Role of oxygen vacancies in the surface charging of CoFe₂O₄ thin films: A temperature dependent X-ray photoemission spectroscopy study of CoFe₂O₄ thin films

We observed large increases in the measured binding energies of the Co and Fe 2p_3/2 levels in X-ray photoemission spectroscopy (XPS) of CoFe₂O₄ thin films at room temperature, due to large photovoltaic surface charging of the film. When the film was heated to 332 ⁰C from room temperature, the binding energies of the 2p_3/2 core levels of Co and Fe atoms decreased significantly by about 5 eV. The binding energies of Co and Fe 2p_3/2 levels, however, did not change when the film was cooled in vacuo from 332 ⁰C to room temperature. This irreversible transition became reversible when the CoFe₂O₄ was annealed in oxygen. The results are significant in that surface charging and hence the surface electronic properties can change with number of oxygen vacancies. We can compare the surface charging seen with CoFe₂O₄ to NiFe₂O₄ and NiCo₂O₄ thin films. The increasing conductivity (or decreasing surface charging) at the surface with increasing temperature may be due to increased carrier concentration, but the nature of the surface oxide and the surface defect density may play a role.