Structure determination of monolayer-by-monolayer grownLa1−xSrxMnO3thin films and the onset of magnetoresistance

Abstract

Surface x-ray diffraction was used to determine the atomic structures of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ thin films, grown monolayer by monolayer on ${\mathrm{SrTiO}}_{3}$ by pulsed laser deposition. Structures for one-, two-, three-, four-, six-, and nine-monolayer-thick films were solved using the Coherent Bragg rod analysis phase-retrieval method and subsequent structural refinement. Four important results were found. First, the out-of-plane lattice constant is elongated across the substrate-film interface. Second, the transition from substrate to film is not abrupt, but proceeds gradually over approximately three unit cells. Third, Sr segregates towards the topmost monolayer of the film: we determined a Sr-segregation enthalpy of $\ensuremath{-}15\phantom{\rule{0.3em}{0ex}}\mathrm{kJ}∕\mathrm{mol}$ from the occupation parameters. Finally, the electronic bandwidth $W$ was used to explain the onset of magnetoresistance for films of nine or more monolayers thickness. Resistivity measurements of the nine monolayer-thick film confirm magnetoresistance and the presence of a dead layer with mostly insulating properties.