Perovskite Manganite Superlattice Strain Analysis

By introducing artificial interface, two-phase structure of ferromagnetic conductor or insulator in rare-earth doped perovskite manganese oxides, it has been found that the low-field magnetoresistance can be enhanced and the sensitivity to temperature and structure can be reduced. However, no significant breakthrough has been made along this direction in recent years, which has hindered the practical applications of functional devices based on manganese oxide films. On the other hand, one can epitaxially grow artificial superlattices to modulate the structures of magnetic multilayer films, by means of pulsed laser deposition technique. The artificial superlattices thus prepared are expected to overpass above-mentioned disadvantages, which makes the functional devices based on manganese oxide films promising for practical applications. Therefore, it is a very interesting topic to make strain analysis of superlattices.Along this direction, we have epitaxially grown superlattices, [Pr0.7Ca0.3MnO3/La0.5Ca0.5MnO3]20 and [La0.8Ca0.2MnO3/La0.5Ca0.5MnO3]20 by pulsed laser deposition technique, characterized their structures by high resolution X-ray diffraction technique, and performed a strain analysis.By means of the pulsed laser deposition technique, we have first grown a La0.5Ca0.5MnO3 buffer (240A thick) on a single-crystal MgO(001) substrate, and then epitaxially grown a series of [Pr0.7Ca0.3MnO3/La0.5Ca0.5MnO3]20 superlattices, where the thickness of the La0.5Ca0.5MnO3 layer is fixed at 48 A, while the thickness of the Pr0.7Ca0.3MnO3 layer is taken to be 24A,48A,72A,96A,120A, so as to examine the strain effect. Our results indicate that, with the increase of the thickness of Pr0.7Ca0.3MnO3 in a cycle, the average in-plane lattice constant will first increase and then decrease, while the average out-plane lattice constant will change in an opposite order, which results from the strain and relaxation. In addition, for all of our superlattices, the average in-plane lattice constant is slightly less than the bulk lattice constant of either La0.5Ca0.5MnO3 or Pr0.7Ca0.3MnO3, which is attributed to the interaction between the buffer and the interface of the superlattices.To examine the effects of different thicknesses of each component in a cycle on the structure and strain relaxation of superlattices with different substrates, we have grown a series of [La0.8Ca0.2MnO3/La0.5Ca0.5MnO3]20 superlattices on the SrTiO3 and LaAlO3 substrates. Tensile strain in the horizontal direction is observed for superlattices on the SrTiO3 substrate, while compressive strain in the horizontal direction is observed for superlattices on the LaAlO3 substrate. When the thicknesses of all components are increased synchronously in the measurable range, the stress in horizontal direction will be released and the lattice strain will be relaxed until the superlattice is in complete relaxation state. If we only vary the thickness of the ferromagnetic La0.8Ca0.2Mno3 layer, since the LaAlO3 substrate has a smaller lattice constant, its mismatch with the superlattice will lead to a compressive strain of the superlattice in the horizontal direction. In contrast, a tensile strain is observed in the vertical direction of the superlattice. Especially, when the thickness of the ferromagnetic layer goes beyond 92A, both the in-plane and out-plane lattice constants will become almost unchanged. The superlattice will thus be almost in a full relaxation state and all of the samples will have perfect structures.