Transition from double exchange ferromagnetic metal to hysteretic insulator mimicking charge-ordering effects in ultra thin films of a perovskite manganite

Magnetic and electric field dependent charge transport in epitaxial films of La0.7Ca0.3MnO3 deposited on (001) SrTiO3 and LaAlO3 substrates is compared as a function of the film thickness. A hysteretic and history dependent insulating behaviour and a strongly field dependent resistivity observed at small thicknesses suggest coexistence of charge ordered and ferromagnetically ordered regions in these films. This inhomogeneous film growth is presumably due to the lattice mismatch induced strain and atomic scale roughness of the substrates. It is shown that on SrTiO3 substrates treated to obtain atomically flat TiO2 terminated surfaces, films retain the metallic character of bulk La0.7Ca0.3MnO3 down to 60 Å thickness