Effect of annealing temperature on the structural and magnetic properties of CeO2 thin films

CeO2 thin films were deposited on silicon(100) substrates via a radio frequency magnetron sputtering technique and subsequently annealed with a 10 degrees C/min-heating step in a vacuum tube furnace between 400 degrees C and 800 degrees C under an argon atmosphere for 2 h to generate a higher concentration of oxygen vacancies. All samples were characterized to determine their phase, morphology, valence states and magnetic properties using grazing incidence X-ray diffraction, Raman spectroscopy, field emission scanning electron microscopy, X-ray photoelectron spectroscopy (XPS) and vibrating sample magnetometry. All samples showed X-ray diffraction peaks corresponding to a cubic fluorite structure with no evidence of impurities. The cubic structure was further confirmed by observation of the Ce-O8 vibrational unit stretching mode of F-28 associated with the CeO2 structure. Evidence for defect/oxygen vacancies was found in the Raman results. The morphology of all films showed aggregated nanoparticles amongst columnar structures with average particle sizes increasing from similar to 59 nm to similar to 68 nm when annealed at temperatures from 400 degrees C to 800 degrees C. The thicknesses of the CeO2 films decreased from 327 +/- 5 nm to 295 +/- 4 nm with increasing annealing temperature. The presence of Ce4+ and Ce3+/oxygen vacancies in all samples was confirmed using XPS. All CeO2 thin film samples showed weak ferromagnetic behavior with a maximum magnetization value of 27.3 X 10(3)A/m for a sample annealed at 700 degrees C. This was approximately 7 times better than that of the as-grown material before annealing. The relationship between higher magnetization values and higher concentrations of oxygen vacancies mediating an exchange mechanism via ferromagnetic coupling was investigated.