Magnetic properties of cobalt-cluster dispersions generated in an electrochemical cell

Magnetization curves of stabilized cobalt-cluster dispersions in tetrahydrofuran with a narrow size distribution have been studied by SQUID and Gouy balance measurements. The cobalt colloids are generated by a newly developed electrochemical method which allows one to generate clusters with mean cluster sizes of about 1000 atoms. The final size distribution of the clusters is examined by small-angle x-ray scattering (SAXS) and high-resolution transmission electron microscopy (HRTEM) measurements. The magnetization curves have been measured with special emphasis on changes at the freezing point of the solution. The liquid phase shows typical superparamagnetism whereas strong deviations from Langevin behavior occur in the solid dispersion which can be understood in terms of magnetic anisotropy effects. These observations are discussed with regard to recent Stern-Gerlach experiments on isolated ferromagnetic clusters in molecular beams. It is shown that cluster size and susceptibility of the dispersions are related. This permits the control of the growth of the clusters during the electrolysis by measuring the susceptibility as a function of the charge that is converted in the cell.