Static and dynamic magnetic behaviour of iron nanoclusters on magnetic substrates

The static and dynamic magnetic behaviour of Fe nanoclusters with controlled sizes in the range 140-270 atoms (1.5-1.8 nm) deposited in situ from a gas aggregation source on magnetic vitrovac amorphous ribbons has been studied using synchrotron radiation. The static magnetization of the cluster films in the exchange field of the substrates was measured as a function of coverage using magnetic linear dichroism in the angular distribution (MLDAD) of the Fe 3p core level photoemission. The switching dynamics were studied on the nanosecond timescale by time-resolved spin-polarized photoemission. For a given particle size, the magnetization of the Fe cluster film increases with coverage and saturates at a coverage of about 0.4 cluster monolayers. Modelling the growth of the magnetization gives an effective exchange field at the interface of similar to20 T. Dense cluster films with several cluster layers have an MLDAD signal at saturation that is similar to5% higher than a molecular beam epitaxy (MBE)-grown film, indicating an enhanced spin moment even when clusters are in contact. Coating an exposed sub-monolayer cluster layer with Co increases the Fe MLDAD signal by 35%, indicating a substantially increased magnetic moment within the Fe clusters. At low coverages, below the percolation threshold, the switching dynamics of the sample remains the same as in the clean substrate. At around the percolation threshold, however, a significant acceleration of the magnetic reversal is observed with a fast component due to a reversal propagating through the cluster film. We show that, on average, each cluster switches in about 10 ns.