Fabrications And Characterizations Of Low-Dimensional Perovskite Manganite Nanostructures

In the perovskite manganites, multi-degrees of freedom such as charge, spin, orbital and crystal lattice coexist simultaneously, and their strong completing interactions result in a series of novel physical phenomena such as the colossal magnetoresistance effect, giant magnetic entropy effect, metal-insulator (M-I) transition, electronic phase separation and charge/orbital ordering, which make them become hot issues in condensed matter physics. Advances in the integration and miniaturization of the electronic devices have resulted in their feature sizes continued to be decreased, and now the feature sizes of electronic devices based on the perovskite manganites are down-scaled into nanometered sizes. Therefore, the fabrications and characterizations of low-dimensional perovskite manganite nanostructures become important especially. In this thesis, low-dimensional perovskite manganite nanostructures are fabricated by sol-gel, chemical solution deposition, and molten salts methods. Their microstructures and physical properties are also characterized.First, La0.7Ca0.3MnO3 nanocrystals were synthesized by sol-gel method and the effects of annealing temperatures and annealing times on the microstructures of the La0.7Ca0.3MnO3 nanocrystals were also investigated. X-Ray diffraction (XRD) patterns reveal that all the samples are well crystallized without impurity phase, and their crystal structures are of orthorhombic system. The volumes of the sample unit cells have a weak tendency of increase with increasing the annealing temperatures. Scanning electron microscopy (SEM) images and energy dispersive X-ray spectra (EDX) results demonstrate that the samples annealed at lower temperatures exhibit sheet morphology, whereas the samples annealed at higher temperatures exhibit particle morphology. All the chemical compositions of the samples obtained the EDX spectra are close to the nominal values. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images reveal that the particle sizes are around 20 nm, which are increased with increasing the annealing temperatures. The clear lattice fringes observed in the HRTEM images of single particles confirm the well crystallinity of the particles. Polycrystalline diffraction rings shown in selected area electron diffraction (SAED) patterns match well with the diffraction peaks shown in the XRD patterns. Based on the measurements by superconducting quantum interference device (SQUID) the ferromagnetic - paramagnetic transition temperatures (Curie temperature, Tc) of the samples annealed @ 600℃ and 650℃ were determined to be 267K and 268K respectively, but the Tc of the sample annealed at 700℃ was only 250K. Tc is increased about 2K when magnetic field of 0.1 T is applied. The M-H loops demonstrate that the samples behave ferromagnetic state at 5K but paramagnetic state at 300K (>Tc). Measurements by physical property measurement system (PPMS) show that the M-I transition temperatures (Tp) of the samples annealed @ 600℃,650℃ and 700℃ are 265.6K,267.7K, and 248.5K, respectively. When a magnetic field of 5 T is applied, the Tp is shifted to low temperature. The MR value of the sample annealed @ 600℃ is decreased when the applied magnetic field is increased.Secondly, perovskite manganite nanoislands are synthesized on YSZ(100) and MgO(100) single crystal substrates by chemical solution deposition (CSD) method, and their microstructure are characterized by atomic force microscopy (AFM) and TEM. The results show that the La0.7Sr0.3MnO3 nanoislands (annealed@1000℃ for 12 h) synthesized by 0.3 M sol deposited on YSZ substrates have a lateral size of ～780 nm, longitudinal size of ～110 nm, and planar island density of 8/μm2. When the sol concentration is decreased to 0.03 M, the lateral size of the nanoislands becomes to ～550 nm, the longitudinal size to ～50 nm, and the planar island density changes to 2/μm2. The La0.875Ca0.125MnO3 nanoislands (annealed@1000℃ for 1 h) synthesized by 0.03 M sol deposited on MgO substrates have lateral size of ～150 nm, longitudinal size of ～17 nm and island density of 11/μm2. When the annealing time is increased to 3 h, the lateral size of the nanoisland is increased up to near 1μm, the longitudinal size increased up to more than 100 nm, the island density decreased to 1.5/μm2. Therefore, the optimized experiment conditions for the synthesis of the La0.7Sr0.3MnO3 nanoislands include annealing @ 1000℃ for 12 h, and sol concentration of 0.03 M. For the fabrication of the La0.7Ca0.3MnO3 nanoislands, the annealing time is less than 1 h.Finally, both La0.7Ca0.3MnO3 and La0.5Ca0.5MnO3 nanocrystals were synthesized by molten salts method. The XRD patterns demonstrate that all the diffraction peaks of the two nanocrystals match well with that in the La0.8Ca0.3MnO3 (PDF card No.44-1040), indicating they are monoclinic system close to cubic phase. Impurity phase of La0.7Ca0.3MnO3 were decreased with increasing the annealing temperatures. The volumes of the unit cells of the two samples have a slight tendency to increase with increasing the annealing temperatures. TEM images reveal that the particle sizes of the samples are nearly 20 nm. High-resolution lattice fringes observed in a single particle also confirm the well crystallinity of the particles. The M-H loops demonstrate that the samples behave ferromagnetic state at 5K but paramagnetic state at 300K (>TC). The Curie temperatures (TC) of the two samples are close to 270K. The saturation magnetization of La0.7Ca0.3MnO3 nanocrystals is higher than that of La0.5Ca0.5MnO3 nanocrystals. The resistivities of the two sample are decreased with increasing the external applied magnetic fields and the M-I transition temperature (TP) is also decreased. Moreover, The TP value is also decreased with increasing the annealing temperature. The TP value of La0.5Ca0.5MnO3 nanocrystals is lower than that of La0.7Ca0.3MnO3 nanocrystals synthesized under the same experimental conditions.