Magnetic x-ray circular dichroism has been used to measure the room-temperature magnetization of cobalt clusters embedded in a copper matrix. It is found that the magnetization depends on the average cluster size and concentration but, in all cases, is significantly less than that of a thin cobalt film even at magnetic fields of 4 T. Various mechanisms for this behaviour are discussed including the possibility that there are significant cluster - matrix and cluster - cluster exchange interactions. Anomalously large values of the ratio of the average orbital to spin moment, measured for some samples, may be a signature of antiferromagnetic exchange coupling. Antiferromagnetic coupling of clusters would necessitate a stronger interaction than can be predicted with RKKY theory and the possibility that this coupling is a `superexchange' mechanism is discussed.

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The ratio of the orbital to the spin magnetic moment has been measured for 2000-atom cobalt clusters in a copper matrix using magnetic x-ray dichroism. Averaging the result over all magnetic fields gives a value of 0.19(5) which is 2.2(6) times larger than the bulk value for face-centred-cubic cobalt. The orbital-to-spin ratio, at the cluster surface, is calculated from the measured high-field value of 0.16(2), using a simple surface model. The resulting value is almost twice that found for a plane surface and various possible reasons for this are given including changes in the nature of the cobalt-copper interface and an increase in the local density of states at the cluster surface (interface). The variation of the orbital-to-spin magnetization with the applied field provides the first evidence of changes in the surface magnetization of clusters in an external field.

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A novel technique for producing intense, parallel beams of mass-selected nanocrystals from a magnetron cluster source has been used to study the low-energy impact of size-selected gold nanocrystals, 2.0-8 nm diameter, on 2 nm thick carbon films. The measurements of the sizes using transmission electron microscopy show that it is possible to deposit, intact, nanocrystals with a very narrow size range ( D(FWHM)/D = 0.06) as long as the impact energy is below 40 eV. The subsequent surface motion of the nanocrystals after impact (with the substrate) results in cluster-cluster collisions, which for large clusters (>4 nm) produces aggregations but for small clusters (<3.5 nm) results in complete fusion and reformation into larger aggregated clusters with approximate spherical symmetry. However, when the energy is reduced to 10 eV, clusters of a size of around 5 nm form some localized ordered arrays rather than random aggregates. This implies that the clusters forming these arrays can undergo local spatial rearrangement during their formation in order to reduce the total energy. The clear absence of sintering other than that expected from random impact collisions requires either a repulsive cluster-cluster interaction or surface passivation of the particles. These issues are discussed in this article.