Theory of Curie temperature enhancement in electron-doped EuO

We present a comparative, theoretical study of the doping dependence of the critical temperature $T_C$ of the ferromagnetic insulator-metal transition in Gd-doped and O-deficient EuO, respectively. The strong $T_C$ enhancement in Eu${}_{1-x}$Gd${}_x$O is due to Kondo-like spin fluctuations on the Gd sites, which are absent in EuO${}_{1-x}$. Moreover, we find that the $T_C$ saturation in Eu${}_{1-x}$Gd${}_x$O for large $x$ is due to a reduced activation of dopant electrons into the conduction band, in agreement with experiments, rather than antiferromagnetic long-range contributions of the RKKY interaction. The results shed light on possibilities for further increasing $T_C$.