Author: Sin, Wai-lun
Title: Synthesis and characterization of magnetic nanoparticles and nano-composites
Degree: Ph.D.
Year: 2008
Subject: Hong Kong Polytechnic University -- Dissertations.
Nanostructured materials -- Synthesis.
Magnetic materials -- Synthesis.
Nanoparticles -- Synthesis.
Department: Department of Applied Physics
Pages: 1 v. (various pagings) : ill. ; 30 cm.
Language: English
Abstract: The design, synthesis, and characterization of particles with nanometer dimensions have attracted intense interests in recent years. The novel electronic, optical, magnetic, and other properties that arise from the quantum size effect and large surface-to-volume ratio can have immense application value. Magnetic nanoparticles are important examples of how a reduction in size changes the properties of a ferromagnetic material. It is a big challenge to understand and control the interfacial magnetism for developing novel devices with strong correlated electron oxides. Indeed spin-polarized ferromagnetic LaxSr1-xMnO3 (LSMO) has been identified as a good candidate for stable room temperature spintronic applications. This project involves a modified hydrothermal method for synthesizing magnetic nanoparticles with controllable chemical, composition and particle morphology. We have demonstrated that nanoscale manganite particles formed from a metal chloride precursor in water under mild hydrothermal conditions yields high crystalline, thermally stable and pure phase particles, whose sizes and morphology can also be tuned through adjustment of reaction temperature, precursor concentration, concentration of mineralizers and the presence of surfactants. Indeed our work represents the first experimental investigation and synthesis of LSMO nanoparticles using cationic surfactant assisted hydrothermal process. Characterization of the nanocrystals was done by Transmission Electron Microscope (TEM), Scanning Electron Microscope (SEM), X-ray Powder Diffraction (XRD) and Vibrating Sample Magnetometer (VSM). Generally, stoichiometric LaxSr1-xMnO3 (LSMO) nanoparticles were obtained at low process temperature of 240oC under the hydrothermal condition. Structural characterization by XRD showed that the particles started to crystallize at 210oC. Crystalline LSMO particles with grain size as small as 20 nm and confined to a narrow size distribution have been obtained. Progressively improved crystalline structure and orientation were seen at higher processing temperature. The nucleation and crystal growth processes mediated by macromolecule (CTAB) finally results in more uniform and controllable products. Based on these magnetic nano-sized perovskite manganites, we have systematically studied their magnetic hysteresis, magnetic anisotropy and exchange coupling. Several magnetometry techniques such as Zero Field-Cooled (ZFC)/ Field-Cooled (FC) measurements, magnetic hysteresis curve above and below the blocking temperature have been employed in these studies. Since LSMO is in a family of large bandwidth and exhibits a colossal magneto-resistivity, a systematic study of the temperature dependence and the magnetic effects on electrical conductivity in LSMO nanoparticle has been made. The experimental results were explained satisfactory by several scattering and hopping models. The effects of exchange coupling between the manganite nanoparticles with antiferromagnetic phase in core-shell structured were also investigated. The composite consists of ferromagnetic La0.67Sr0.33MnO3 core and antiferromagnetic LaMnO3 shell. Formation of this manganites core-shell structure has been done by a two-step hydrothermal process, which involves the use of two precursor solutions in succession. To ensure the homogeneous overcoat the shell layer, cationic surfactant CTAB has been used to modify the surface of core particle. Structural properties of the core-shell arrangement are confirmed by TEM and XRD experiment. We are able to show that a 5 nm thin layer is clearly coated on LSMO particles to form the desired core-shell architecture. The effects on the magnetic properties such as the coercive field enhancement are also discussed. Finally, LSMO/Poly(vinyl alcohol) (PVA) composites have been prepared by simple ultrasonic mixing of as-prepared nanoparticles and polymer solution. Then, films of LSMO/PVA composites have been fabricated on insulating substrate by traditional spin coating method. The microstructure, magnetic and magnetoresistivity properties of these films have been studied also. The experimental resistivity data of the present investigation are fitted to a simple empirical equation in order to reveal conduction mechanism in these composites.
Rights: All rights reserved
Access: open access

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