Structural and magnetic evolution in self-assembling process of nanometer-sized Co clusters

We have studied the structure and magnetic properties of monodispersed Co cluster assemblies with mean diameter, d = 6-13 nm. In these Co cluster assemblies, an fcc phase is predominant, whereas the volume fraction of an hcp phase increases with increasing d from 6 to 13 nm. At a thin deposition stage (average thickness, t = 1 nm), Co clusters are nearly isolated. At 300 K, they are superparamagnetic for d = 6 nm, while ferromagnetic for d = 8.5 and 13 nm. With increasing t, the superparamagnetic to ferromagnetic transition is detected for d = 6 nm. For d = 6-13 nm, the magnetic coercivity, H_c, rapidly increases at t = 2-4 nm, near or below the geometrical and electrical percolation thresholds. However, H_c is almost independent of t up to t = 100 nm.