| The research work in this dissertation, was focused on the fabrication of magnetic nanoparticles and nanoflakes by high-energy ball milling (HEBM), the understanding of their formation and the study of their fundamental and hard magnetic properties as a function of size and shape. A successful fabrication of (Sm, Nd & Pr)-Co, and Nd2Fe14B anisotropic nanoparticles is needed for the development of anisotropic nanocomposite permanent magnets using the bottom-up approach. A novel approach has been employed for the fabrication, which is surfactant-assisted high-energy ball milling. A two-step HEBM was used to prepare the nanoparticles and the nanoflakes. In the first step the coarse powders were brought into the nanocrystalline state by using HEBM in heptane without the presence of surfactants. Surfactants during the milling process leads to the dispersion of crashed particles in the appropriate organic solvent, preventing them from welding, and enabling them to a better size refinement.;A strong correlation between the grain size of the nanoflakes and the size of the nanoparticles has been observed. The data suggests that the nanoparticles break from the nanoflakes along the grain boundaries during milling.;The intrinsic properties of the nanoparticles, including the saturation magnetization MS, the magnetocrystalline anisotropy K and the spin reorientation temperature TSP have been found to decrease drastically with decreasing particle size. This behavior has been attributed mostly to surface disorder and thermal effects.;Magnetically hard Nd2Fe14B nanopowders with RT coercivity in the range of 1.2 - 4 kOe have been produced. The coercivity shows a strong dependence on particle size. As the particle size decreases, the coercivity is found to decrease substantially. This effect is related the reduced magnetocrystalline anisotropy of the nanoparticle and to thermal effects. |
