Fabrication Of Superparamagnetic And Ferromagnetic Nanoparticles

Monodisperse magnetic nanoparticles with controlled size and geometry have drawn great attention in the last decade for fundamental scientific studies and for their potential applications in advanced materials and devices such as ultra high-density magnetic recording media, exchange-coupled nanocomposite magnets, biomedicine and nanodevices. This dissertation focuses on the fabrication and characterization of superparmagnetic and ferromagnetic nanoparticles of hard magnetic materials (FePt, SmCo5, Sm2Co17 and Nd2Fe14B) and soft magnetic materials (CoFe2O4, NiFe2O4 and FeCo). Novel preparation techniques including salt-matrix annealing, surfactant-assisted ball milling and magnetic field milling have been adopted in this study. The FePt nanocrystals with a variety of morphologies including nanowires, nanorods, spherical nanoparticles, nanocubes and nanosized multi-pods, were synthesized by a polyol reduction process. The size and shape control was achieved by adjusting synthesis parameters. The as-synthesized FePt nanocrystals have chemically disordered fcc structure and are superparamagnetic at room temperature. Upon heat treatment, the nanoparticles were transformed into hard magnetic films with ordered fct structure and high coercivity up to 25 kOe was achieved. Monodisperse L10 FePt nanoparticles from 3 to 15 nm were prepared by a salt-matrix annealing technique. Size dependent phase transition and chemical ordering of FePt nanoparticles were systematically investigated. Magnetic properties of L10 FePt nanoparticles, including magnetization and coercivity are strongly dependent on both the particle size and the chemical ordering and increases with particle diameter. The giant coercivity up to 35 kOe was achieved for 8 nm L10 FePt nanoparticles with faceted shape. Monodisperse CoFe2O4 and NiFe2O4 nanoparticles of different sizes ranging from 3 to 20 nm were also synthesized by a polyol reduction process. Air stable FeCo nanoparticles with controllable particle size and narrow size distribution were prepared by reductive salt-matrix annealing of CoFe2O4 nanoparticles. Size and temperature dependent magnetic properties of CoFe2O4, NiFe2O4 and FeCo have been reported. Sm2Co17, SmCo5 and Nd2Fe14B nanoparticles with narrow size distribution were fabricated by high energy ball milling in the presence of surfactants and followed by subsequent size selection process. Significant room-temperature coercivity up to 3.2 kOe was achieved with the Sm2Co17 nanoparticles of an average size of  20 nm. Nd2Fe14B and Sm2Co17 sub-micron particles were also prepared by ball milling in a magnetic field. Particles milled in a magnetic field, consisting of nano-sized grains, exhibit strong magnetic anisotropy compared with the particles milled without a magnetic field.