Formation of the (La0.67Sr0.33)2MnO4 phase in La-Sr-Mn-O thin films by pulsed laser deposition

La0.67Sr0.33MnO3 thin films were grown on LaAlO3 substrate in vacuum using pulsed laser deposition to investigate the effect of changing oxygen content. Transmission electron microscopy studies showed that the epitaxial (La0.67Sr0.33)2MnO4 phase with K2NiF4 structure formed unexpectedly as a matrix with a square-shaped nanometer-sized MnO phase distributed in a regular pattern throughout the whole film like self-assembled quantum dots. The MnO phase grew epitaxially from the LaAlO3 substrate to the top of the film with no outgrowth. High-resolution image simulation indicated that Sr ions take up only positions in every other La layer in the (La0.67Sr0.33)2MnO4 structure. Basing our theory on the composition and structure of the matrix phase, we propose that it is possibly electron-doped with a mixed valence of Mn2+/Mn3+ instead of the Mn3+/Mn4+ as in the hole-doped case.