S.C. Thornton

The construction of a gas aggregation source for the preparation of size-selected nanoscale transition metal clusters

The design and operation of a gas aggregation source is described. The source combines the attributes of high-temperature operation (enabling preparation of transition metal clusters), mass selection, ultrahigh vacuum compatibility, and transportability. This makes it ideally suited to in situ studies such as scanning tunneling microscope or synchrotron radiation experiments. Data are presented to illustrate the performance of the source; recent results obtained in synchrotron radiation studies are highlighted.

The construction of a gas aggregation source for the preparation of mass-selected ultrasmall metal particles

The design and operation of a high temperature gas aggregation source, capable of in situ deposition of mass-selected atomic clusters of transition metals onto a substrate in an ultrahigh vacuum chamber, is described. Mass-selection is achieved by an ultrahigh mass quadrupole filter operating at masses up to 3×104 amu.

Enhancements in magnetic moments of exposed and Co-coated Fe nanoclusters as a function of cluster size

Abstract X-ray magnetic circular dichroism has been used to measure the magnetic moments of exposed and Co-coated Fe nanoclusters in the size range 180–690 atoms. The clusters were deposited in situ onto highly oriented pyrolytic graphite substrates using a ultra-high vacuum-compatible gas aggregation cluster source. Enhancements in both the spin and orbital magnetic moments, m S and m L , respectively, are observed for the exposed clusters. As the cluster size is reduced, m S gradually increases from the bulk value, and for the 180-atom clusters, it is enhanced by around 10%. The degree of enhancement in m L also increases as the cluster size decreases, although m L starts to decrease again for the 180-atom clusters; the maximum value observed for m L corresponds to an increase of ∼75% relative to bulk Fe. Coating the Fe clusters with Co further raises their magnetic moments, as a result of the increases in m S across the size range. For the 180-atom clusters, m S is 20% greater than in the bulk.

Realizing high magnetic moments in fcc Fe nanoparticles through atomic structure stretch.

We describe the realization of a high moment state in fcc Fe nanoparticles through a controlled change in their atomic structure. Embedding Fe nanoparticles in a Cu(1-x)Au(x) matrix causes their atomic structure to switch from bcc to fcc. Extended x-ray absorption fine structure (EXAFS) measurements show that the structure in both the matrix and the Fe nanoparticles expands as the amount of Au in the matrix is increased, with the data indicating a tetragonal stretch in the Fe nanoparticles. The samples were prepared directly from the gas phase by co-deposition, using a gas aggregation source and MBE-type sources respectively for the nanoparticle and matrix materials. The structure change in the Fe nanoparticles is accompanied by a sharp increase in atomic magnetic moment, ultimately to values of ~2.5 ± 0.3 μ(B)/atom .

Magnetism in Fe nanoclusters: From isolated particles to nanostructured materials

We have studied the evolution of the magnetic behaviour of Fe nanoclusters from isolated particles adsorbed on a surface and exposed in UHV to dense interacting assemblies. The clusters were produced by a UHV-compatible gas aggregation source and were studied in situ by synchrotron radiation techniques based on dichroism and ex situ after transfer into a UHV-capable vibrating sample magnetometer. Isolated and exposed 250-atom Fe clusters deposited on HOPG substrates exhibit enhanced orbital and spin magnetic moments that decay towards the bulk value with increasing size or with increasing particle density on the surface. The individual particles appear to be magnetically isotropic but a strong in-plane anisotropy develops as the coverage is increased to a complete cluster monolayer. At low temperature, thick cluster films have an asperomagnetic configuration that develops into a two-dimensional correlated super spin glass with increasing temperature.

