【作者】 高艳卿；

【导师】 李志青；

【作者基本信息】 天津大学 ， 材料物理与化学， 2007， 硕士

【摘要】 用溶胶-凝胶法制备了La_0.67Ca_0.33MnO₃多晶块体样品,并对其结构、磁性、电和热的输运性质及红外透射光谱进行了系统的研究。X射线衍射结果表明,样品为单相的正交结构,其晶格常数分别为a=5.3229 （?）、b=7.6821 （?）、c=5.6072 （?）。磁性质和电输运性质的测量结果表明,样品的居里温度和金属-绝缘体转变温度均在260 K附近。我们对上述样品在不同外磁场下的导热系数和电导率随温度的变化规律进行了系统的测量,发现,在居里温度附近,导热系数和电导率随温度的降低均有一突然增加,同时,外磁场使得样品在居里温度附近的导热系数明显增加。针对导热系数在居里温度附近随温度的降低而突然增加这一现象,我们对样品的在不同温度的红外透射光谱作了详细的测量,结果表明,有效载流子浓度在居里温度附近随着温度的降低而突然增加,结合以前本研究组对Nd_0.75Na_0.25MnO₃样品导热系数的测量结果及Park等人（Phys. Rev. Lett. 76, （1995） 4215）对La_0.67Ca_0.33MnO₃电子能态密度的测量结果,我们提出导热系数在居里温度附近随温度的降低而突然增加是由于载流子的退局域化而导致的。此外,我们发现在180 K以上La_0.67Ca_0.33MnO₃的磁电阻数据很好的符合Wagner修正的Mott模型,说明在居里温度附近很大的温度范围内载流子在其内部磁团簇之间的跳跃起主导作用;而在140 K以下由于晶粒间自旋相关的隧穿效应起主导作用,使得磁电阻数据偏离Wagner修正的Mott模型。通过改变退火时间、基底温度、溅射气压、溅射功率等工艺参数,我们用射频溅射法在（100）取向的LaAlO3单晶基底上生长了La_0.67Ca_0.33MnO₃薄膜,并对薄膜的结构、电输运性质做了详细的研究。我们发现,当基底温度为780°C、溅射气压为0.5 Pa、溅射功率为184 W,并且在900°C的氧气氛围中退火24 h时,X射线衍射花样中只出现了正交结构的La_0.67Ca_0.33MnO₃的（101）、（202）及（303）晶面衍射峰,而在选定的断面任意区域作电子衍射,其电子衍射花样都相同,并且都是单晶布拉格衍射斑点,这就证明了在该条件下制备的La_0.67Ca_0.33MnO₃薄膜为沿[101]方向生长的单晶薄膜。断面高分辨电镜图像进一步证明了上述结论。该薄膜的金属-绝缘体转变温度为253 K,说明我们制备的La_0.67Ca_0.33MnO₃薄膜中各离子含量接近理想配比。磁电阻的研究表明,该单晶薄膜在很大的温度范围内载流子在磁团簇之间的跳跃为主要导电机制。更多还原

【Abstract】 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.更多还原