Synthesis And Characterization Of Ferromagnetic Nanoparticles
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Ferromagnetic nanoparticles and their bimagnetic nanoparticles with uniform size distribution have drawn great attention in the past decades because of their unique magnetic properties and potential applications in high density recording media, exchange-spring permanent magnets and biotechnology. In this thesis, synthesis and characterization of ferromagnetic nanoparticles (FePt, Sm-Co system) and bimagnetic nanoparticles (FePt/Fe3O4, FePt/Au) are reported. FePt nanoparticles with different sizes and compositions were successfully synthesized via chemical synthesis. FePt particle size was tuned to a range of from 3 nm to 8 nm with 1 nm accuracy by changing the surfactant to Pt precursor ratio. By optimizing the amount of Fe3O4 phase in exchange-coupled FePt/Fe3O4 nanoparticles, the highest (BH)max of 17.6 MGOe was achieved in 6 nm FePt / Fe3O4 particles after proper annealing, which is 35 % higher than 13 MGOe for single-phase isotropic FePt nanoparticles. As-synthesized FePt particles with average size of 8 nm were annealed using Rapid Thermal Annealing (RTA) treatment with various temperatures and time. It was found that FePt nanoparticles can be transferred from disordered A1 phase to ordered L10 phase in several seconds at above 600 ºC, which is much shorter than the time needed for furnace annealing. The transmission electronic microscopy (TEM) and X-ray diffraction studies revealed that the average particle size of RTA treated FePt nanoparticles is smaller than that of the furnace annealed particles and no excessive particle sintering and agglomeration were observed in the RTA treated particles. FePt/Au core/shell nanoparticles have been successfully synthesized using seed mediated chemistry. The high resolution TEM (HRTEM) with Selected Area Electron Diffraction (SAED) and Nano-Beam Diffraction (NBD) analyses confirmed the core/shell structure of FePt/Au nanoparticles. The VU-Vis measurement revealed that FePt/Au core/shell nanoparticles show a red shift of surface plasmon absorption band compared to pure Au nanoparticles. The FePt/Au core/shell nanoparticles showed ferromagnetism after annealing at optimum temperature. Moreover, the water soluble FePt/Au core/shell nanoparticles were achieved by ligands exchange. Sm-Co particles were prepared via surfactants-assisted ball milling and separated into slurries and nanoparticles by size selection process. As-milled particles were then annealed by different types of annealing techniques to improve the magnetic properties. XRD studies revealed that after forming gas and hydrothermal annealing the Sm-Co particles were oxidized and decomposed into Sm2O3 and magnetically soft phase (FeCo, Co), resulting in the decrease of coercivity. Coercivity of Sm2Co17 slurries increased by vacuum annealing when the ball milling time is less than 5 h. Highest Hc of 7.5 kOe was achieved in 1 h ball-milled particles, which is 35 % higher than the coercivity of non-annealed particles. For the nanoparticles, it was difficult to increase or even maintain the magnetic properties after annealing because of high oxidation affinity of nanoparticles.