Structure determination of monolayer-by-monolayer grown La_1−xSr_xMnO₃ thin films and the onset of magnetoresistance

Surface x-ray diffraction was used to determine the atomic structures of La_1−xSr_xMnO₃ thin films, grown monolayer by monolayer on SrTiO₃ 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 −15 kJ∕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.