Effect of strains on electrical and optical properties of thin La0.67Ca0.33MnO3 films

Thin films of La0.67Ca0.33MnO3 (LCMO) (d = 15-200 nm) were grown epitaxially at 750 degrees C under oxygen pressure of 20-25 Pa on lattice-matched NdGaO3 (1 0 0), LaAlO3 (1 0 0), SrTiO3 (1 0 0) and SrTiO, (1 1 0) using pulsed laser deposition. Highly oriented LCMO (1 0 0) films have been additionally prepared on MgO (1 0 0) substrate. Asymmetric (grazing incidence) X-ray diffraction revealed various strain states appearing ina film plane under biaxial compressive stress (LaAlO3) and tensile stress (SrTiO3 (1 0 0) and SrTiO3 (1 1 0)). Existence of nonuniform strains was found for the films grown on MgO crystal exhibiting the largest lattice mismatch. The paramagnetic to ferromagnetic transition temperature, T-p, found from resistivity measurements increased with film thickness. Measurements of optical transmission and reflectance revealed a strain-dependent absorption band at 1.2-1.4 eV. The slightly higher T-p values indicated for the LCMO (1 1 0)/STO (1 1 0) films compared to those of similar LCMO (1 0 0)/STO (1 0 0) layers have been explained taking into account different textures related to a slightly distorted (orthorhombic) LCMO crystal when grown epitaxially on different planes of cubic SrTiO3. (C) 2000 Elsevier Science B.V. All rights reserved.