Morphological and magnetic characteristics of monodispersed Co-cluster assemblies

Co-cluster-assembled films have been prepared using a size-controllable clu ster beam deposition system, by which monodispersed Co clusters with a mean diameter, d=6-13 nm are available. Their morphology and magnetic propertie s have been studied by scanning electron microscopy (SEM), small-angle x-ra y scattering (SAXS) and magnetization measurements. The SEM images show tha t the film has a porous structure consisting of fine grains without a colum nar texture and its density is about 25% of the bulk Co. The SAXS measureme nts indicate that monodispersivity of the incident clusters is maintained t hrough their assembling process only for d=13 nm. All the specimens exhibit ferromagnetic behavior at room temperature and the magnetic coercive field H-c rapidly increases with decreasing temperature: H-c=168 kA/m (2.1 kOe) at 5 K. Such an enhancement in H-c is ascribed to the exchange anisotropy w hich arises from the antiferromagnetic Co-oxide layers covering the Co clus ters, and to the assemblies of single-domain ferromagnetic clusters with th e structure modification and magnetic interaction among them. The monotonic increase in H-c at 300 K with increasing d is simply understood in terms o f the single-domain particle theory. (C) 1999 American Institute of Physics . [S0021-8979(99)04722-2].