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

一氧化铕是一种典型的铁磁性半导体。由于铕过量和二价铕离子被离子半径较小的三价离子（如La或Gd）取代，电子掺杂和晶格收缩使铁磁转变温度（居里温度，T_C）从~70 K提高到~150 K或更高。讨论了其成因是eu4f和o2p的杂化、载流子介导的4f电子间的间接交换相互作用和/或磁极化子效应。在本研究中，我们成功地分别通过改变薄膜厚度和载流子掺杂来控制杂化效应和间接交换相互作用。EuO的T_C由这些相互作用的组合决定。

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.