Raman Spectroscopy of Colossal Magnetoresistive Manganese Oxides

Abstract

A number of members of the colossal magnetoresistive manganite family have been studied using Raman Spectroscopy across ranges of temperature and rare-earth/alkaline-earth substitutions. In the case of the pseudo cubic manganite La1-xCaxMnO3 (x=0.1, 0.265 & X=0.3), we have tested the suitability of a temperature-dependent phonon model specifically developed for pseudo-cubic manganites. We find that, contrary to recently published works, those models are not as good at describing the observed data as a simple anharmonic-potential model. Our studies of six samples of the layered manganites focus on Nd2-2xSr1+2xMn2O7 (x=0.25, 0.4), Pr2-2xSr1+2xMn2O7 (x=0.4, 0.5) and Dy2-2xSr1+2xMn2O7 (x=0.4, 0.5).We have assigned the broad modes in the mid-frequency region to non-zone-centre disorder-induced scattering from a full optical O(Mn) branch. We testify to the simple nature of the second- and third-order overtone modes which has recently been challenged, seen in the high-frequency region of the Raman spectra. We make the case that the orbiton-scattering model works best at low x concentration and the multi-phonon model works best for partially-doped materials. In a similar vein to the pseudo-cubic results, we find that a recent Frank-Condon model for second-order scattering does not agree with our data, which is explained better by conventional Raman scattering models. We have also observed the existence of charge-ordering in PrSr2Mn2O7 at ~ 200 K, and in Nd1.2Sr1.8Mn2O7 at T 100 – 150 K, and data from the PrSr2Mn2O7 sample suggests that it has a lower doping fraction than x=0.5.