Strain effect on electronic transport and ferromagnetic transition temperature in La_0.9Sr_0.1MnO₃ thin films

We report on a systematic study of strain effects on the transport properties and the ferromagnetic transition temperature T_c of high-quality La_0.9Sr_0.1MnO₃ thin films epitaxially grown on (100) SrTiO₃ substrates. Both the magnetization and the resistivity are critically dependent on the film thickness. T_c is enhanced with decreasing the film thickness due to the compressive stain produced by lattice mismatch. The resistivity above 165 K of the films with various thicknesses is consistent with small polaronic hopping conductivity. The polaronic formation energy Ep is reduced with the decrease of film thickness. We found that the strain dependence of T_c mainly results from the strain-induced electron-phonon coupling. The strain effect on E_p is in good agreement with the theoretical predictions.