Ferromagnetism and point defects of undoped and Cobalt doped tin oxide nanopowders

The influence of annealing temperature and atmosphere on the structural and magnetic properties of undoped SnO2 and Co- doped SnO2 nanoparticles synthesized using a coprecipitation technique has been studied. For the undoped case the as-prepared SnO2 nanoparticles were annealed at 350-1000 °C in air and a mixture of Ar/N under ambient pressure. Annealing in an Ar/N atmosphere at different temperatures, substoichiometric SnOx phases were observed to coexist with the stable tetragonal rutile SnO2 structure. Micro Raman spectra measurements recorded at room temperature exhibit sharp Raman modes in the lower frequencies of the spectrum confirming the presence of SnOx phases. At high frequencies the fundamental Raman modes reveal remarkable red shift and broadening with decreasing in the annealing temperature. The presence of both Sn4+ and Sn2+ as well as the presence of oxygen vacancies was confirmed by the shape analysis of corresponding XPS Sn3d5/2 and O1s spectral line. Magnetic measurements indicate room temperature ferromagnetism for samples annealed at 800 and 1000 °C. When samples were annealed in oxygen atmosphere, the magnetic properties of the sample reduce greatly. As no magnetic impurities are present, these results together suggest that the observed FM in SnO2 nanopowders is arises from oxygen vacancies. We also investigate the effect of substitutional Co on the structural and magnetic properties of Sn1-xCoxO2 nanoparticles (with doping percentage varying from 0 to 10 %) which were verified to have the tetragonal SnO2 cassiterite structure after annealing in an Ar/N atmospheres at 600 °C. Room temperature Raman spectra revealed that with increasing Cobalt content, the A1g, B2g and A2u(LO) modes undergo large shift to lower frequencies and significant linewidth broadening. The redshift in the Raman A1g mode was explained by alloy potential fluctuation (APF) using a spatial correlation model (SC). Secondary phases of cobalt are excluded by XRD, Raman, and X-ray photoelectron spectroscopy (XPS). Oxygen related defects have been clearly studied from XPS and Raman spectroscopy. Magnetic measurement shown that samples annealed in the reduced atmosphere exhibited room temperature ferromagnetism (RTFM). Ours results suggest that here also the ferromagnetism is strongly correlated with the increase of oxygen vacancies in SnO2.