Influence of misfit stress on the magnetoresistive properties of La0.7Ca0.3MnO3-δ thin films

For this study La0.7Ca0.3MnO3-delta (LCMO) films of various thicknesses were deposited on SrTiO3 (100), LaAlO3 (100), and MgO(100) substrates. The substrate-film lattice mismatch causes changes in the microstructure of the films and can therefore influence their magnetoresistive behavior. The structure of the films was characterized by x-ray diffraction and high-resolution transmission electron microscopy. Magnetic and electric properties were determined using standard techniques. Samples grown on SrTiO3 show a strongly distorted growth. A large number of defects are incorporated in the initial LCMO layers. Beginning at a thickness of approximately 500 nm films on these substrates show strong deviations in electric behavior and develop macroscopic cracks. In the case of LaAlO3 substrates the films show only a few lattice defects. The substrate-film lattice mismatch is compensated by large stresses in the LCMO films. The best samples with the least amount of defects are grown on MgO substrates even though the substrate-film mismatch is largest in this case. The misfit is completely compensated in a narrow region near the substrate interface. From there on the LCMO grows with only few lattice defects and little strain. These films exhibit electric and magnetic properties similar to bulk materials. The results demonstrate that the colossal magnetoresistance properties of thin films are significantly influenced by strain caused by the substrate. They behave different than bulk samples under hydrostatic pressure. These aspects must be considered when interpreting the properties of such films and comparing the results of individual experiments. (C) 1999 American Institute of Physics. [S0021-8979(99)74108-3].