Raman Scattering in Perovskite Manganites

Abstract

Raman spectroscopy is shown to be a useful tool for the study of phase transitions in lanthanum manganese compounds of the perovskite type. In this work doped and undoped La1−x R x MnO3 (R = Ca, Sr) single crystals and thin films deposited on LaAlO3 and NdGeO3 substrates are studied in polarized Raman scattering as functions of doping (0 < x < 1) and temperature (5 < T < 423K). Raman spectra of single crystals exhibit a significant dependence on the amount and kind of the dopant R, and they are sensitive to structural and magnetic transitions driven by both doping and temperature. Raman spectra of the doped La1−x R x MnO3 crystals have weakly-resolved features and intensities comparable to those of second-order spectra in cubic crystals. The small distortion of the nearly cubic lattice of doped La1−x R x MnO3 single crystals results in structured phonon Raman spectra composed of two parts. These are attributed 1) to the distorted non-cubic perovskite lattice and are consistent with the selection rules for a tetragonal structure, and 2) to the density of vibrational states manifested as second-order Raman scattering. An effect of spin-lattice interaction corresponding to a paramagnetic to canted anti-ferromagnetic phase transition was found in the Raman spectra of undoped LaMnO3. A correlation of lattice transformation as well as magnetic ordering was found in thin films of La1−x R x MnO3. Structural changes, preferential orientation of films as well an influence of deposition conditions can be ascertained from the phonon Raman spectra. The effect of spin-lattice interaction due to spin ordering below T c, was observed in spectra of La1−x R x MnO3 films. A simple frequency dependence of the totally symmetric external mode with doping x was found for La1−x Sr x MnO3 crystals. This dependence is demonstrated to be a useful diagnostic tool for determining the doping value in La1−x Sr x MnO3 crystals. The results of the present work may be extended to other related manganite structures, in particular La1−x Sr x MnO3 films.KeywordsRaman SpectrumRaman ScatRaman ShiftRaman FeaturePerovskite ManganiteThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.