Is there an intrinsic limit to the charge-carrier-induced increase of the curie temperature of EuO?

Rare earth doping is the key strategy to increase the Curie temperature (T_C) of the ferromagnetic semiconductor EuO. The interplay between doping and charge carrier density (n), and the limit of the T_C increase, however, are yet to be understood. We report measurements of n and T_C of Gd-doped EuO over a wide range of doping levels. The results show a direct correlation between n and T_C, with both exhibiting a maximum at high doping. On average, less than 35% of the dopants act as donors, raising the question about the limit to increasing T_C.