Local Valency of Transition Metals by ELNES

The properties of magnetoelectronic devices based on thin mixed-valence manganite films showing colossal magnetoresistance depend strongly on the local electronic environment of Mn atoms. The analysis of the near edge fine structure of electron energy loss spectra (ELNES) measured in a TEM provides a tool to probe chemical composition and bonding character of materials at high spatial resolution. In particular, in Mn oxides the Mn-L2,3 and O-K ELNES allow to retrieve the Mn valency. However, due to the highly local measurement the signals are very noisy, which demands a thorough analysis.

Using MnxOy and La1-xSrxMnO3 reference compounds we have compared several Mn valence-sensitive ELNES quantities and analyzed their robustness against varying experimental parameters. As a result the energy separation of Mn-L3 relative to O-Ka, which can be determined for a single spectrum, resolves smaller valency shifts than the white-line intensity ratio I(L3)/I(L2). The latter is more susceptible to the influence of specimen thickness and even the kind of method to extract the white-line intensities from the raw spectra has an impact on the valence sensitivity and on the error of I(L3)/I(L2). The valency determination via ELNES contributes to clarify the local electronic structure in manganite based thin films.

References:

T. Riedl, T. Gemming, K. Wetzig: "Extraction of EELS white-line intensities of manganese compounds: Methods, accuracy, and valence sensitivity", Ultramicroscopy 106 [4-5] (2006) 284-291.  (doi:10.1016/j.ultramic.2005.09.002)

T. Riedl, T. Gemming, W. Gruner, J. Acker, K. Wetzig: "Determination of manganese valency in La1-xSrxMnO3 using ELNES in the (S)TEM", Micron 38 [3] (2007) 224-230.  (doi:10.1016/j.micron.2006.06.017)

 

The figure shows a comparison of the relative changes and sensitivity on Mn valence of various ELNES quantities. O-Ka,b denote details of the O-K edge.
The figure shows a comparison of the relative changes and sensitivity on Mn valence of various ELNES quantities. O-Ka,b denote details of the O-K edge.