A Study On The Microstructure And Electrical Transport Properties Of La_0.70Ca_0.30MnO₃ Films

The colossal magnetoristance(CMR) effect in manganese perovskites has aroused considerable interest, due to its properties. It has become one of the frontier objects of condensed physics and electronic science. A few studies of vacuum annealing related to the properties of the La_0.70Ca_0.30MnO₃ films have been conducted, as they are listed as followed:The study background and application perspectives of manganites are introduced.The La_0.70Ca_0.30MnO₃ (LCMO) films which have been made by pulsed laser ablation can be annealed in an oxygen atmosphere of different pressure and temperature.The XRD results show that the LCMO film grains are grown epitaxially at about 700℃, and the lattice mismatch between film and substrate is small, which can be as an experimental guide for the LCMO thin film annealing.The metal-semiconductor resistance-temperature (R-T) curves and transition temperature of the films are systematically investigated. The Vacuum annealing treatment can change the oxygen content and the ratio of Mn⁴⁺/Mn³⁺ of the films effectively. However, the resistance versus temperature relationship depends on the oxygen content and the ratio of Mn⁴⁺/Mn³⁺ of the films. With the annealing temperature decreases, the film oxygen content and the ratio of Mn⁴⁺/Mn³⁺ increase, so the resistance of the films decreases. The LCMO films which have been made by pulsed laser ablation can be annealed in an oxygen atmosphere of 2×10^-4-1×10^-3Torr at 300-600℃. For the samples which were in-situ annealed at 600℃, 500℃and 400℃, it shows a metal-semiconductor transition at 170K, 193K and 211K, respectively. However, for the samples which were ex-situ annealed at 500℃, 400℃, 300℃, it shows a metal-semiconductor transition at 179K, 181K and 237K, respectively. It is found that the metal-semiconductor resistance-temperature (R-T) curves and transition temperature of the films which have been in-situ or ex-situ annealed can be changed obviously.The oxygen content plays an important role in determining the transport properties of films. Vacuum annealing can obviously decrease the transition temperature T_p of the LCMO films.