Infrared absorption in colossal-magnetoresistance materials

Full text: Phonons and the electron-phonon interaction play an important role in the phenomenon of colossal magneto-resistance (CMR) observed in lanthanum perovskites. The phonon energies fall in the range 20-100 meV and so are ideally suited to investigation by infrared spectroscopy. We have reported on the reflectivity of La0.7Ca0.3MnO3 and La0.8Li0.2MnO3. The 'external' phonon mode, arising from (La, Ca/Li) vibration against the MnO3 octahedra, is observed to shift between the samples. This is explained on the basis of the compositional differences. Most infrared studies on CMR materials to date have employed reflectivity measurements. Recently, Li et al. reported transmission measurements using KBr as a carrier. We have extended such measurements, also employing CsI as carrier. Relative to La0.7Ca0.3MnO3, phonon energies associated with the Mn site are found to increase as Li is substituted for Mn. In the absence of Ca substituting for La, a new phonon mode is observed. We have studied La0.7Ca0.3MnO3 and La0.5Ca0.5Mn0.9Li0.1O3 over the temperature range 10-300 K, i.e. through the Curie temperature Tc. In contrast to the earlier reflectivity data of Kim et al., but in agreement with the more recent reflectivity data of Franck et al., our transmission measurements reveal no distinct shift in the energies of the 'bending' and 'stretching' phonon modes at Tc