Coherent waves of magnetic polarons propagating in La1-xCaxMnO3: An inelastic-neutron-scattering study

Inelastic neutron scattering has been performed on semiconducting La1-xCaxMnO3 single crystals, which exhibit a slightly canted magnetic structure at a low doping rate (x=0.05,0.08). The dynamic study reveals the coexistence of two excitation branches with very different characteristics, and local modes for x=0.05; The first branch is anisotropically dispersed and is readily attributed to spin waves; the Mn spins are coupled by exchange forces, indicating a strong decrease of the antiferromagnetic interplane coupling by doping. The second branch is weakly and isotropically dispersed, which suggests a coupling induced by the hole hopping. Its unusual q-dependent intensity reveals that this excitation is associated with a very short ferromagnetic correlation length (approximate to 8-10 Angstrom), reminiscent of a picture of mobile ''magnetic polarons.''.