M.J. Maher

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.

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.

Magnetism of exposed and Co-capped Fe nanoparticles

Abstract The effect of capping a dilute assembly of nanoscale mass-selected Fe clusters with a Co thin film has been studied using X-ray magnetic circular dichroism (XMCD). The clusters, containing around 400 atoms, were deposited in situ from a gas-aggregation source onto highly oriented pyrolytic graphite. The exposed clusters possess magnetic moments that are enhanced compared to the bulk, by around 4% for m spin and around 75% for m orb . In addition, a surface core level shifted component is observed in the L 3,2 XMCD spectrum. Upon adding the Co layer, the surface component disappears, m orb is decreased for the Fe clusters, and m spin increases. The exposed clusters are magnetically isotropic but a strong in-plane anisotropy is observed after depositing the Co overlayer. We attribute this to the shape of the Co islands in which the Fe clusters are embedded.

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.

Magnetism in exposed and coated nanoclusters studied by dichroism in X-ray absorption and photoemission

We have studied the evolution of the magnetic behaviour of size-selected Fe nanoclusters from isolated particles adsorbed on a surface 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. Isolated and exposed Fe clusters deposited on highly oriented pyrolitic graphite substrates exhibit orbital and spin magnetic moments that are, respectively, up to 250% and 10% higher than the values measured in thick Fe films. The moments decay towards the bulk value with increasing cluster size or with increasing particle density on the surface. The total moment in the supported clusters is not as high as observed in free clusters of the same size. The individual particles appear to be magnetically isotropic but a strong in-plane coherent anisotropy develops as the coverage is increased to a complete cluster monolayer. Coating the deposited Fe clusters with a Co film substantially increases the spin moment on the Fe.

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.

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.