The isotope effect and phase separation in (La0.5Pr0.5)(0.7)Ca0.3MnO3 films: Optical data

A new approach to the study of phase separation in lanthanum manganites is proposed based on the combined investigation of their optical and magnetoop tical characteristics providing information about the conducting and ferrom agnetic regions, respectively. Effects of the O-18 isotope substitution for O-16 in the epitaxial films of (La0.5Pr0.5)(0.7)Ca0.3MnO3 (grown on SrTiO3 or LaAlO3 substrates) upon the IR absorption spectra and the equatorial Ke rr effect measured in the 1.5-3.8 eV range were studied. A giant drop in th e temperature of maximum resistance of the film grown on SrTiO3 and disappe arance of the metal-insulator transition in the film on LaAlO3 , observed u pon the isotope exchange, are accompanied by a decrease in the contribution of free charge carriers to the absorption spectra, by the appearance of ba nds due to localized states, and by a decrease in magnitude of the equatori al Kerr effects. Measurements of the Kerr effect and the temperature variat ion of the optical transmission show evidence of the presence of ferromagne tic metal regions in the O-18-isotope-substituted (La0.5Pr0.5)(0.7)Ca0.3MnO 3/LaAlO3 film at low temperatures, with a general semiconductor character o f the resistivity behavior in the entire temperature range studied. Changes observed in the absorption spectra are explained based on a model of the p seudo-Jahn-Teller polar centers and phase separation. The optical and magne tooptical data show evidence of a percolation nature of the giant isotope e ffect in manganites. (C) 2001 MAIK "Nauka/Interperiodica".