  The effect of Gd doping and O deficiency on the electronic structure,
exchange interaction, and Curie temperature of EuO in the cubic and tetragonal
phases is studied by means of density functional theory. For both defects, the
Curie temperature is found to exhibit a distinct maximum as a function of the
defect concentration. The existence of optimal defect concentrations is
explained by the interplay of the on-site, RKKY, and superexchange
contributions to the magnetism.
