Brillouin light scattering in CMR manganite films

Thin films of La_0.7A_0.3MnO₃ (A=Ce, Ca, and Sr) and La_0.55Ho_0.15Sr_0.3MnO₃ manganites have been investigated by Brillouin light scattering at room temperature. The phonon spectra for the Ce-doped sample showed an interesting three-peak structure at frequency shifts of 11.4, 13.8 and 16.1 GHz, with negligible dispersion and in-plane anisotropy. For a fixed scattering angle, the phonon spectrum of the other samples showed a broad peak at a smaller frequency (<10 GHz). It is shown that the observed phonon frequency decreases with increasing ionic character of the La–Mn bonds resulting from the replacement of part of the atoms of La by Ce, Ca, Sr and Ho.