Annealing effects on the ferromagnetism of Ce0.97Co0.03O2-δ compounds

Diluted magnetic oxide of Ce0.97Co0.03O2-δ powder was synthesized by standard solid-state reaction method. Oxidized or reduced samples were obtained via O2 or Ar/H2 annealing, respectively. Magnetic investigations found that the as sintered Ce0.97Co0.03O2-δ is ferromagnetic at room temperature. The saturation magnetic moments (MS) are highly enhanced by Ar/H2 annealing, while sharply diminished by O2 treatments. X-ray photoelectron spectroscopy (XPS) measurements show that Ce4+ ions are partially reduced to Ce3+ with additional oxygen vacancies (VO) being introduced by Ar/H2 annealing. The increase of VO by Ar/H2 annealing is further proved by Raman spectra in which a new Raman scattering peak appearing at the frequency of 465 cm−1. This fact contributes to the enhanced ferromagnetism observed in reduced Ce0.97Co0.03O2-δ. On the other hand, VO density is reduced via O2 annealing and thus leading to a decreased MS. On all accounts MS is found to be in direct ratio-dependent on VO density in Ce0.97Co0.03O2-δ. The experimental results are well consistent with the F-center mediated ferromagnetism mechanism recently found in some transitional metal (TM) doped oxides, and thus is very important in understanding the mechanism of the ferromagnetism (FM) origin in many TMs-doped insulating oxides.