Optical and magnetoresistive properties of polycrystalline La_0.7Sr_0.3MnO₃ films on crystalline Al₂O₃ and Gd₃Ga₅O₁₂ substrates

The influence of thickness and heat treatment on the optical, magnetic and magnetoresistive properties of polycrystalline La_0.7Sr_0.3MnO₃ (LSMO) films deposited onto Al₂O₃ (012) and Gd₃Ga₅O₁₂ (111) crystalline substrates was investigated. It was shown that the optical properties of the annealed films are similar in the investigated range of photon energies 1–6 eV, independently of their thickness and substrate material. The optical conductivity spectra of the annealed films exhibit the same structure as epitaxial LSMO films but shifted to higher energies by ∼0.5 eV. The temperatures of broadened metal–insulator and ferromagnetic–paramagnetic transitions in the LSMO films are size-dependent and considerably lower as compared with the epitaxial films. The magnetoresistive effect is ∼15% in the field of 10 kOe and at the temperature of metal-insulator transition. These features of the films under study are explained by the presence of grain boundaries and the influence of the film-substrate interface.