Self-purifying La2/3Sr1/3MnO3 epitaxial films: observation of surface precipitation of Mn3O4 particles for excess Mn ratios

20-25 nm thin films based La2=3Sr1=3MnO3 (LSMO) are prepared via Oxide Molecular Beam Epitaxy setup (MBE). Different ways of effusion cell shutter opening intervals are used to produce samples in co-deposition and shuttered mode. In-situ Reflection High-Energy Electron Diffraction (RHEED) intensity measurements in dependence of evaporation time are performed. The RHEED intensities exhibit distinct oscillations, indicating a stacking of layers with a stoichiometry controlled by the shutter opening times, in particular of the La2=3Sr1=3O vs. MnO content. Inside the thin LSMO films, vertical stoichiometric constant and gradient structures are produced. Low Energy Electron Diffraction (LEED) and X-Ray Diffraction (XRD) exhibit the Bragg reflections expected for epitaxial growth of the thin films. XRR analysis is in agreement with the nominal layer thickness and composition. To determine the magnetic layer thickness and to see whether a magnetic gradient inside the structural gradient takes place, Polarized Neutron Reflectometry (PNR) measurements are performed and evaluated. The depth-dependent magnetization behavior does not render the anticipated sample structure. A combined refinement of XRR and PNR data requires MnOx excess towards the surfaces in the model of the scattering length density. Additional High-Resolution Transmission Electron Microscopy (HRTEM) images reveal the existence of pure homogeneous perovskite LSMO layers with enclaved MnOx precipitates. Detailed SQUID measurements indicate these particles to have a Mn3O4 stoichiometry. Due to the combination of different experimental methods, the difference between the nominal and the actual layer composition can be identified showing that LSMO prefers to grow in pure La2=3Sr1=3MnO3 perovskite phase on SrTiO3. The observation of this phase separation effect will be discussed.