Microstructure and direct ordering of FePt nanoparticles produced by nanocluster beam technology

The equiaxed or polyhedral morphology of FePt nanoparticles can be produced by tuning the temperature of the nanoparticle- forming chamber. Weak beam dark- field ( WBDF) and high resolution transmission electron microscopy ( HRTEM) techniques were used to analyze the microstructures and three- dimensional ( 3D) shapes of the particles. The equiaxed nanoparticles formed at 170 K were icosahedra. The nanoparticles with the square shapes in bright- field TEM image were octahedra. The nanoparticles with hexagonal shapes were either truncated octahedral or icosahedral particles. The rhombic shapes in TEM images originated from both truncated octahedral and decahedral particles. As- deposited FePt nanoparticles showed superparamagnetic behavior. The FePt nanoparticles were partially chemically ordered and showed ferromagnetic behavior by annealing the nanoparticles in transit through the furnace from the source to the substrate. The coercivity of the polyhedral FePt particle assembly was about 600 Oe, whereas that of the equiaxed FePt particles was 450 Oe. The larger coercivity was attributed to the increased particle size for particles formed at the lower temperature of the nanoparticle- forming chamber, and better long- range order for the particles that went through the on- line heating chamber.