Heteroepitaxial interfaces in thin film complex oxides have attracted considerable attention in recent years due to their influence on the physical properties of these materials, in particular due to the possibility of tuning bulk functional properties. The most common strain relaxation mechanism between lattice mismatched heterostructures is the formation of misfit dislocation arrays at the interface. These misfit dislocations play an interesting role as they can form self-organized patterns on the nanometre scale which can behave differently from bulk material. An understanding of the structural and chemical consequences associated with the strain fields of dislocations at oxide interfaces is important as they may determine the functionality of these oxides in ultrathin films and multilayers. In this work, misfit dislocations in epitaxial films of La0.7Sr0.3MnO3 (LSMO), a half-metal ferromagnet, grown on [001] LaAlO3 (LAO) single crystal substrates were investigated. A detailed study of atomic-scale structural and chemical changes associated with the strain field of dislocations was performed using aberration-corrected scanning transmission electron microscopy (STEM) combined with atomic resolution spectroscopy techniques; electron energy-loss (EELS) spectrum imaging and energy dispersive x-ray (EDS) spectral mapping. Special attention was paid to ultrathin films of only a few nanometers thickness where the strain field of the dislocations affects surface topography and current. A STEM high angle annular dark field (HAADF) of a cross-section of a 7nm film exhibiting dislocated interface is shown in Figure 1a, where spacing between dislocations around 20 nm can be observed. A STEM-HAADF image of the dislocation core structure with the Burgers circuit yielding a Burgers vector bx = aLAO[100], parallel to the interface, is shown in Figure 1b. The lattice strain maps (Figure 1c), taken from the experimental images using the Geometrical Phase Analysis (GPA) method, show core splitting in two partials, along with compressive and tensile regions extending into the substrate and the film, respectively. Spectroscopy analyses of cross-sections reveal a chemically rough interface between the substrate and film and show that the dislocations are not located at the interface but one or two unit cells above the interface with the substrate. Analysis of the EELS spectrum images suggest that the lanthanum composition at the vicinity of the dislocation core is enhanced. These results provide an insight into how the intrinsic strain associated with a defect, in this case misfit dislocations, may produce chemical changes which may be useful in the development of ordered functional patterns in complex oxide thin films.

We acknowledge financial support from the Spanish MINECO (MAT2011-29081-C02, MAT2012-33207 and MAT2013-47869-C4-1-P). N. B. thanks to the Spanish MINECO for financial support through the FPI program.

Resumen del trabajo presentado al 16th European Microscopy Congress, celebrado en Lyon (Francia) del 28 de agosto al 2 de septiembre de 2016.

Peer Reviewed