Monodisperse FePt 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 synthesis and characterization of FePt nanoparticles and their use in potential applications.

The FePt nanoparticles of different size (2 to 16 nm) and shape (spherical, cubic, rod) were synthesized by a chemical solution method. The size and shape of these particles were controlled by adjusting reaction parameters. The as-synthesized FePt nanoparticles have chemically disordered fcc structure and are superparamagnetic at room temperature. Upon heat treatment the nanoparticles were transformed into ordered L1₀ structure, and high coercivity up to 27 kOe was achieved. Magnetic properties of annealed FePt nanoparticles including magnetization and coercivity were strongly dependent on particle size, shape, composition and annealing temperature.

FePt/Fe₃O₄ bimagnetic nanoparticles with two different morphologies, core/shell and heterodimer, were prepared by coating or attaching Fe₃O₄ on surface of FePt nanoparticles. The size of FePt and Fe₃O₄ was tuned very finely to obtain most effective exchange coupling. The heterodimer nanoparticles resulted in relatively poor magnetic properties compared to the core/shell nanoparticles due to insufficient exchange coupling. By optimizing the dimensions of the FePt and Fe₃O ₄ in core/shell bimagnetic nanoparticles, energy products up to 17.8 MGOe were achieved.

FePt/Fe₃O₄ core/shell and FePt+Fe₃O ₄ mixed nanoparticles with similar magnetic properties were compacted under 2.0 GPa at 400°C, 500°C and 600°C. A density up to 84% of the full density was achieved. After annealing at 650°C in forming gas, the FePt/Fe₃O₄ compacted samples were converted into L1₀ FePt/Fe₃Pt magnetic nanocomposite. The nanoscale morphology was retained before and after annealing for bulk samples made from both core/shell and mixed nanoparticles. After annealing, the highest energy product in the bulk samples was 18.1 MGOe based on the theoretical density. The core/shell nanoparticle compacted samples had more effective exchange coupling than the mixed nanoparticle compacted samples.

FePt/Au core/shell nanoparticles were successfully synthesized where Au shell was coated by reduction of gold acetate on surface of FePt nanoparticles. The FePt/Au core/shell nanoparticles show ferromagnetism after annealing at optimum temperature without any significant sintering. Also, FePtAu nanoparticles were prepared by doping Au into FePt nanoparticles during the synthesis. By tuning right stoichiometry of the Fe_xPt_yAu_100-x-y nanoparticles, the phase transition temperature from fcc to L1 ₀ was reduced by more than 200°C. After annealing at 500°C, the highest coercivity of 18 kOe was obtained from the Fe₅₁Pt ₃₆Au₁₃ nanoparticles compared to 2 kOe from Fe₅₁Pt ₄₉ nanoparticles without any sacrifice in saturation magnetization.