Mn valence instability inLa2∕3Ca1∕3MnO3thin films

Abstract

A Mn valence instability on ${\mathrm{La}}_{2∕3}{\mathrm{Ca}}_{1∕3}\mathrm{Mn}{\mathrm{O}}_{3}$ thin films, grown on $\mathrm{La}\mathrm{Al}{\mathrm{O}}_{3}$ (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 $800\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ in oxygen, exhibit a Curie temperature well below that of the bulk material. Upon air exposure a reduction of the saturation magnetization, ${M}_{S}$, of the films is detected. Simultaneously a ${\mathrm{Mn}}^{2+}$ spectral signature develops, in addition to the expected ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Mn}}^{4+}$ 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 $1000\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ in air, immediately after growth, improves the magnetic and transport properties of such films towards the bulk values and the ${\mathrm{Mn}}^{2+}$ signature in the spectra does not appear. The Mn valence is then stable even to prolonged air exposure. We propose a mechanism for the ${\mathrm{Mn}}^{2+}$ ions formation and discuss the importance of these observations with respect to previous findings and production of thin films devices.