Raman measurements on thin films of the La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> manganite: a probe of substrate-induced effects

We report on a Raman study of the phonon spectrum of La <Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Sub script> thin films epitaxially grown on LaAlO<Subscript>3</Subscript>. The spectrum, as a function of film thickness d, does not change over the 1000–100 Å range, whereas a strong hardening of the phonon frequencies of both bending and stretching modes is apparent in ultra-thin films (d>100 Å) 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 MnO<Subscript>6</Subscript> 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. Copyright EDP Sciences/Società Italiana di Fisica/Springer-Verlag 2005