Surface alloying and iron selenide formation in Fe/Bi 2 Se 3 ( 0001 ) observed by x-ray absorption fine structure experiments

The atomic structure of ultrathin iron films deposited on the (0001) surface of the topological insulator ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ is analyzed by surface x-ray absorption spectroscopy. Iron atoms deposited on a ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ (0001) surface kept at 160 K substitute bismuth atoms within the first quintuple layer. Iron atoms are neighbored by six selenium atoms at a distance in the 2.4 \AA{} range indicating substantial atomic relaxations. Mild annealing up to 520 K leads to the formation of $\ensuremath{\alpha}$-FeSe, characterized by a local order extending up to the sixth shell (5.80 \AA{}). Ab initio calculations predict a noncollinear magnetic ordering with a transition temperature of 3.5--10 K depending on the iron concentration and the number of the layers in which Fe is located.