Abstract
Cerium-doped manganite thin films were grown epitaxially by pulsed laser deposition at and oxygen pressure and were subjected to different annealing steps. According to x-ray diffraction (XRD) data, the formation of as a secondary phase could be avoided for . However, transmission electron microscopy shows the presence of nanoclusters even in those films which appear to be single phase in XRD. With annealing, the metal-to-insulator transition temperature increases, while the saturation magnetization decreases and stays well below the theoretical value for electron-doped with mixed 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 to . The dominating Mn valences in the films are and , and only a small amount of ions can be observed by XAS. and 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.
3 More- Received 26 November 2008
DOI:https://doi.org/10.1103/PhysRevB.79.054416
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