Thickness dependent Mn valence in La$_{\mathrm{0.7}}$Sr$_{\mathrm{0.3}}$MnO$_{\mathrm{3}}$ thin films*

The Mn valence in thin film La$_{\mathrm{0.7}}$Sr$_{\mathrm{0.3}}$MnO$_{\mathrm{3}}$ is studied as a function of film thickness in the range of 1-16u.c. Using a combination of bulk and surface sensitive x-ray absorption spectroscopy techniques, the layer-by-layer Mn valence is determined for these film thicknesses. It is found that while the bulk averaged valence hovers around its expected value of 3.3, a significant deviation occurs within several unit cells of the surface and interface, where the surface and interface valence are determined to be 4 and 2.68, respectively. These results are supported by theoretical calculations. The change in valence from the expected bulk value arises from the polar discontinuity at the film-substrate interface.