  The nature of the polarons in the optimally doped colossal magnetoresistive
(CMR) materials La0.7Ba0.3MnO3 (LBMO) and La0.7Sr0.3MnO3 (LSMO) is studied by
elastic and inelastic neutron scattering. In both materials, dynamic nanoscale
polaron correlations develop abruptly in the ferromagnetic state. However, the
polarons are not able to lock-in to the lattice and order, in contrast to the
behavior of La0.7Ca0.3MnO3. Therefore ferromagnetic order in LBMO and LSMO
survives their formation, explaining the conventional second order nature of
the ferromagnetic--paramagnetic transition. Nevertheless, the results
demonstrate that the fundamental mechanism of polaron formation is a universal
feature of these ferromagnetic perovskite manganites.
