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Vorträge und Posterpräsentationen (mit Tagungsband-Eintrag):

W. Wallisch, M. Stöger-Pollach, S. Löffler, E. Navickas:
"EELS experiments influenced by the CMR effect";
Vortrag: 13th Multinational Congress on Microscopy, Rovinj, Kroatien; 24.09.2017 - 29.09.2017; in: "13th Multinational Congress on Microscopy", (2017), S. 181 - 182.



Kurzfassung englisch:
For a wide range of applications, double perovskites are promising candidates due to their structural, physical and electronic properties. The La2CoMnO6 (LCM) composition exhibits a huge negative magnetoresistance (1), which is known as colossal magnetoresistance (CMR). The CMR effect is an interesting consequence of a metal-insulator transition, which describes the change of the resistance in the presence of a magnetic field. Transmission electron microscopes (TEMs) equipped with energy filters are powerful tools and their main advantage for probing band gaps is the high spatial resolution. It is primarily limited by the inelastic delocalization and the excitation of Čerenkov light (2). In the low energy part of the electron energy loss spectrometry (EELS) spectrum the optical properties and band gaps can be investigated by a technique called valence EELS (VEELS). An additional advantage of TEM is the opportunity to detect energy loss magnetic chiral dichroism (EMCD) with high spatial resolution and chemical sensitivity (3). In this study, the influence of the CMR effect on the EELS signal is shown. The change of the band structure in the low losses caused by the CMR effect and the magnetic behaviour of the material are measured. Different LCM thin film layers are investigated to ensure the reproducibility of our evaluation. The experimental results are confirmed by density functional theory (DFT) calculations. The preparation of the LCM thin films are done by pulsed laser deposition. The LCM layers are epitaxially grown on different substrates, SrTiO3 and LaAlO3. The cross section specimens for the TEM analysis are prepared by focused ion beam milling and the analytical TEM investigations are performed by using two TECNAI TEMs. In order to study the influence of the CMR effect, the magnetic field and the temperature are varied. The EMCD 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 code. In Figure 1A
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the 40 keV and the 200 keV spectra are shown at a temperature of 85 K and a difference in the energy loss range of 1.5 eV to 4 eV is observed. An intensity variation caused by the CMR effect is detected by 40 keV VEELS spectra at different temperatures (Figure 1B). The chemical sensitivity of EMCD is shown in Figure 2. 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 (RT) exhibits no EMCD effect. The consequences of the CMR effect on the band gap are detected by observing the LCM oxide.

Erstellt aus der Publikationsdatenbank der Technischen Universität Wien.