Magnetic moment and coupling mechanism of iron-doped rutile TiO2 from first principles

The magnetic ground state of Fe-doped rutile TiO2, an oxide-based dilute magnetic semiconductor, has been investigated within the hybrid-exchange approximation to density-functional theory. FexTi1-xO2 with x=0.125 has been simulated by means of a 24-atom supercell with Fe doped substitutionally for Ti and this is established as the dilute limit through explicit comparison with calculations on a 192-atom supercell (x=0.0156). A detailed study of the nature and stability of the predicted ground state with respect to variations in the oxidation state of the Fe ion, the delocalization or self-trapping of holes donated to the lattice, and the treatment of electronic exchange and correlation is presented. The ground state is found to be well described by a model based on an Fe3+-d(5) in a high spin state coupled to a partially delocalized hole accommodated in the 2p states of neighboring oxygen ions. No evidence is found for ferromagnetic coupling suggesting that the observed ferromagnetism in this system is dependent upon additional structural and/or electronic defects.