Oxygen doping and polaron magnetic coupling in Alq3 films

The understanding of the physics underlying the performances of spin-valve devices comprising organic semiconductors is still incomplete. According to some recent models, spin transport takes place in an impurity band inside the fundamental gap of the organic semiconductor. This seems to be confirmed by recent experiments performed with La0.7Sr0.3Mn0.3/A1q(3)/AlOx/Co devices. The reported results suggest a possible correlation between the magnetoresistance and the variable oxygen doping in the Alg(3) spacer. In this paper we investigate the electronic and magnetic properties of oxygen molecules and ions in Alg(3) films by means of first-principles calculations to establish whether they indeed play any important role for spin transport in La0.7Sr0.3Mn0.3/A1q(3)/AlOx/Co devices. The conclusion is that they probably do not. In fact, we show that O-2 molecules do not form an impurity band and there is no magnetic interaction between them. In contrast, we suggest that spin-transport may be enabled by the direct exchange coupling between Alg(3)(-) ions.