Structure And Physical Properties Of Hole-doped Manganite La_0.67Ca_0.33 MnO₃ Bulk And Thin Film

Polycrystalline bulk sample La_0.67Ca_0.33MnO₃ was prepared by sol-gel method. The structure, magnetic property, electrical transport property, thermal transport property, and infrared transmission spectra of La_0.67Ca_0.33MnO₃ have been studied systematically. X-ray diffraction patterns show that the sample is single phase crystallized in the orthorhombic structure. The corresponding lattice constants are a =5.3229 ?, b =7.6821 ?, and c =5.6072 ?, respectively. The Curie temperature and the metal-insulator temperature obtained from the magnetic and electrical transport properties measurement are both around 260 K. The temperature dependence of the thermal conductivity and the electrical conductivity of the La_0.67Ca_0.33MnO₃ sample under different magnetic fields reveal that they both increase sharply on cooling near the Curie temperature. Synchronously, under the influence of the magnetic field, the thermal conductivity increases distinctively on cooling around the Curie temperature. In order to clarify the reasons for the sudden increase of the thermal conductivity near the Curie temperature, infrared transmission spectra of the sample are measured in detail at different temperatures. The results indicate that the charge carriers increased sharply near the Curie temperature. Combining the previous results of the Nd_0.75Na_0.25MnO₃ and the density state results of La_0.67Ca_0.33MnO₃ reported by Park et al (Phys. Rev. Lett. 76, (1995) 4215) we ascribe the abrupt change of the thermal conductivity to the delocalization of the charge carriers below the Curie temperature. Furthermore, above 180 K the magnetoresistance (MR) data can be fitted well according to Wagner's modified Mott model, which indicates that the hopping of the charge carriers between magnetic clusters is dominant in a wide range near the Curie temperature. However, below 140 K, the calculated data deviate from the experimental ones due to the spin polarized tunneling effect between the grains.La_0.67Ca_0.33MnO₃ thin film is deposited on LaAlO3(100) single crystal substrate using radio frequency sputtering by changing annealed time, substrate temperature, sputtering pressure and sputtering power. The structure and electrical transport properties of the film are investigated systemically. Under the following conditions: 780°C substrate temperature, 0.5 Pa sputtering pressure, 184 W sputtering power and annealed 24 h under 900°C in oxygen, the X-ray diffraction (XRD) results of the film only show (101), (202) and (303) diffraction peaks of the orthorhombic La_0.67Ca_0.33MnO₃. At the same time, the random selected area diffraction patterns of the cross-sectional sample are all uniform single crystalline Bragg diffractions. All these results reveal that the film is epitaxial grown along [101] direction, which is consistent with the result of the cross-sectional image of high-resolution transmission electron microscopy (HREM). The metal-insulator transition temperature of the film is 253 K, which reveals that the ions contents of our La_0.67Ca_0.33MnO₃ film are close to the ideal stoichiometry. The manetoresistance of the film is also studied, and the results indicate that the hopping of the charge carriers between the inside magnetic clusters is dominant.