Magnetic behavior of nanostructured films assembled from preformed Fe clusters embedded in Ag

We have observed the magnetic behavior of nanostructured magnetic materials produced by co-depositing pre-formed Fe nanoclusters from a gas aggregation source and Ag vapor from a Knudsen cell. Films containing particle volume fractions from <1% (isolated clusters) to 100% (pure clusters with no matrix) have been prepared in UHV conditions and, after capping with a thin Ag layer for removal from the deposition chamber, have been studied at temperatures in the range 2-300 K by magnetometry and field-cooled/zero-field-cooled measurements. The results have been interpreted with the help of a Monte Carlo simulation of the cluster-assembled films that includes exchange and dipolar interactions. At elevated temperatures (>50 K) the lowest concentration films display ideal superparamagnetism with an H/T scaling of the magnetization. With increasing cluster density the films pass through an interacting superparamagnetic phase in which the effective blocking temperature and the initial susceptibility above the blocking temperature increase, in contrast to predictions of nanoparticle systems interacting via dipolar forces only. It is concluded that the exchange interaction becomes important even at volume fractions of 10% as clusters that are in contact behave as a single larger particle. This is confirmed by the theoretical model. At high volume fractions, well above the percolation threshold, the cluster assemblies form correlated superspin glasses (CSSG's). At 2 K, the magnetization curves in all films, irrespective of cluster concentration, have a remanence of approximate to50% and an approach to saturation that is characteristic of randomly oriented, particles with a uniaxial anisotropy, in agreement with the remanence. In the most dense Ag-capped films there appears to be a "freezing out" of the interparticle exchange interaction, which is attributed to temperature-dependent magnetoelastic stress induced by the capping layer. An uncapped 100% cluster film measured in UHV remains in the CSSG state at all temperatures and does not show the low-temperature decoupling of particles evident in the Ag-capped samples.