High-moment films produced by assembling nanoclusters

We have used X-ray magnetic circular dichroism to determine the orbital and spin moments in exposed and embedded mass-selected Fe nanoclusters (250-550 atoms) deposited in situ using a gas aggregation source. The isolated and exposed particles have a total moment that increases with decreasing cluster size and reaches 2.4 /spl mu//sub B/ in 250-atom clusters. Although this is enhanced relative to the bulk, it is smaller than found in free clusters of the same size. Coating the exposed clusters with Co, however, increases the spin moment to 2.6 /spl mu//sub B/. The prospects for producing high-moment materials by cluster assembly are discussed.

Magnetic behavior of nanostructured Fe films measured by magnetic dichroism

The magnetic properties of Fe nanostructured films have been studied using magnetic linear and circular dichroism in x-ray photoemission spectroscopy. The samples were prepared by the deposition of nanoscale Fe clusters, size 1–4 nm, onto thin Cu films. The linear dichroism, which is used to measure the in-plane magnetization, increases with increasing film thickness, with a sharp increase between 1 and 1.5 ML coverage. The circular dichroism, which measures the out-of-plane magnetization, is zero within the experimental error at all thicknesses studied. Capping an Fe film with an ultrathin Pd layer results in a factor of 3 decrease of the linear dichroism response.

Synchrotron radiation study of mesoscopic manganese particles

Abstract We present a synchrotron radiation photoemission study of supported mesoscopic manganese particles deposited in situ from a gas aggregation source. Investigations have been carried out on both mass-filtered and unfiltered particle beams. With the source conditions used the unfiltered beam shows a log-normal distribution of particle sizes with the peak at 2.5 nm. Mass selection was achieved by a quadrupole filter set to pass particles of 3.8 × 10 4 amu, i.e. at the peak of the distribution. Exposed Mn clusters deposited on highly oriented pyrolytic graphite (HOPG) showed significant changes in the photoemission line shape of the 3s core level relative to bulk Mn, which is interpreted as an increase in the atomic magnetic moment. Unfiltered Mn clusters were also incorporated into a continuous Vanadium matrix to make a nano-scale granular film revealing a satellite structure in the photoemission spectrum of the V 3s core level which we ascribe to a magnetic polarisation of the V atoms in the vicinity of the Mn clusters.

MAGNETIC BEHAVIOUR OF EXPOSED AND EMBEDDED NANOCLUSTERS

Abstract—Cluster-assembled films made by depositing nanoscale Fe clusters from a UHV-compat-ible gas aggregation source have been studied in situ using STM, X-ray dichroism and magnetometry.We follow the evolution of the properties of the films supported on substrates and exposed in UHV,as a function of cluster size and coverage. The STM images show that there is no significant diffusionof the Fe clusters deposited onto Si(111) surfaces at room temperature and that dense films of clustersdo not coalesce but maintain their granular nature, X-ray magnetic circular dichroism measurmentsshow that isolated 300-atom clusters adsorbed on graphite have an orbital moment per atom that isdouble the bulk value and a spin moment enhanced by 4% giving a total moment 10% greater thanthe bulk. Both moments tend towards the bulk value as either the cluster size or the density of clusterson the surface increases. The isolated clusters are superparamagnetic above 10K but the films havean increasing magnetic remanence at higher temperatures as the cluster density is increased. Thickcluster films have a strong in-plane anisotropy. Films of Fe clusters embedded in Cu and Ag show agiant magneto-resistance that peaks sharply with volume fraction.

X-ray photoemission and absorption spectroscopy of supported nanoscale iron clusters

Fe particles with sizes in the range 1–5 nm, formed by a gas-aggregation method and deposited onto graphite and C60 supports, were studied by x-ray photoemission spectroscopy, x-ray absorption spectroscopy, and magnetic linear dichroism. Clusters deposited onto a C60 coated graphite substrate become embedded within the fullerene film, and have an increased resistance to oxidation compared to exposed clusters supported on a graphite surface. No evidence for hybridization between the electronic states of Fe and C60 is seen. The magnetic dichroism signal of the exposed clusters increases sharply with the film thickness because of the increased cluster interactions.