Epitaxial Growth of Thin Film Strontium Cobaltite: A Feasibility Study.

摘要

In this work we present a feasibility study of epitaxial growth of thin films of strontium cobaltite, SrCoO3-delta. The properties of strontium doped lanthanum cobaltite, La 1-xSrxCoO 3, 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 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 SrCoO3-delta 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.