Magnetic resonance studies of Co ²⁺ ions in nanoparticles of SnO ₂ processed at different temperatures

Cobalt doping (≤1%) produces ferromagnetism at room temperature in semiconducting Sn O2, presumably due to oxygen vacancies and/or changes in carrier concentration. Electron paramagnetic resonance (EPR) is a sensitive technique to investigate the Co ionic states and their local environments and/or interactions. This paper reports EPR studies of Co2+ ions doped in chemically synthesized nanoparticles of Sn O2 carried out at 5 K. EPR spectra were recorded from 600 °C prepared Sn O2 with Co concentrations of 0.5%, 1%, 3%, 5%, 8%, and 12% and from 1% Co-doped Sn O2 prepared at temperatures of 150, 250, 350, 450, 600, and 830 °C. Each EPR spectrum in samples with cobalt doping can be simulated as an overlap of spectra due to two broad ferromagnetic resonance lines and those due to interstitially and substitutionally incorporated Co2+ ions with effective spin S=12 characterized by their particular g and A tensors. It is concluded that the Co2+ ions occupy substitutional as well as interstitial sites of Sn O2 and that a fraction, albeit small, of the doped Co2+ spins contribute to the ordered ferromagnetic state. The relative concentrations of these different components depend on the annealing temperature and Co concentration of the samples.