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colossal magnetoresistance; manganites; Raman scattering; strains; thin films
Abstract:
Thin films of La0.5Ca0.5MnO3 (LCMO) deposited on SrTiO3 (STO) with orientations (100) and (111) and on (001) SrLaGaO4 (SLGO) substrates have been studied by Raman spectroscopy. Three different thicknesses for each substrate were studied at room temperature. The energy and the width of the A(g) tilting mode have shown thickness and substrate dependence, implying an effect of the lattice mismatch on the tilting angle. Low temperature measurements (80 K) have been carried out at different scattering polarizations for the thicker films (290 nm). The spectra of LCMO/STO(100) films are strongly affected by the paramagnetic to ferromagnetic (FM) or charge ordered (CO) anti ferromagnetic (AF) transitions. The Jahn-Teller (JT) modes gain intensity close to the transition temperature, where a considerable softening of the tilting mode has been also observed. New bands appear below Neel temperature (T-N) for LCMO/STO(100), while this transition cannot be observed on the other films. A possible explanation is the phase coexistence which is discovered at the FM to CO/AF region (210-150 K) for LCM/STO(100) films. However, a strain effect seems to destroy the FM phase on contrary to the bulk. The JT modes do not appear (partially hidden by the spectrum of the substrate) in the spectra of LCMO/STO(111) films, which show a metallic-like spectrum, consistent with the resistivity measurements. The JT distortion appears decreased in LCMO/SLGO(001) films and the transition temperatures are affected from the strains. (C) 2007 Elsevier B.V. All rights reserved.
