Spin waves and phonons in La1-xSrxMnO3 (x=0.09, 0.125):: Dynamical signatures of low-temperature phase transitions for x=0.125 -: art. no. 214430

Neutron scattering has been used to study static and dynamic magnetic properties of La1-xSrxMnO3 with x=0.09 and 0.125. The x=0.09 compound appears as a limit case of an inhomogeneous canted antiferromagnetic (CAF) A-type structure. Related to the mean AF medium, anisotropic superexchange integrals are determined from the measurement of a high energy spin wave branch. Coexisting with this mean state, ferromagnetic (F) hole-rich platelets spread in the basal F plane, close to the limit of percolation. Related to these F inhomogeneities, an anisotropic magnetic coupling is determined from an anisotropic low energy spin wave branch. The x=0.125 compound exhibits, below T-C, a ferromagnetic and metallic phase (FM). At (TOO')-O-', a magnetostructural transition occurs leading to a F and insulating phase (FI) with unusual properties: in the whole q range, a splitting of the spin wave spectrum is observed. A large gap opens at (0,0,0.5). At this point and beyond, the energy of the spin waves locks on the phonon energy values. The temperature evolution of the splitted magnetic modes and of the phonons is presented, with marked signatures of both transitions. These results are discussed in the frame of existing theories.