Coherent waves of magnetic polarons propagating inLa1−xCaxMnO3:An inelastic-neutron-scattering study

Inelastic neutron scattering has been performed on semiconducting ${\mathrm{La}}_{1-x}{\mathrm{Ca}}_x{\mathrm{MnO}}_3$ 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 (≈8–10 Å), reminiscent of a picture of mobile “magnetic polarons.”