Cerium-doped manganite thin films were grown epitaxially by pulsed laser deposition at $720°\text{C}$ and oxygen pressure $p_{{\text{O}}_2}=1–25\text{Pa}$ and were subjected to different annealing steps. According to x-ray diffraction (XRD) data, the formation of ${\text{CeO}}_2$ as a secondary phase could be avoided for $p_{{\text{O}}_2}\ge 8\text{Pa}$. However, transmission electron microscopy shows the presence of ${\text{CeO}}_2$ nanoclusters even in those films which appear to be single phase in XRD. With ${\text{O}}_2$ annealing, the metal-to-insulator transition temperature increases, while the saturation magnetization decreases and stays well below the theoretical value for electron-doped ${\text{La}}_{0.7}{\text{Ce}}_{0.3}{\text{MnO}}_3$ with mixed ${\text{Mn}}^{3+}/{\text{Mn}}^{2+}$ valences. The same trend is observed with decreasing film thickness from 100 to 20 nm, indicating a higher oxygen content for thinner films. Hall measurements on a film which shows a metal-to-insulator transition clearly reveal holes as dominating charge carriers. Combining data from x-ray photoemission spectroscopy, for determination of the oxygen content, and x-ray absorption spectroscopy (XAS), for determination of the hole concentration and cation valences, we find that with increasing oxygen content the hole concentration increases and Mn valences are shifted from $2+$ to $4+$. The dominating Mn valences in the films are ${\text{Mn}}^{3+}$ and ${\text{Mn}}^{4+}$, and only a small amount of ${\text{Mn}}^{2+}$ ions can be observed by XAS. ${\text{Mn}}^{2+}$ and ${\text{Ce}}^{4+}$ XAS signals obtained in surface-sensitive total electron yield mode are strongly reduced in the bulk-sensitive fluorescence mode, which indicates hole-doping in the bulk for those films which do show a metal-to-insulator transition.

• Received 26 November 2008

DOI:https://doi.org/10.1103/PhysRevB.79.054416