Synthesis and characterization of magnetic nanoparticles and nano-composites.

摘要

This project involves a cationic surfactant assisted hydrothermal method for synthesizing magnetic manganite nanoparticles. We have demonstrated that manganite nanoparticles 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.;Generally, stoichiometric LaxSr1-xMnO 3 (LSMO) nanoparticles were obtained at low process temperature of 240°C under the hydrothermal condition. Crystalline LSMO particles with grain size as small as 20 nm and confined to a narrow size distribution have been obtained. 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, temperature dependent magnetization curve have been employed in these studies. A systematic study of the temperature dependence and the magnetic effects on electrical conductivity in LSMO nanoparticle has been made also. The experimental results were explained satisfactory by several scattering and hopping models.;The composite consists of ferromagnetic La0.67Sr0.33 MnO3 core and antiferromagnetic LaMnO3 shell has been prepared. 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. 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.