Magnetic behaviour of thin films produced by depositing pre-formed Fe and Co nanoclusters

We have studied the magnetic behaviour of ultra-thin films produced by depositing pre-formed gas phase Fe and Co nanoclusters, containing typically a few hundred atoms, in ultra-high vacuum (UHV) conditions. Two types of sample were prepared, that is, clusters embedded at very low volume fractions (⩽2%) within Ag matrices to obtain the isolated particle properties, and pure cluster-assembled films with no matrix that were transferred without a capping layer into the magnetometer in UHV. The dilute assemblies both display ideal superparamagnetism, with an H/T scaling of the magnetization curves, above 50 K for Fe clusters and 150 K for Co clusters. Fitting the magnetization data above these temperatures to Langevin functions enabled an accurate determination of the size distribution and gave a median size of 3 nm for the Fe and 2.8 nm for the Co clusters. At 2 K the magnetic isotherms are characteristic of assemblies of blocked particles with a uniaxial anisotropy axis and anisotropy constants of 2.6×10⁵ and 7.7×10⁵ J m^-3 for Fe and Co particles respectively. The magnetic behaviour of the pure cluster films was analysed using a random anisotropy model including parameters determined from the isolated cluster films. The approach to saturation of the Fe and Co cluster films indicates that the ground state is a correlated super-spin glass over the temperature range 10-300 K in both cases.