The stoichiometry and Surface termination of La2/3Sr1/3MnO3 film on SrTiO3 (001) substrate *

It has been observed that films of La2/3S1/3MnO3 (LSMO) are insulating when the films are below a certain critical thickness. The nature of such thickness-driven metal-to-insulator transition is still under debate: is this an intrinsic (dimensional confinement) or an extrinsic (oxygen vacancies or change of stoichiometry) effect. We have investigated the thickness-dependent composition variation in the LSMO films on SrTiO3 (001) by Low Energy Electron Diffraction (LEED), Angle-resolved XPS (ARXPS), and scanning transmission electron microscopy (STEM). Both ARXPS and STEM results show that the LSMO films prefer to be terminated with (La/Sr)-O layer and exists a thickness-dependence of Sr vs La concentration ratio in the film as well as the Sr surface segregation, incorporated with the thickness-dependence of the out-of-plane buckling in the MnO2 basal plane. These changes should have profound effect on the electronic and magnetic property, including the emergent insulating behavior in ultrathin films. * Supported by U.S. DOE under Grant No. DOE DE-SC0002136