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

Abstract.

We present a detailed experimental study of Fe and Co colloidal particles stabilized by organic ligands. Comparison is made between the magnetic properties of pure and gold-coated particles. In all cases, evidence of superparamagnetic behaviour is found from dc susceptibility and hysteresis loops, with blocking at quite different temperatures. The magnetic anisotropy, as obtained from the blocking temperature and the coercive field, is larger for the pure particles than for the bulk material. Saturation magnetization values obtained at low temperatures are smaller than the bulk. We attribute these 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 colloids coated by gold remains larger than the bulk value, probably because of the formation of an inhomogeneous Fe/Au alloy.