Spatially inhomogeneous metal-insulator transition in doped manganites

Scanning tunneling spectroscopy was used to investigate single crystals and thin films of La1-xCaxMnO3 (with x of about 0.3), which exhibit colossal magnetoresistance, The different spectroscopic signatures of the insulating (paramagnetic) and metallic (ferromagnetic) phases enable their spatial extent to be imaged down to a Lateral scale of the order of 10 nanometers. Above the bulk transition temperature T-c, the images show mostly insulating behavior. Below T-c, a phase separation is observed where inhomogeneous structures of metallic and more insulating areas coexist and are strongly field dependent in their size and structure. Insulating areas are found to persist far below T-c. These results suggest that the transition and the associated magnetoresistance behavior should be viewed as a percolation of metallic ferromagnetic domains.