Oxygen doping and polaron magnetic coupling in Alq$_3$ films

The understanding of the Physics underlying the performances of organic spin-valve devices is still incomplete. According to some recent models, spin transport takes place in an impurity band inside the fundamental gap of organic semiconductors. This seems to be confirmed by recent experiments performed with La$_{0.7}$Sr$_{0.3}$MnO$_3$/Alq$_3$/AlO$_x$/Co devices. The reported results suggest a possible correlation between the magnetoresistance and the variable oxygen doping in the Alq$_3$ spacer. In this paper we investigate by means of first-principles calculations the electronic and magnetic properties of O$_2$ molecules and ions in Alq$_3$ films to establish whether oxygen plays any important role for spin transport in La$_{0.7}$Sr$_{0.3}$MnO$_3$/Alq$_3$/AlO$_x$/Co devices. The conclusion is that it does 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 Alq$_3^-$ ions.