Polaron ordering in ferromagnetic colossal magnetoresistive oxides

We review our recent x-ray and neutron scattering studies that reveal stati c diffuse scattering due to polarons in the paramagnetic phase of the colos sal magnetoresistive manganites La2-2xSr1+2x,Mn2O7, with x = 0.40 and 0.44. We show that the polarons exhibit short-range incommensurate correlations that grow with decreasing temperature, but disappear abruptly at the combin ed ferromagnetic and metal-insulator transition in the x = 0.40 system beca use of the sudden charge delocalization, while persisting at low temperatur e in the antiferromagnetic x = 0.44 system. The "melting" of the polaron or dering as we cool through Te occurs with the collapse of the polaron scatte ring itself in the x = 0.40 system. The polaron order is characterized by a n ordering wave vector q = (0.3,0,1) that is almost independent of x for x greater than or equal to 0.40, and is consistent with a model of disordered stripes.