Raman measurements on thin films of the La0.7Sr0.3MnO3 manganite:: a probe of substrate-induced effects

We report on a Raman study of the phonon spectrum of La 0.7Sr0.3MnO3 thin films epitaxially grown on LaAlO3. The spectrum, as a function of film thickness d, does not change over the 1000-100 angstrom range, whereas a strong hardening of the phonon frequencies of both bending and stretching modes is apparent in ultra-thin films (d < 100 angstrom) where substrate-induced effects are remarkable. This behaviour, which appears to be related with the measured d-dependence of the insulator-to-metal transition temperature, is ascribed to co-operative effects of MnO6 octahedra rotation and charge-localization. Raman spectroscopy proves to be a simple and powerful tool to monitor subtle structural modifications hardly detectable with conventional diffraction techniques in ultra-thin films.