Nanocrystallized thin films of transition metal obtained by low energy cluster beam deposition

Nanocrystallized thin films of iron obtained from low energy cluster beam deposition (LECBD) have been produced by adjusting different source parameters and elaboration conditions in an attempt to control the size distribution of supported clusters, In all cases, the films exhibit a nanostructure characteristic of the LECBD technique (mean 5 nm-diameter of the particles) and only the percentage of oxide in the form of a thin skin surrounding the particles changes. The specific magnetic behavior of these weakly correlated entities is described by ferromagnetic resonance and magnetization measurements. We showed that the magnetic properties of these cluster assembled materials can be interpreted using the random anisotropy model with a scale law related to grain size.

The increasing interest of the scientific community in the magnetism of “small particles”¹ is mainly driven by their potential application as high density storage devices^2,3. From the Stoner criterion, the size and the first-neighbor number reduction would be favourable to ferromagnetism in transition metals. In this context, metallic clusters are interesting for their large $\text{surface}\text{volume}$ ratio but they are very oxygen reactive. By varying the elaboration conditions (vacuum quality, substrate temperature, size distribution of the incident clusters…) we obtained nanostructured iron-films by Low Energy Cluster Beam Deposition (LECBD). This paper is mainly concerned with the structural and magnetic properties of these original cluster assembled materials.

The laser vaporization source operating in the laboratory of Lyon⁴ allows the production of an intense cluster beam in a wide range of size (from few to a thousand atoms per cluster), to synthetize thin films of any kind of materials even the most refractory. Roughly, the plasma created at the laser impact on the iron rod, is thermalized by a synchronized high pressure (5 bars) helium pulse. The nascent clusters are then rapidly quenched during the following isentropic expansion into the vacuum (10⁻⁴ torr) chamber containing the source. The size distributions of clusters are checked by time-of-flight mass spectrometry before deposition. The deposit of neutral clusters at low energy on various substrates are subsequently realized after the retraction of the time-of-flight acceleration grids. This type of source produces cold clusters and we succeeded previously in the stabilisation of very low size fullerenes⁴. In particular we deposited C₂₀-clusters without fragmentation on the substrate in order to produce original diamondlike structures⁴.