Transmission electron microscopy and x-ray structural investigation of La0.7Ca0.3MnO3 thin films

The structural changes and magnetoresistance (MR) properties of as-grown and post-annealed La0.7Ca0.3MnO3 films were investigated by transmission electron microscopy (TEM) and x-ray diffraction (XRD). The data for the films were compared to that for bulk La0.7Ca0.3MnO3 post-annealed under the same conditions. The main structure of the as-grown films was face-centered pseudo-cubic with a doubled perovskite unit cell, of size similar to 2a(p) x similar to 2a(p) x 2a(p), where a(p) is the single perovskite parameter. The phase showed a cube-on-cube epitaxy with the underlying LaAlO3 substrate. Upon annealing to a saturation point, a minor primitive pseudo-tetragonal structure evolved, of cell parameters similar to root 2a(p) x similar to root 2a(p) x similar to 2a(p). A total of four possible orientations of the two structures was observed by TEM, comprised of one orientation of the similar to 2a(p) x similar to 2a(p) x similar to 2a(p) cell, i.e., the cube-on-cube epitaxy, giving rise to (OOl) peaks in x-ray, and three orientations of the similar to root 2a(p) x similar to root 2a(p) x similar to 2a(p) cell, giving rise to a single (OOl)/(hkO) peak in x-ray. The bulk La0.7Ca0.3MnO3 sample also contains the similar to root 2a(p) x similar to root 2a(p) x similar to 2a(p) structure. The difference between the bulk and the film and the effects of annealing on films can be ascribed to the influence of strain between the film and substate, induced by lattice mismatch.