Surface effects on the magnetic properties of ultrafine cobalt particles

Monodispersed nanoparticles of cobalt have been prepared by an original method using the decomposition under hydrogen of an organometallic precursor in the presence of a stabilizing polymer. Two colloids (Coll-I and Coll-II) have been obtained by changing the organometallic concentration in the polymer. Observation by high-resolution transmission electronic microscopy (HRTEM) showed Co particles well isolated and regularly dispersed in the polymer with a very narrow size distribution centered around 1.5 nm (Coll-I) and 2 nm (Coll-II) diameter. These particles are superparamagnetic above the blocking temperature 9 K (Coll-I) and 13.5 K (Coll-II). The particle size deduced from the analyses of the magnetic susceptibilities and magnetization curves are consistent with those measured by HRTEM. Magnetization at 5 K seems to saturate in fields up to 5 T leading to an enhanced mean magnetic moment per atom for both samples, where $〈{\ensuremath{\mu}}_{\mathrm{Co}}〉=1.94\ifmmode\pm\else\textpm\fi{}0.05$ ${\ensuremath{\mu}}_{B}$ for the smallest particles. High-field magnetization measurements, up to 35 T, increases nearly linearly with the applied field. This is equivalent to an increase of the mean magnetic moment with $〈{\ensuremath{\mu}}_{\mathrm{Co}}〉=2.1\ifmmode\pm\else\textpm\fi{}0.1$ ${\ensuremath{\mu}}_{B}$ at 35 T for the smallest particles. The effective magnetic anisotropies are found to be larger than that of the bulk materials and decrease with increasing particle size. This set of data allows us to conclude that the enhanced magnetization, its increase with applied magnetic field, and the enhanced effective magnetic anisotropy are associated with the large influence of the surface atoms and are more significant with decreasing size.