Magnetization measurements on fine cobalt particles

Abstract We measure the magnetization of fine cobalt particles by SQUID and pulsed magnetic fields up to 35 T. These measurements have been made on two samples (C1, C2) with nonagglomerated particles. The analysis of the magnetic meaurements evidences very narrow log-normal size distribution centered around 1.5 nm (≅150 atoms) and 1.9 nm (≅310 atoms) for C1 and C2, respectively. Magnetization at 4.2 K seems to saturate in fields up to 5 T leading to an enhanced mean magnetic moment per atom compared to bulk value (1.72 μ B ). However, magnetization measurements up to 35 T do not permit to reach saturation, and show a continuous increase of μ Co reaching 2.1±0.1 μ B (C1) and 1.9±0.1 μ B (C2). The effective magnetic anisotropies are found to be larger than those of bulk materials and decrease with increasing particle size. These features are associated with the large influence of the surface atoms.