Spin and Lattice excitations in Ferromagnetic Insulating Manganites*

Though double-exchange interaction has been recognized as a major driving force for the couple magnetic and electronic phase transition, the nature of insulating ground state with ferromagnetic ordering in low-doping manganites is still not fully understood. Here we report on an inelastic neutron scattering study of spin and lattice excitations in the ferromagnetic insulating (FMI) phase of La$_{1-x}$Ca$_{x}$MnO$_{3}$ with x(Ca) = 0.2. Dispersion relations for both phonons and spin waves along high-symmetry directions were obtained for temperatures of 5 and 225 K, respectively. At low temperatures, our results indicate an anomalous softening and broadening of the magnons near the zone boundary, especially when the magnon energy E $\sim $ 20 meV, where a longitudinal optical phonon is present. Additional phonon and magnon branches observed will also be discussed.