Fabrication of Superparamagnetic and Ferromagnetic Nanoparticles
by Poudyal, Narayan, Ph. D., The University of Texas at Arlington, 2008,240 pages; AAT 3339158
Abstract
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, biomedicines and nanodevices. This dissertation focuses on the fabrication and characterization of superparamagnetic and ferromagnetic nanoparticles of hard magnetic materials (FePt, SmCo 5 , Sm 2 Co 17 , and Nd 2 Fe 14 B) and soft magnetic materials (CoFe 2 O 4 , NiFe 2 O 4 , 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 L 1 0 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 also systematically investigated. Magnetic properties of L 1 0 FePt nanoparticles including magnetization and coercivity are strongly dependent on both the particle size and the chemical ordering and increase with particle diameter. The giant coercivity up to 35 kOe was achieved for 8 nm L 1 0 FePt nanoparticles with faceted shape.
Monodisperse CoFe 2 O 4 and NiFe 2 O 4 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 CoFe 2 O 4 nanoparticles. Size and temperature dependent magnetic properties of CoFe 2 O 4 , NiFe 2 O 4 , and FeCo have been reported. Sm 2 Co 17 , SmCo 5 , and Nd 2 Fe 14 B 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 Sm 2 Co 17 nanoparticles of an average size of ∼ 20 nm. Nd 2 Fe 14 B and Sm 2 Co 17 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.