Abstract
In this thesis, we have studied the correlation between magnetism and oxygen vacancies in yttrium oxide based diluted magnetic oxide (DMO) systems. Yttrium oxide nanoparticles (NPs) doped with 3d transition metals (TM) cobalt and manganese are synthesized by thermal decomposition of yttrium acetylacetonate (Y(acac)3) in oleylamine. Thermal annealing in either oxygen or forming gas was employed to adjust the concentration of oxygen vacancies. The nanoparticles' size and structures of the Y2O3 host were investigated by x-ray powder diffraction (XRD) and high resolution transmission electron microscopy (HR-TEM). Cobalt and manganese K-edge x-ray absorption near-edge structure (XANES) spectra were used to determine the oxidation state of the dopant atoms. Local environments surrounding Co and Mn were probed by using extended x-ray absorption fine structure (EXAFS) techniques. Both Co and Mn impurity atoms were found to locate on interstitial sites with O nearest neighbors and shorter TM-O bond as compared to the Y-O bond in the Y2O3 host. Forming gas (5%H2/95%N2) annealing has significantly increased the concentration of the oxygen vacancies while oxygen gas annealing has decreased the oxygen vacancies in all samples. As revealed by SQUID measurements performed at temperatures 10K and 300K, the Co- and Mn-doped samples show combinations of ferromagnetism and paramagnetism while an undoped sample is diamagnetic. This indicates that the observed ferromagnetism is indeed due to the doping of transition-metal atoms in the samples. Our experimental results on the correlation between oxygen vacancies and magnetism appear to be rather consistent with the bound magnetic polaron model.