Direct Demonstration of a Magnetic Dead Layer Resulting from A-Site Cation Inhomogeneity in a (La,Sr) MnO₃ Epitaxial Film System

The degradation of the functional properties of epitaxial oxide films and the performance of related devices have often been attributed to the presence of so-called interfacial “dead layers”. Extensive efforts have been made to understand the origin of such dead layers and to avoid their formation. However, the results of these efforts have not been fully satisfactory, largely as a result of the complex origin of dead layers. Here, the dead layer is studied in a sample that contains hillocks of nominally La_0.7Sr_0.3MnO₃ (LSMO) sandwiched between a ferroelectric PbTiO₃ film and a Nb-doped SrTiO₃ substrate using aberration-corrected (scanning) transmission electron microscopy, nanobeam electron magnetic circular dichroism spectroscopy, and related techniques. The results here reveal the presence of a 5 nm thick layer in the LSMO hillocks, close to the LSMO/Nb-SrTiO₃ interface, which exhibits distinct structural and physical properties. The atomic, electronic, and magnetic structures and local chemistry of the interfacial layer are determined. It is found that octahedral rotations are suppressed in ultrathin regions at the edges of the hillocks, providing evidence for a strong effect of the adjacent Nb-SrTiO₃ and PbTiO₃. The formation of the dead layer is discussed in the light of lattice strain and valence changes of Mn ions.