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Vorträge und Posterpräsentationen (mit Tagungsband-Eintrag):

W. Wallisch, M. Stöger-Pollach, S. Löffler, E. Navickas:
"Influences of the CMR effect on energy loss signal";
Vortrag: Microscopy Conference 2017, Lausanne, Schweiz; 21.08.2017 - 25.08.2017; in: "MC 2017 Lausanne - Microscopy Conference", (2017), S. 698 - 699.



Kurzfassung englisch:
Introduction: Double perovskite oxides are promising in a wide spectrum of applications due to their
structural, physical and electronic properties. In particular, the colossal magnetoresistance (CMR) of
La2CoMnO6 (LCM) is a consequence of a metal-insulator transition [1]. It describes the change of the
resistance in the presence of a magnetic field. These days, transmission electron microscopes (TEMs)
equipped with energy filters are powerful tools. Their main advantage for probing band gaps is the high
spatial resolution, limited primarily by the inelastic delocalization and the excitation of Čerenkov light [2].
The optical properties and band gaps show up in the low energy part of the electron energy loss
spectrometry (EELS) spectrum and the technique called valence EELS (VEELS). The opportunity to
detect energy loss magnetic chiral dichroism (EMCD) with high spatial resolution [3, 4] and chemical
sensitivity [5] is an additional advantage of EELS.
Objectives: In our study, we show that the physical consequences of the CMR effect also influence the
EELS signal. We are able to detect the change of the band structure in the low losses caused by the
CMR effect and observe the magnetic behaviour of the material with chemical sensitivity by using EMCD.
Different LCM thin film layers are investigated to ensure the reproducibility of our evaluation. To confirm
the experimental results, we use density functional theory (DFT) calculations.
Material and Methods: The LCM thin film samples are prepared by pulsed laser deposition. The LCM
layers are epitaxially grown on different substrates, SrTiO3 and LaAlO3. For the TEM analysis the
preparation of the cross section specimens is done by focused ion beam milling. Analytical TEM
investigations are performed by using two TECNAI TEMs.
We varied the magnetic field and the temperature, in order to study the influence of the CMR effect on
the EELS spectrum. The band gaps and optical properties are detected in the low energy range of the
energy loss spectrum by VEELS. The beam energy is set in the range from 20 keV to 200 keV. The
magnetisation of the material is measured by EMCD, which allows a characterisation in the nanometre
range. The experiments are performed in the classical scheme to achieve a three-beam diffraction
geometry. By considering the theoretical aspects the experimental results are supported by DFT
calculations using the WIEN2k [6] code.
Results: The 40 keV and the 200 keV spectra are shown at a temperature of 85 K in Fig. 1. A difference
in the energy loss range of 1.5 eV to 4 eV is observed. Concerning the comparison of the 40 keV VEELS
spectra at different temperatures (Fig. 2), an intensity variation is caused by the CMR effect and not by
the Čerenkov effect. The chemical sensitivity of EMCD is shown in Fig. 3 and Fig. 4. The EMCD effect
can be observed at the Co L-edge (779 eV) in the 85 K experiment, whereas the same edge at room
temperature exhibits no EMCD effect.
Conclusion: The influences of the CMR effect on the band gap are detected by observing the double
perovskite LCM and the element specific magnetisation by means of EELS. Consequently, EELS can be
employed at least qualitatively to investigate electronic properties of CMR materials.

Erstellt aus der Publikationsdatenbank der Technischen Universität Wien.