Evolution of strain-dependent transport properties in ultrathin La _0.67Sr_0.33MnO₃ films

We report on a systematic investigation of strain-induced lattice distortion effects on the crystal structure and transport properties of as-grown and postannealed ultrathin La_0.67Sr_0.33MnO₃ epitaxial films grown on LaAlO₃ (001) substrates by pulsed laser deposition. The resistivity of the as-grown films is critically dependent on the thickness, i.e., 100 Å thick films show insulating behavior, 300 Å thick films show metal-insulator transitions, and 500 Å thick films show metallic behavior. However, all the annealed films show identical metallic behavior. Conventional θ-2θ x-ray scans with momentum transfer (q) along the [001] direction, and θ-2θ scans of the (103) and (113) peaks, with q having a component perpendicular to the growth direction, were used to measure the out-of-plane and in-plane lattice parameters of the sample. Φ scans of the (103) and (113) peaks revealed a fourfold symmetry which is consistent with a tetragonal unit cell. These data conclusively show that significantly elongated tetragonal structures (c/a=1.02-1.04) are insulating whilst films with cubic unit cells, relaxed either due to annealing or as a function of thickness, have metallic characteristics.