Extracting the near surface stoichiometry of BiFe _0.5Mn _0.5O ₃ thin films; A finite element maximum entropy approach

The surface and near-surface chemical composition of BiFe _0.5Mn _0.5O ₃ has been studied using a combination of low photon energy synchrotron photoemission spectroscopy, and a newly developed maximum entropy finite element model from which it is possible to extract the depth dependent chemical composition. In the uppermost few unit cells, an overabundance of Bi, and a deficiency of Fe and Mn are observed. In deeper layers, the measurements are consistent with bulk-like stoichiometry. Additionally, a definitive identification of all the observed species together with their abundance and depth dependence is given, and the mixed Fe and Mn valencies are estimated. In addition to the expected bulk valencies Mn ^{3 +} and Fe ^{3 +}, some Fe ^{2 +} and a small amount of Mn ^{4 +} are also observed. The maximum entropy finite element model demonstrated here is also discussed in more general terms and its potential application to the broader field of perovskite thin films is made apparent.