Distribution and magnetization of Co near the rutile TiO2 (110) surface: A first-principles study

The room-temperature ferromagnetism demonstrated by Co-doped TiO₂ films remains a challenge to our understanding, notwithstanding intensive experimental and theoretical investigations. We have calculated the binding energy and spin-polarization of Co atoms doped in the rutile TiO₂ (110) surface using first-principles method, aiming to elucidate the relationship between structure and magnetism of Co-doped TiO₂ films. In a defect-free surface the binding energy of Co substituting for Ti reduces slightly with the depth into the surface, suggesting a very minor Co depletion near the surface. More interestingly, the stability of ferromagnetic coupling over anti-ferromagnetic coupling of Co atoms decreases rapidly when it goes from surface to the bulk region. The residual ferromagnetism in the surface will give rise to a non-vanishing average magnetic moment of the thin film. The calculated pairing energy of Co is 0.12 eV/Co in the surface layer and 0.20 eV/Co in the middle layer of the film, an indication that Co atoms have a tendency to aggregate in both regions and that pairing will not modify the Co concentration in the direction vertical to the surface.