Evidence for Mn ²⁺ ions at surfaces of La _{0. 7}Sr _0.3MnO ₃ thin films

We present a detailed investigation of the valence of manganese sites at the surface of colossal magnetoresistance La _0.7Sr _0.3MnO ₃ (LSMO) thin films by x-ray absorption spectroscopy (XAS). The XAS Mn L-edge spectra of epitaxial LSMO films usually show a peak or shoulder at 640 eV. Differences in the intensity of this feature are commonly attributed to slight changes in the Mn ³⁺/Mn ⁴⁺ ratio or the crystal field strength. By comparison of different XAS spectra of LSMO thin films with the known multiplet structure of Mn ²⁺ in a cubic crystal field, we assign this 640-eV feature to Mn ²⁺ ions. XAS with increased surface sensitivity, combined with photon energy-dependent photoelectron spectroscopy measurements of the Mn(3s) exchange splitting, show that the Mn ²⁺ species are mainly located at the surface. The Mn ²⁺ scenario indicates significant modification of the LSMO surface with respect to the bulk properties that should be taken into account in all the charge and spin tunneling and injection experiments.