Preparation and magnetic characteristics of size-monodispersed Fe–Co alloy cluster assemblies

Size-monodispersed Fe70Co30 alloy clusters were obtained by a plasma-gas-condensation (PGC)-type cluster deposition method. The structural and magnetic characteristics of Fe70Co30 alloy cluster-assembled films were systematically studied. The as-obtained Fe70Co30 clusters have a body-centered-cubic (bcc) α-(Fe,Co) structure and their average sizes can be adjusted from 8 to 14 nm. Based upon the temperature dependence of magnetic coercivity, we estimated the effective magnetic anisotropy constant, Keff, and magnetic exchange length, Lex, as a function of initial mean cluster diameter, d, in the Fe70Co30 cluster-assembled films which were formed by soft landing of the clusters and have a low cluster-packing fraction of about 30%. The estimated Keff and Lex values depend strongly on the d values of the cluster-assembled films: Keff decreases while Lex increases with decreasing d of the Fe70Co30 clusters. For the Fe70Co30 cluster-assembled films with d = 14.3 nm, the Lex value is close to the initial mean cluster size, whereas for the films with d = 9.7 nm the Lex value is about two times of the initial mean cluster size.