AVS 60th International Symposium and Exhibition
    Electronic Materials and Processing Friday Sessions
       Session EM+NS+TF-FrM

Paper EM+NS+TF-FrM9
Understanding the Origin of the Dead-layer at the La0.66Ca0.33MnO3/SrTiO3 Interface: A Grazing Incidence X-ray Diffraction and Hard X-ray Photoelectron Spectroscopy Study

Friday, November 1, 2013, 11:00 am, Room 102 A

Session: Nanoelectronic Interfaces, Materials, and Devices/Crystalline Oxides on Semiconductors
Presenter: G.R. Castro, SpLine Spanish CRG beamline at the European Synchrotron Radiation Facility, France
Authors: J. Rubio-Zuazo, SpLine Spanish CRG beamline at the European Synchrotron Radiation Facility, France
A. de Andres, Institute Materials Science of Madrid-CSIC, Spain
G.R. Castro, SpLine Spanish CRG beamline at the European Synchrotron Radiation Facility, France
Correspondent: Click to Email
La1-XCaxMnO3–type perovskite-manganese oxides exhibit a wide variety of interesting physical properties which originate from mutual coupling among spin, charge and lattice degrees of freedom. The perovskite-manganese oxides have, in the Ca doping range between 0.15 and 0.5, a ferromagnetic –paramagnetic (F–M) phase transition accompanied by a metal – insulator (M–I) transition that results in a colossal magneto-resistance behaviour. In bulk La0.66Ca0.33MnO3 (LCMO), the transition temperature TC, TMI rises for 33% Ca doping level reaching values close to room temperature. We have studied a series of epitaxial LCMO films with thickness between 2.4 and 27 nm grown on SrTiO3(001) (STO). The magnetic measurements show a severe decrease of TC as the film thickness is reduced below 2.4 nm. The atomic structure, as obtained by grazing incidence X-ray diffraction shows that the LCMO films adopt the substrate STO in-plane lattice parameter (1% mismatch) inducing a pseudomorphic growth. The 27 nm film presents a bulk-like crystal structure and space group, but also magneto-transport bulk behavior, while the 2.4 nm film shows a different crystallographic space group and the film is an insulator within the whole temperature range (i.e., no Tc transition is present). The structure, also observed at the LCMO-STO interface of thicker LCMO films, is based on an anti-correlation between Mn-O octahedra along the three crystallographic directions. This could explain the origin of anomalous magneto-transport or dead-layer behavior in LCMO hetero-structures. These results evidence the strong influence of the interface. The mismatch may be accommodated by the formation of facets, by structural defects or by diffusion-induced changes in stoichiometry or oxygen vacancies. However, in our study no facets were found, but bi-dimensional in-plane reciprocal space maps show a clear lattice relaxation of about 1% suggesting the existence of a stoichiometry change as strain relaxation mechanisms. To understand the relaxation mechanisms a non-destructive compositional depth profile analysis was performed using the Hard X-ray Photoelectron Spectroscopy (HAXPES). The data was fitted with a model in which the La is diffused into the interface, while the Ca is segregated to the surface. The substitution of the Ca2+ cation for La3+ results in an LCMO lattice enlargement reducing the mismatch between LCMO and STO. Hence, the first La enriched layer close to the interface will grow pseudomorphic, and in the successive layer Ca concentration increase and consequently a lattice parameter reduction until the bulk lattice parameter is achieved, without energy strain accumulation.