The studies of the growth of thick films of La_1.85Sr_0.15CuO₄ (LSCO) by pulsed laser deposition on the SrLaAlO₄ substrates indicate the presence of variable built-in strain, ranging from compressive to tensile. While the compressive strain is induced by the mismatch to the substrate, the origin of tensile strain is unclear, and we have suggested recently that it may be caused by the thin layer of the oxygen or strontium-deficient material which grows in the initial stages of the deposition [1]. To check this hypothesis in the present study we investigate the properties of several groups of LSCO films of thicknesses ranging from 20 to 300 nm. The X-ray diffraction and the atomic force microscopy are used to evaluate the strain and the microstructure, and the superconducting properties are measured using dc resistivity and ac susceptibility. For each group of films of given thickness, several films are studied. The films in each group grow with variable degree of strain. In thin films the strain ranges from compressive to close to zero indicating variable compressive strain relaxation. In thick films the strain ranges from zero to tensile strain. Surprisingly, the maximum Tc observed in each group is almost independent of the film thickness. This observation rules out the possibility that the tensile strain is caused by the oxygen or strontium-deficient layer close to the substrate interface. Instead, it is possible that the grain-to-grain misalignment in thick films contributes to the building of tensile strain inside the these films.

1. M. Z. Cieplak, M. Berkowski, A. Abal'oshev, S. Guha and Q. Wu, Supercond. Sci. Technol. 19, 564 (2006).

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