Low field magnetoresistance at the metal-insulator transition in epitaxial manganite thin films

We present a study of the magnetotransport behavior around T-c of epitaxial La0.7Ca0.3MnO3 thin films with controlled chemical defects introduced in order to change the electron-lattice coupling (lambda). We found unexpected enhancement of the very low field (below 250 Oe) magnetoresistance around T-c and relate it to a reduction of the magnetic domain size. This process is strongly favored by the presence of chemical defects that also promote weak localization of the carriers into polaron clusters that can be overcome by relatively low magnetic fields. This behavior is characteristic of temperatures near T-c where the bandwidth is reduced and localization, due to intrinsic disorder and chemical defects, is more probable. In addition, the increase of lambda raises the polaron binding energy and reduces the temperature at which percolation occurs. (C) 2002 American Institute of Physics.