Role of Ti 3d carriers in mediating the ferromagnetism of CoTiO₂ anatase thin films

We study the surface and bulk electronic structure of the room-temperature ferromagnet CoTiO₂ anatase films using soft- and hard-x-ray photoemission spectroscopy with probe sensitivities of ∼1 and ∼10nm, respectively. We obtain direct evidence of metallic Ti3+ states in the bulk, which get suppressed to give a surface semiconductor, thus indicating the difference in electronic structure between surface and bulk. X-ray absorption and resonant photoemission spectroscopy reveal Ti3+ electrons at the Fermi level (E_F) and high-spin Co2+ electrons occurring away from E_F. The results show the importance of the charge neutrality condition: Co2++VO2-+2Ti4+ Co2++2Ti3+ (V_O is oxygen vacancy), which gives rise to the elusive Ti 3d carriers mediating ferromagnetism via the Co 3d-O 2p-Ti 3d exchange interaction pathway of the occupied orbitals.