Abstract
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 (C1, C2) have been obtained by changing the organometallic concentration in the polymer. Observation by HRTEM and WAXS showed that Co particles have diameter around 1.5 nm (C1) and 2 nm (C2). They are well isolated and regularly scattered in the polymer, with an original bcc structure. These particles show a superparamagnetic behavior above the blocking temperature (T B ). Analysis of the magnetization curves above T B confirms the very weak volume dispersion of the particles centered around 150±10 atoms (C1) and 310±20 atoms (C2), and their homogeneous distribution in the polymer observed by HRTEM. Enhanced effective anisotropy energy with uniaxial symmetry has been deduced from magnetization and ferromagnetic resonance measurements in the FIR range (100-700 Ghz). Magnetization at 5 K seems to saturate in fields up to 5 T leading to an enhanced mean magnetic moment per atom, = 1.94±0.05 μ B for the smallest particles. These values still increase in field up to 35T reaching = 2.1±0.1 μ B (C1), exhibiting a canted spin structure due to high surface anisotropy. This set of data allows us to conclude that the enhanced magnetization, its increase with applied magnetic field, and the enhanced effective magnetic anisotropy, these effects being more significant with decreasing size, are associated to the large influence of the surface atoms.
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