Magnetic properties of nanosized transition metal colloids: the influence of noble metal coating

We present a detailed experimental study of Fe and Co colloidal particles s tabilized by organic Ligands. Comparison is made between the magnetic prope rties of pure and gold-coated particles. In all cases, evidence of superpar amagnetic behaviour is found from de susceptibility and hysteresis loops, w ith blocking at quite different temperatures. The magnetic anisotropy, as o btained from the blocking temperature and the coercive field, is larger for the pure particles than for the bulk material. Saturation magnetization va lues obtained at low temperatures are smaller than the bulk. We attribute t hese effects to surface pinning by the ligands. The magnetic anisotropy of the cobalt particles coated with a gold layer is found to be highly reduced , to a value very close to the bulk. By contrast, the anisotropy of iron co lloids coated by gold remains larger than the bulk value, probably because of the formation of an inhomogeneous Fe/Au alloy.