FePt nanoparticles from the gas phase: Preparation, structure and magnetism

The subject of this thesis is the investigation of the morphology, the structure, and the magnetic properties of FePt nanoparticles. The particles were prepared by DC sputtering in an Ar/He gas mixture at pressures in the range 0.5 mbar 2.0 mbar and sintered in-flight at sintering temperatures of T <= 1273 K. At p = 0.5 mbar, we obtain monodisperse FePt nanoparticles with a mean diameter of d = 6 nm independent of the sintering temperature. HRTEM investigations show that the particles exhibit spherical morphologies and are of icosahedral structures. The are superparamagnetic at room temperature with a blocking temperature of T = 50 K. At higher pressures p > 1.0mbar, the particles form agglomerates due to an increased concentration of particles in the carrier gas. The agglomerates can be compacted in-flight at elevated sintering temperatures 673 K < T < 1073 K, and the mean particle size increases from d = 5 nm at T = 673 K to d = 8 nm at T = 1073 K. From the variation of the particle size d with the sintering temperature T, an activation energy of roughly E = 0.5 0.7 eV is estimated for the growth process. This indicates that surface diffusion and / or grain boundary diffusion are the predominant sintering mechanisms in the temperature range 673 K < T < 1073 K. At T < 1073 K, we observe the onset of intra-particle re-crystallisation, which leads to the formation of an increasing amount of single crystals and to the onset of L10 ordering within the particles. An analysis of both the diffusion lengths and sintering times shows that in this temperature regime, volume diffusion dominates. The magnetic investigations reveal that, concurrently with the increasing amount of L10 ordered particles, the room temperature coercivity increases form HC(RT) = 0.5 kOe for particles sintered at T = 1073 K to HC(RT) = 1.2 kOe for particles which are sintered at T = 1273 K