Thickness evolution of the twin structure and shear strain in LSMO films

Abstract

X-ray diffraction analysis and orientation contrast scanning electron microscopy imaging of La_0.7Sr_0.3MnO₃ epitaxial layers grown on (001)-SrTiO₃ substrates have been used to track the shear strain and twin domain period as a function of the thickness of the films, t. To this end, the diffraction by a periodically modulated twinned structure is analyzed in detail. In contrast with current equilibrium models, here we demonstrate the occurrence of a critical thickness, t_τ ∼ 2.0–2.5 nm, for twin formation in rhombohedral perovskite films. The absence of twinning below t_τ is explained by the formation of a monoclinic interfacial phase presumably driven by electronic interactions between film and substrate not taken into account in theoretical models. Above t_τ, twin domains develop concomitantly with the build-up of misfit shear strains associated with the formation of the rhombohedral structure. At a thickness ∼10 nm, the in-plane and out-of-plane shear strain components exhibit similar values, as imposed by the rhombohedral symmetry. However, upon increasing the film thickness, both strain components are found to follow divergent trajectories indicating a progressive perturbation of the octahedral framework which allows the in-plane lattice parameters to remain fully strained within the explored thickness range (up to 475 nm). Despite these structural perturbations, the twin size follows a t^1/2 dependence as predicted for homogeneous films by equilibrium models.