin Films
Stoian, Graţiela M. (author)
Van Molnár, Stephan (professor directing dissertation)
Stiegman, Albert (university representative)
Crede, Volker (committee member)
Dobrosavljevic, Vladimir (committee member)
Xiong, Peng (committee member)
Department of Physics (degree granting department)
Florida State University (degree granting institution)
Dilute magnetic semiconductor oxides are complex systems in which ferromagnetic and semiconducting properties coexist, potentially making these materials suitable for Spintronics applications. Understanding the fundamental physical processes taking place in these intricate materials could offer a better way to implement these structures into real life Spintronic devices. This dissertation is based on the study of the physical properties of a magnetic oxide system, namely SnO2 thin films doped with a few atomic percent Co, grown by two different methods. The ultimate goal of this study is to understand the physical mechanisms of high temperature ferromagnetism (RTFM) in SnO2-δ:Co. Two different methods for growing epitaxial SnO2-δ: Co thin films were employed: pulsed laser deposition (PLD) and RF Sputtering. In both cases, films were deposited on R-cut Al2O3 substrates under well controlled growth conditions in order to produce epitaxial thin films which behave ferromagnetically at room temperature and are semiconducting. Detailed structural and magnetic characterization demonstrates that PLD SnO2-δ:Co films grown under optimal oxygen pressure are single phase SnO2 with no presence of Co-based nanoclusters in the oxide matrix. Moreover, a direct relationship between structure, resistivity and magnetization is demonstrated, the number of structural defects influencing greatly the electromagnetic properties of the studied materials. Highly crystalline samples have a larger resistivity than less ordered materials. In addition, films grown with an optimal deposition rate have a saturation magnetization that is comparable to the Co2+ low spin state (1μB/Co). This is consistent with the XPS measurements which suggest that the valence of Co in SnO2 is +2, proving that Co2+ ions substitute for octahedral Sn4+ ions in the SnO2 lattice. Temperature dependent magnetic investigations indicate the presence of a granular-type magnetism, which is believed to occur due to formation of small Co-doped SnO2 grains with different particle sizes and blocking temperatures, existing in a dielectric matrix. Annealing samples in O2 enhances the ferromagnetism dramatically due to the strong intergranular interaction produced after the thermal treatment. Electrical transport data reveal there is a hopping-type activated conduction mechanism up to room temperature. Non-diffusive electrical transport is observed up to 350 K. The study done on RF sputtered films deposited by two methods: RF co-sputtering and sputtering from a 5 at.% Co - doped target, has shown that by changing the deposition parameters, one can produce films which are highly crystalline and insulating, some of which exhibit a paramagnetic signature, originating from the presence of noninteracting Co2+ ions. It was concluded that the magnetism in sputtered SnO2:Co thin films is not carrier mediated but is due to the superexchange interaction which couples the nearest neighbor Co spins antiferromagnetically, depending on the local lattice distortions and the spin environment. The moments per Co ion in these films determined from magnetic measurements are lower than the theoretical value of low spin state Co2+ ions. Moreover, XPS and TEM analysis suggests that Sn4+ is substituted by Co2+ ions. Annealing some of the samples in oxygen environment modifies their structure and enhances the magnetic moment per Co ion. The sputtered materials studied in this project may not be suitable for Spintronics applications since magnetism is not carrier mediated due to the very low carrier concentration achieved in these samples. The deposition process was found to be unreliable because of the difficulty to control the growth. This yielded discrepancies in the structural and magnetic properties, characteristics that will be outlined in this thesis.
Defects, Dilute Magnetic Semiconductor Oxide, Ferromagnetism, high Curie temperature, SnO2:Co, Spintronics
November 18, 2013.
A Dissertation submitted to the Department of Physics in partial fulfillment of the requirements for the degree of Doctor of Philosophy.
Includes bibliographical references.
Stephan von Molnár, Professor Directing Dissertation; Albert Stiegman, University Representative; Volker Crede, Committee Member; Vladimir Dobrosavljevic, Committee Member; Peng Xiong, Committee Member.
Florida State University
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