Stuart Thornton

Casimir force between a metal and a semimetal

We present here measurements of the Casimir force gradient in the 60–300 nm range using a commercial Atomic Force Microscope operating in Ultra High Vacuum (UHV). The measurements were carried out in the sphere-plate geometry between a Au sphere and plates consisting of two different classes of material, that is a metal (Au) and a semimetal (HOPG). The variation in the optical properties of the materials produces clearly observed differences in the Casimir force as predicted by calculations based on the quantum theory of optical networks and the Lifshitz theory.

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 ~20 T. Dense cluster films with several cluster layers have an MLDAD signal at saturation that is ~5% 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.

Measurement of the Casimir effect under ultrahigh vacuum: Calibration method

In this article, the authors present a strategy to measure the Casimir effect with an atomic force microscopy in an ultrahigh vacuum system. The key parameters including the absolute distance, the contact potential difference, and the calibration factor of the probe are determined by electrostatic interaction without contact. The strategy has been developed with the main purpose of performing a reliable relative measurement, that is, comparison of the Casimir force between different surfaces. As an example of the method, the authors estimate the accuracy and the precision of measurements performed on a Au sample.

Control of gas phase nanoparticle shape and its effect on MRI relaxivity

We have used a sputtering gas aggregation source to produce Fe@FeO nanoparticles with different shapes, by annealing them at different temperatures in the gas phase. Without annealing, the most common shape found for the nanoparticles is cubic but annealing the nanoparticles at 1129 °C transforms the cubes into cuboctahedra. Measurements of the MRI relaxivity show that the cubic nanoparticles have a higher performance by a factor of two, which is attributed to a higher saturation magnetization for this shape. This indicates that the shape-control enabled by gas-phase synthesis is important for obtaining optimal performance in applications.

Magnetism in Nanoscale Fe Clusters Studied by Dichroism in X-ray Absorption and Photoemission

Dichroism has been used to study magnetism in mass-selected Fe nanoclusters adsorbed on surfaces in UHV as a function of particle size and density. XMCD shows enhanced orbital, m L and spin, m S magnetic moments in exposed 300-atom particles adsorbed on graphite that decrease towards the bulk value with increasing particle size or density on the surface. Coating exposed 400-atom clusters with Co in situ reduces m L but increases m S yielding a total moment 13% higher than the bulk. The isolated clusters show a cubic anisotropy with a blocking temperature below 10K and an anisotropy constant about 10 times the bulk value. Dense cluster films develop an in-plane anisotropy and remanence at higher temperatures.