Morphology and magnetic properties of size-selected Ni nanoparticle films

We present the results of a study on the morphology and magnetic properties of size-selected Ni nanoparticles films grown on Si/SiOx substrates. The films were produced by deposition of preformed Ni nanoparticles, using a gas aggregation nanocluster source and an electric quadrupole mass filter. The diameter d of the produced particles ranged between 3 and 10 nm. The morphology of the films, with average thickness t varying from t = 0.5 up to t = 7nm, was studied with Atomic Force Microscopy and Scanning Electron Microscopy, combining in this way information about height and lateral topography. We observed the presence of some small aggregates made of 2 o 3 particles at the early stage of film formation, probably due to some degree of cluster diffusion on the substrate, and particle average flattening. For increasing values of t, large agglomerates are formed in the films, resulting in a porous structure. Information about the magnetic properties was obtained with Field Cooled-Zero Field Cooled (FC/ZFC) magnetization curves. We observed a reversibility-irreversibility transition at temperatures 70 K < TI < 80 K, and a significant deviation from the superparamagnetic behavior at T>TI, even for the lowest coverage studied (t = 2 nm for ZFC/FC measurements, <d> = 5.5 nm). A comparison with Montecarlo simulations of the FC/ZFC curves reveals the concurrence between inter-particle exchange interaction and single particle random anisotropy.