We have investigated thin films of lanthanum manganites epitaxially grown by pulsed laser deposition on single-crystalline substrates. X-ray diffraction (XRD) studies show that on the axial line of Laue pattern simultaneously occur bright well-expressed reflexes, due to coherent scattering of X-rays by planes parallel to the film surface exhibiting perfect long-range order, and weak smudged reflexes caused by coherent diffusive scattering of X-rays on fragmented planes with mesoscopic ordering. It appears that the characteristic feature of the investigated films is a superposition of coherently coupled fractions with differently scaled atomic order: long range order in the basic single-crystalline matrix, and mesoscopic order in the clusterized structure encompassing the Mn-O layers where the long-range order is disturbed by sequentially changing regions of order (clusters) and disorder (cluster/matrix interfaces). Varying the film growth parameters induces substantial changes in the matrix and in the cluster structure. The fact that cluster boundaries are smeared facilitates local changes of atomic order. Detailed analysis of diffusive scattering shows that the size of metallic clusters in the dielectric matrix varies from 40-70 A to 140-200 A, and their volume concentration from 1-10% to 20-50%. In the first case (small size and concentration), the metallic clusters in low temperature behave as quantum dots and their presence leads to tunneling mechanism of conductivity with characteristic R(T)=const dependence at low enough temperatures. In the second case (large cluster size), the discrete energy levels become smeared and at high enough cluster concentration percolative conduction mechanism takes over. It appears also that the optical, transport and magnetic properties of the investigated films depend critically on the distribution of Mn2+, Mn3+ and Mn4+ ions in the clusters.
This work was supported by Polish Government (KBN) Grant PBZ-KBN-013/T08/19.