High-resolution transmission electron microscopy study on strained epitaxial manganite thin films and heterostructures

Pulsed laser deposition has been used to grow epitaxial La_2/3Sr_1/3MnO₃ and La_2/3Ba_1/3MnO₃ thin films as well as La_2/3Ba_1/3MnO₃/SrTiO₃ heterostructures. The microstructure was studied by four-circle X-ray diffraction and high-resolution transmission electron microscopy. Strain effects due to lattice mismatch between substrate and film, and between different layers in the heterostructures were found to play a key role in determining the thin film properties. La_2/3Ba_1/3MnO₃ films on SrTiO₃ substrates show perfect epitaxy and grow coherently strained over the whole film thickness. In contrast, La_2/3Sr_1/3MnO₃ films on SrTiO₃ substrates were found to form two layers with different lattice parameters separated by an intrinsic interface region containing a high density of defects. La_2/3Ba_1/3MnO₃/SrTiO₃ superlattices are coherently strained and show a very low density of structural defects and sharp interfaces. The observed interface roughness was below 1 nm allowing the controlled growth of continuous two unit-cell thick La_2/3Ba_1/3MnO₃ layers. We also discuss the influence of strain on the electrical transport properties.