Structural, electrical and magnetic properties of epitaxial La 0.7 Sr 0.3 CoO 3 thin films grown on SrTiO 3 and LaAlO 3 substrates

Abstract

La 0.7 Sr 0.3 CoO 3 (LSCO) thin films have been epitaxially grown on SrTiO 3 (STO) and LaAlO 3 (LAO) substrates by metal organic deposition. The effects of the strain - induced by clamping - on the structural and physical properties of the films were studied. For that, we have performed resistivity and magnetization studies as a function of temperature and magnetic field as well as X-ray diffraction and Raman spectroscopy measurements. Our X-ray results are similar for both substrates showing that the 20 nm films are fully strained while thicker films have two components corresponding to a fully strained and a relaxed component. Relaxation induced by increasing film thickness (up to 100 nm) results in a systematic evolution of the out of plane crystallographic cell parameter toward the bulk LSCO values. Raman spectra of the thinner films exhibit specific modes which are not present in the bulk LSCO spectra. These modes disappear for thicker films which are totally relaxed. All the samples show similar magnetic behavior independently of the thickness and the substrate with a Curie temperature (T C ) around 210 K. Relative changes in resistivity due to the film thickness are larger than 3 orders of magnitude with a relatively small influence of the type of strain induced by the substrate (compressive or tensile). Moreover whereas the relaxed film (100 nm thick) shows similar transport properties as the bulk sample, the fully strained film (20 nm thick) shows a 3D variable range hopping conduction with a higher degree of localization which is a direct result of the strain state. © 2014 Elsevier B.V. All rights reserved.