Evidence of magnetization-dependent polaron distortion

X-ray-absorption fine-structure measurements at the Mn (Formula presented) edge as a function of temperature were performed on (Formula presented) and (Formula presented)(Formula presented)Mn(Formula presented). All samples have a metal-insulator (MI) transition near the ferromagnetic transition, except the (Formula presented) sample, which does not have a MI transition. Near (Formula presented) for the samples exhibiting a MI transition, the Debye-Waller (Formula presented) for the Mn-O and Mn-Mn atom pairs increases rapidly with temperature. Since the change in (Formula presented) for the Ca/La atoms is much smaller, this nonthermal disorder must involve mainly the Mn and O atoms. These results strongly suggest that small polarons delocalize as the magnetization increases in materials which exhibit a MI transition.