Evaluation of the strains in charge-ordered Pr_(1-x)Ca_xMnO_3 thin films using Raman spectroscopy

摘要

Thin films of Pr_(1-x)Ca_xMnO_3 (x=0.5,0.6) deposited on LaAlO_3 and SrTiO_3 substrates have been studied by Raman spectroscopy at low temperatures in order to investigate the effect of strains from the Ca doping or the substrate. A detailed assignment of the observed bands is suggested based on the present observations and published results on manganites. We assign the low frequency bands to modes involving only displacements of the A-site ions from their mass dependence by the Ca substitution for Pr (Pr/Ca-O modes). The A_g(2) mode, which is related to the tilting angle of the MnO_6 octahedra, appears strongly coupled with the carriers and is very sensitive to the strain effects. Based on the Raman data obtained from the film cross sectional area, we extract the strain distribution across the film. Besides, we calculate in the pseudocubic approximation the phonon deformation potentials, the Grueneisen parameter, and the bulk modulus of the film, which are in good agreement with the ultrasonic measurements.