Magnetic clusters Clusters constitute a finely divided state of solid matter, at the nanometer-scale, that can be used as a bench test for fundamental concepts in physics, like for instance the effect of quantum confinement on electron energy levels. Technological research is interested as well in nanometer-scale systems, in its never ending race for smaller and smaller devices for data communication, analysis and storage. However at this length scale many pitfalls arise, most often related to the fundamental physical properties of the systems. To tackle these problems, anything goes: Reducing size? Fine. Looking for new materials and processes? Sure! But why not also building up the devices from tiny, purposely crafted elemental bricks? This is where clusters can do well. And how to get clusters? The trick is simple; you just have to put your head in the clouds: Coming from the Atlantic, the wind soaked up with humidity, that is, water. Meeting the cliffs and mountains surrounding Grenoble it raises and, going over the ridges, it expands and cools. There, in the colder air, water vapour condenses to form clusters, Huh..., micro-droplets: the mist that gives the plateau its peacefulness and beauty.
Figure 1: Cobalt sputtering target (center). Grey foam of loosely packed clusters can be seen around the target.
In a similar way, we grow clusters by etching a metallic target with argon plasma and condensing the sputtered vapour in a cooled gas (figure 1). The size range for the obtained clusters is below ten nanometers, that is, less than 10 000 atoms.
Figure 2: Deposition chamber. Clusters are from the left. Right side: Entrance for the time of flight spectrometer (for size measurement). Far side: Microbalance for clusters flux measurement. Bottom: Magnetron sputtering gun for protective layer deposition.
Size of clusters and cluster beam intensity are done in-situ, on free-flying clusters, before they are deposited on a substrate (figure 2).
Figure 3: cobalt clusters - with 6 nm average diameter - deposited on silicon with controlled density (TEM image, 250 nm x 250 nm).
The deposited clusters density can be tuned (figure 3), which is important given the fact that their physical properties vary according to whether they are isolated or interacting due to proximity – or in close contact, as in a full layer.
Clusters can also be diluted in a metallic matrix (from 0.1 up to 100 %vol.) or covered any type of metal or oxide. Mix clusters, made of many elements, can also be obtained. We have full control of the size and properties of the clusters, especially their oxidation state.
Last update : 06/19 2009 (486)