Influence of Post-Deposition Annealing on Lattice Strain, Electrical Transport and Magnetic Properties in Epitaxial La_0.8Ca_0.2MnO₃ CMR Films

Abstract

The effect of annealing on 3-dimensional lattice strain, crystallographic domain structure, magnetic and electrical properties of both 250 Å and 4000 Å thick epitaxial La0.8Ca0.2MnO3 (LCMO(x=0.2)) thin films grown on (001) LaAlO3 substrates have been studied. While short annealing time (∼2hrs. at 950 °C in oxygen of 1 atm. pressure) leads to anomalous increase of the peak temperature (Tp) and Curie temperature (Tc) above room temperature and that of the bulk material, longer annealing time (∼10 hrs.) restores the Tp and Tc to almost the same values as that of the as-grown films. Furthermore, as the annealing time is increased, the lattice strain relaxes with film's lattice parameter approaching the bulk value. In-plane and out-of-plane lattice parameters and strain states of the as-grown and annealed films were measured directly using normal and grazing incidence x-ray diffraction. A clear correlation is observed between Tp and perovskite unit cell volume for both the films. Tp is found to increase with the decrease of perovskite unit cell volume. This is attributed to the enhancement of overlap between Mn d orbitals and oxygen p orbitals leading to increased bandwidth and conductivity. Crystalline quality of the films as determined by the full width at half maximum (FWHM) of the x-ray rocking curves, improves with the annealing time. This work highlights the importance of controlling the 3-dimensional lattice strain for optimizing the properties of CMR films.