  A Mn valence instability on La2/3Ca1/3MnO3 thin films, grown on LaAlO3
(001)substrates is observed by x-ray absorption spectroscopy at the Mn L-edge
and O K-edge. As-grown samples, in situ annealed at 800 C in oxygen, exhibit a
Curie temperature well below that of the bulk material. Upon air exposure a
reduction of the saturation magnetization, MS, of the films is detected.
Simultaneously a Mn2+ spectral signature develops, in addition to the expected
Mn3+ and Mn4+ contributions, which increases with time. The similarity of the
spectral results obtained by total electron yield and fluorescence yield
spectroscopy indicates that the location of the Mn valence anomalies is not
confined to a narrow surface region of the film, but can extend throughout the
whole thickness of the sample. High temperature annealing at 1000 C in air,
immediately after growth, improves the magnetic and transport properties of
such films towards the bulk values and the Mn2+ signature in the spectra does
not appear. The Mn valence is then stable even to prolonged air exposure. We
propose a mechanism for the Mn2+ ions formation and discuss the importance of
these observations with respect to previous findings and production of thin
films devices.
