Theoretical study of magnetism in transition-metal-dopedTiO2andTiO2−δ

Abstract

In recent years there has been an intense search for room temperature ferromagnetism in semiconductors doped with dilute magnetic impurities, in particular, oxides. In this work we study the structural, electronic, and magnetic properties of doped rutile $\mathrm{Ti}{\mathrm{O}}_{2}$ for two different impurity concentrations (25% and 6.25%), considering different distributions of the impurities in the host lattice. Calculations were performed with ab initio methods, assuming that the magnetic impurities substitutionally replace the Ti ions. Our results show that a local magnetic moment appears in the cases of Mn, Fe, and Co impurities, but not in the cases of Ni and Cu impurities. They also show that in the system ${\mathrm{Ti}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}{\mathrm{O}}_{2}$ the magnetic ions align ferromagnetically, while in ${\mathrm{Ti}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}{\mathrm{O}}_{2}$ and ${\mathrm{Ti}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}{\mathrm{O}}_{2}$ the antiferromagnetic alignment is energetically favorable. We have also studied the effect of oxygen vacancies, which turned out to be very important. Their presence decreases the energy required to introduce the impurities in the host lattice and reciprocally, the presence of impurities is related to a higher vacancy concentration. The pairs impurity-nearest-neighbor oxygen vacancy seem to be the energetically preferred structures and to produce the highest local magnetic moments. Ni and even Cu impurities acquire magnetic moments in this environment.