0 1000 2000 3000 o 4000 5000 Thickness(A)

Figure 14. Thickness dependence of the perovskite unit cell volume of epitaxial Laa8Caa2MnO3 films on (001)-LaAlO3 (triangles) and (001)-SrTiO3 (circles). Large deviations of the lattice parameters from those of the bulk are observed. As film thickness increases, both in-plane and out-of-plane lattice parameters tend to deviate away from those of the substrates toward the bulk value. Reprinted with permission from [140], R. A. Rao et al., Appl. Phys. Lett. 73, 3294 (1998) © 1998, American Institute of Physics.

respectively, as a function of the film thickness [121]. For example, in the case of Nd2/3Sr1/3MnO3 grown on SrTiO3, the out-of-plane parameter increases from a value of 0.38 nm for a 20 nm thick film to 0.386 nm for a 100 nm film, which is close to the bulk value [141]. The scenario is the opposite when the film is compressively strained as in La07Sr03MnO3 on (100)-LaAl03 [142]: the out-of-plane parameter decreases from 0.394 nm for a 30 nm thick film to 0.39 nm for a 450 nm thick film, while at the same time the in-plane parameter changes from 0.382 to 0.388 nm. The film is not completely relaxed until it reaches a thickness on the order of 100 nm. In Nd05Sr05MnO3 deposited on (001)-LaAl03 [143], two regimes were observed using XRD: one which was strained (close to the substrate), and a quasi-relaxed component in the upper part of the film, the latter increasing with film thickness.

As previously reported, increasing film thickness leads to a change of the symmetry of the film. This was systematically studied by looking at various film compositions vs thickness by Yu et al. [144]. They found a strong tetragonal lattice strain using HRTEM and XRD characterization. This is more important for a composition in which the bulk structure is orthorhombic, as in (La1-xPrx)07Ca03MnO3, as compared to La1-xNaxMnO3 or La0 7Sr03MnO3 where the structure is rhombohedral [144].

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