La-doped EuO: A rare earth ferromagnetic semiconductor with the highest Curie temperature

We report the fabrication of single-crystalline La-doped EuO thin films with a Curie temperature (TC) of about 200 K, the highest among rare-earth compounds without transition metals. From first-principle band calculation and x-ray diffraction measurement, the observed increase in TC cannot be explained only by the increase in hybridization intensity due to lattice contraction and the increase in up-spin electrons of the Eu 5d state caused by the electron doping. Hybridization between the Eu 4f and donor states and/or Ruderman–Kittel–Kasuya–Yoshida interaction mediated by the doped La 5d state is a possible origin of the increase in TC.