Interaction effects in dilute cluster-assembled magnetic nanostructures

We have prepared dilute nanostructured magnetic samples by codeposition of preformed cobalt clusters with a narrow size distribution around 40 atoms per cluster in silver matrices. Magnetoresistance measurements are used to derive information about the magnetic structure of the samples. Effects of cluster size distribution or anisotropy can be neglected in our samples. Deviations from simple Langevin-type magnetization are observed as a function of temperature and identified as due to intercluster interactions. Pairwise magnetostatic and indirect exchange interactions as well as the model of interacting superparamagnets are found to be not adequate to explain the observed temperature dependences. We propose an interpretation of a correlated spin glass, which shows that for small clusters, spin glass behavior can be observed even at high dilutions.