Study on physical properties of europium monoxide as a ferromagnetic semiconductor at room temperature

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Europium monoxide EuO is a typical ferromagnetic semiconductor. The ferromagnetic transition temperature (Curie temperature, T_C) is increased from ~70 K to ~150 K or higher by electron doping and lattice contraction owing to Eu-excess and substitution of divalent Eu ions to trivalent ions with smaller ionic radius such as La or Gd. The origin has been discussed to be the hybridization between the Eu 4f and O 2p, the indirect exchange interaction between 4f electrons mediated by carriers, and/or a magnetic polaron effect. In this study, we succeeded to control separately the hybridization effect and the indirect exchange interaction by changing the thickness of films and carrier doping, respectively. The T_C of EuO is concluded to be determined by the combination of these interactions.