A large amount of our program is built around a custom made nanoparticle source: The Leicester University Mesoscopic Particle Source (LUMPS). With this source we can produce metallic nanoparticles of roughly a few hundred atoms in size that exhibit novel structural and magnetic properties. These clusters are not only fundamentally interesting but also have incredible potential as components in a new generation of nanoscale devices.

We are currently developing a more advanced cluster source to produce so called 'nano-onion' structures which are predicted to have a wide range of technological applications. On a more fundamental level we are probing the quantum nature of the vacuum itself in a project on the Casimir force.

Magnetic nanoclusters

Nanoscale clusters of magnetic materials such as Fe or Co have recently attracted much attention, not only due to a fundamental interest in mesoscopic physics but also due to their potential use in commercial devices.

The novel electronic and magnetic properties of nanoclusters arises in part from the high proportion of surface atoms with a reduced coordination; this causes a narrowing of the valence bands and an increase in the density of states at the Fermi level. Both these effects enhance the spin magnetic moment towards the high spin atomic limit. In addition, the orbital magnetic moment is increased due to spin-orbit coupling and the reduced symmetry in a cluster, which leads to a less effective quenching of the orbital magnetism by the crystal field. Further novel behaviour arises from the quantum size effect and modified valence electron screening behaviour.

Cluster-assembled materials, in which clusters are embedded in a matrix of another material, can be prepared by co-deposition from a molecular beam of the matrix material and an intense size-selected cluster beam. This technique allows independent control of the cluster size and volume filling fraction, which leads to significant flexibility in the production of "new" magnetic materials.

Cluster Deposition Technology

At Leicester, we have designed and built a gas aggregation cluster source (image: left) which produces high fluxes of magnetic nanoclusters such as Fe or Co under UHV conditions. Size selection is achieved via an axially mounted quadrupole filter, also built here at Leicester.

Oxford Applied Research have based their commercially available filter on our design. The source is readily transportable thus enabling in situ work on clusters at other laboratories.

Measurements

Extensive use is made of x-ray techniques at synchrotron sources e.g. ESRF at Grenoble, DIAMOND at RAL in order to probe the magnetic behaviour of the cluster assembled materials. XMCD (x-ray magnetic circular dichroism) is a particularly valuable technique for assessing magnetic materials as it is element specific and also allows determination on a per atom basis of both the orbital and spin magnetic moments. In situ XMCD measurements performed at Grenoble on dilute assemblies of size-selected Fe clusters on HOPG (highly oriented pyrolytic graphite) substrates show that the orbital magnetic moment approaches three times that of bulk Fe while the spin moment is enhanced by 10%. Coating the clusters with Co further enhances the spin moment by 10%, pointing the way towards producing materials with record saturisation magnetisation.

Core-Shell clusters

We are involved in a project to manufacture and study the behaviour of magnetic nanoparticles comprising a core and multiple-shells. These so-called nano-onions have far ranging applications in magnetic recording, quantum devices and medical nanotechnology.

Casimir Force

We are co-ordinating an EPSRC funded project to study one of the most fundamental forces in the universe: The Casimir force arises directly from the quantum zero point energy of the vacuum. The ultimate goal of the project is to use the Casimir force to produce a contactless transmission in a nano-machine.

Image (left): AFM chamber.

The quantum zero-point energy density of empty space is infinite. To understand the Universe in a classical framework the zero-point energy is simply rescaled to zero, however the vacuum energy imposes itself in subtle ways and has directly measurable consequences such as the Casimir force.

In 1948 Hendrik Casimir showed that two reflecting surfaces in the vacuum modify the zero-point vacuum energy density between the surfaces relative to the unperturbed vacuum. This difference in energy density varies with the separation between the mirrors and thus constitutes a force between them which becomes measurable at sub-micron scales.