  Oxygen vacancies can be of utmost importance for improving or deteriorating
physical properties of oxide materials. Here, we studied from first-principles
the electronic and magnetic properties of oxygen vacancies in the double
perovskite Sr$_2$FeMoO$_6$ (SFMO). We show that oxygen vacancies can increase
the Curie temperature in SFMO, although the total magnetic moment is reduced at
the same time. We found also that the experimentally observed valence change of
the Fe ions from $3+$ to $2+$ in the x-ray magnetic circular dichroism (XMCD)
measurements is better explained by oxygen vacancies than by the assumed mixed
valence state. The agreement of the calculated x-ray absorption spectra and
XMCD results with experimental data is considerably improved by inclusion of
oxygen vacancies.
