Interaction Effects between Nearly Magnetic Cobalt Impurities in Gold

The magnetization of $\mathrm{Au}-\mathrm{C}\mathrm{o}$ alloys was studied over a large range of concentrations ($0.05<c<\mathrm{}4\mathrm{}$-at.% Co), fields ($0<H<\mathrm{}70\mathrm{}$ kOe), and temperatures ($0.05 \mathrm{K}<T<\mathrm{}200\mathrm{} \mathrm{K}$). Magnetization and specific-heat results are well represented by a model with the following characteristics: (a) The isolated Co atoms are not magnetic. (b) The isolated pairs are also not magnetic, but have a Kondo temperature nine times smaller than that of isolated Co atoms. (c) The groups of three or more atoms are magnetic and lead to magnetic ordering at very low temperatures.