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

Abstract

CeO2 thin films were deposited on silicon(100) substrates via a radio frequency magnetron sputtering technique and subsequently annealed with a 10 °C/min-heating step in a vacuum tube furnace between 400 °C and 800 °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 F2g 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 ~59 nm to ~68 nm when annealed at temperatures from 400 °C to 800 °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 × 103 A/m for a sample annealed at 700 °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.