Influence on the Physical Properties

Much work has been done on the influence of strain on the transport properties, but we will also describe how the magnetic properties can be changed. Many groups have focused their studies on the modification of the physical properties [122] by the strain effects since the most common properties such as the insulator-to-metal (TIM) transition and the Curie temperatures (TC) are affected. In Figure 13, Koo et al. show this clear correlation between the substrate

Figure 12. Electron diffraction pattern taken along a direction perpendicular to the substrate plane of a Pr0 5Ca0 5MnO3 films on (a) SrTiO3 and (b) LaAlO3. Note the different orientation between both substrates.

Figure 13. (a) p(T) of La07Ca03MnO3 on various substrates under 0 and 1 T magnetic field. (b) MR defined as (p(0) — p(1T))/p(0) normalized to the value at T = 317 K. Reprinted with permission from [123], T. Y. Koo et al., Appl. Phys. Lett. 71, 977 (1997) © 1997, American Institute of Physics.

and the physical properties for La07Ca03MnO3 films: the TIM and maximum MR shift to a higher temperature when changing a SrTiO3 to a LaAlO3 substrate [123].

Since the crystallinity of these films can be changed, as previously discussed, Gillman et al. [113] have prepared La1—xCaxMnO3 (x = 0.41) films on substrates with different lattice parameters by liquid delivery metal-organic chemical vapor deposition. Films on LaAlO3, closely lattice matched with the substrate, exhibit a high degree of crystallization and a high magnetoresistance ratio as compared to films deposited on Al2O3 or Y-ZrO2. Similar results were reported for La08Sr0.2MnO3, which is epitaxial when grown on (100)-LaAl03 and polycrystalline when grown and (100)-Si [122]. Moreover, the TIM increases by 20 K when using (011)-LaAl03 rather than (001)-LaAl03. Similar results have been reported for La2/3MnO3—s films grown on both Al2O3 and SrTiO3 [124] and also for La0 67Ca033MnO3 [125]. The TIM is higher and the transition is sharper for material grown on STO (300 K) than on ALO (200 K). Even if the TIM varies a lot, the Curie temperature is found to remain almost constant, independent of the substrate, for many compounds such as La2/3MnO3—s [124] or La07Sr03MnO3 [126].

The above evidence implies that TIM is directly connected to the substrate. Often, the transition is at higher temperature and sharper when the mismatch between the film and the substrate is smaller, probably due to a high degree of epitaxy.

Strain not only influences the transport transitions but also the direction of the magnetization (via lattice deformation): it is found to be in-the-plane for films under tensile stress (for example on SrTiO3) and out-of-plane for compressive stress (as in the case of LaAlO3) [87,126]. Using a wide-field Kerr microscope, magnetic domain orientation and contrast of Laa67Sra33MnO3/SrTiO3 suggest a magnetic anisotropy with (110) easy axes [127]. The easy direction is along [110] of the pseudocubic unit cell, that is, diagonal to the O-Mn-O bond direction for La07Ca03MnO3 film grown on untwinned paramagnetic NdGaO3 (001) [128].

The substrate-induced strain can also influence the optical properties, as for Laa67Ca033MnO3 [129]. This can be explained by the fact that the substrate-induced strain results in modification of the Mn-O bonds and Mn-O-Mn bond angles and, thus, in both the corresponding phonon modes and electron-phonon interactions leading to changes in the phonon frequencies and optical conductance. Note that the strains can also induce a surface magnetization as for La07Sr03 MnO3 [130].

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