Epitaxial Growth of thin film strontium cobaltite: a feasibility study.

Abstract

In this work we present a feasibility study of epitaxial growth of thin films of strontium cobaltite, SrCoO_3-\delta. The properties of strontium doped lanthanum cobaltite, La_1-xSr_xCoO₃, have been widely studied for dopant concentration x<0.5, but little work has been performed on the x=1 member of the series. The main issue is that this is not a thermodynamically preferable state and close to stochiometric SrCoO₃ in polycrystalline samples can only be obtained under high pressures of oxygen (>10kbar) or by electrochemical oxidation. However, theoretical calculations predict a phase change with</DISS_para> <DISS_para>respect to strain in epitaxially grown samples, from ferromagnetic-metallic behaviour in the bulk state to insulating-ferroelectric-antiferromagnetic behaviour for strongly strained films. This provides strong motivation for epitaxial growth of SrCoO_3-d films. In this work we will present a feasibility study by using the methods of high-pressure oxygen sputtering (typically 1.0-4.0mbar) on SrTiO₃(001) and LaAlO₃(001) substrates. As anticipated, the presence of oxygen vacancies is a severe problem, but also epitaxial stabilization of non-cubic phases, an unexpected issue, arises. These are found to grow in multiple orientations. Overall, the samples exhibit only weak or no ferromagnetism, even though bulk SrCoO₃ is known to be a strong ferromagnet. Based on the results, we present an outline for suggested further research on this topic.