Magnetic behaviour of thin films produced by depositing pre-formed Fe and Co nanoclusters

We have studied the magnetic behaviour of ultra-thin films produced by depositing pre-formed gas phase Fe and Co nanoclusters, containing typically a few hundred atoms, in ultra-high vacuum (UHV) conditions. Two types of sample were prepared, that is, clusters embedded at very low volume fractions (≤2%) within Ag matrices to obtain the isolated particle properties, and pure cluster-assembled films with no matrix that were transferred without a capping layer into the magnetometer in UHV. The dilute assemblies both display ideal superparamagnetism, with an H/T scaling of the magnetization curves, above 50 K for Fe clusters and 150 K for Co clusters. Fitting the magnetization data above these temperatures to Langevin functions enabled an accurate determination of the size distribution and gave a median size of 3 nm for the Fe and 2.8 nm for the Co clusters. At 2 K the magnetic isotherms are characteristic of assemblies of blocked particles with a uniaxial anisotropy axis and anisotropy constants of 2.6×105 and 7.7×105 J m-3 for Fe and Co particles respectively. The magnetic behaviour of the pure cluster films was analysed using a random anisotropy model including parameters determined from the isolated cluster films. The approach to saturation of the Fe and Co cluster films indicates that the ground state is a correlated super-spin glass over the temperature range 10-300 K in both cases

Enhanced capability in a gas aggregation source for magnetic nanoparticles

We describe the characterization of a high-temperature (2000 K) thermal gas aggregation source that is ultrahigh vacuum compatible and can cleanly deposit transition metal clusters with partial pressures of contaminants in the 10(-11) mbar range allowing codeposition with highly reactive matrices. In particular, we investigate the effect of varying (i) the bath gas pressure and composition on the size distribution and flux of clusters produced and (ii) the position of the crucible within the source. The mass spectra of Fe clusters produced, recorded using a quadrupole filter, show that changing the operating conditions and configuration of the source allow a wide range of cluster sizes-3000-320 000 amu (similar to 50-6000 atoms for Fe or Co) to be produced. We demonstrate the cleanliness of the source by producing uncontaminated Fe clusters in rare-earth matrices

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

Structure and magnetic properties of Fe/Fe oxide clusters

Fe clusters have been synthesised in ultra-high-vacuum chamber using a gas-stabilized cluster aggregation method that ensures good control of the cluster size and naturally oxidized in order to obtain Fe/Fe oxide core-shell nanoparticles. The morphology of an individual nanoparticle, as revealed by transmission electron microscopy, consists of a Fe core of an average diameter of 4.4 nm surrounded by an oxide shell of uniform thickness of about 1.2 nm in average. The nanoparticles may be assimilated with a ferro-/antiferromagnetic (FM/AF) system. The morpho-structural features have been correlated with magnetic measurements on the core-shell nanoparticles. A significant exchange bias effect has been measured, when the sample was field-cooled under an applied field of 3 T. As the morphology of core-shell nanoclusters is much more complicated than in FM/AF bilayers of regular thickness due to the particular geometry of the coronal AF layer, the shape and surface anisotropy have to be taken into account for a correct interpretation of the magnetic data

Extended x-ray absorption fine structure studies of the atomic structure of nanoparticles in different metallic matrices

It has been appreciated for some time that the novel properties of particles in the size range 1–10 nm are potentially exploitable in a range of applications. In order to ultimately produce commercial devices containing nanosized particles, it is necessary to develop controllable means of incorporating them into macroscopic samples. One way of doing this is to embed the nanoparticles in a matrix of a different material, by co-deposition for example, to form a nanocomposite film. The atomic structure of the embedded particles can be strongly influenced by the matrix. Since some of the key properties of materials, including magnetism, strongly depend on atomic structure, the ability to determine atomic structure in embedded nanoparticles is very important. This review focuses on nanoparticles, in particular magnetic nanoparticles, embedded in different metal matrices. Extended x-ray absorption fine structure (EXAFS) provides an excellent means of probing atomic structure in nanocomposite materials, and an overview of this technique is given. Its application in probing catalytic metal clusters is described briefly, before giving an account of the use of EXAFS in determining atomic structure in magnetic nanocomposite films. In particular, we focus on cluster-assembled films comprised of Fe and Co nanosized particles embedded in various metal matrices, and show how the crystal structure of the particles can be changed by appropriate choice of the matrix material. The work discussed here demonstrates that combining the results of structural and magnetic measurements, as well as theoretical calculations, can play a significant part in tailoring the properties of new magnetic cluster-assembled materials

Structure and magnetism in Cr-embedded Co nanoparticles

We present the results of an investigation into the atomic structure and magnetism of 2 nm diameter Co nanoparticles embedded in an antiferromagnetic Cr matrix. The nanocomposite films used in this study were prepared by co-deposition directly from the gas phase, using a gas aggregation source for the Co nanoparticles and a molecular beam epitaxy (MBE) source for the Cr matrix material. Co K and Cr K edge extended x-ray absorption fine structure (EXAFS) experiments were performed in order to investigate atomic structure in the embedded nanoparticles and matrix respectively, while magnetism was investigated by means of a vibrating sample magnetometer. The atomic structure type of the Co nanoparticles is the same as that of the Cr matrix (bcc) although with a degree of disorder. The net Co moment per atom in the Co/Cr nanocomposite films is significantly reduced from the value for bulk Co, and decreases as the proportion of Co nanoparticles in the film is decreased; for the sample with the most dilute concentration of Co nanoparticles (4.9% by volume), the net Co moment was 0.25 μ B/atom. After field cooling to below 30 K all samples showed an exchange bias, which was largest for the most dilute sample. Both the structural and magnetic results point towards a degree of alloying at the nanoparticle/matrix interface, leading to a core/shell structure in the embedded nanoparticles consisting of an antiferromagnetic CoCr alloy shell surrounding a reduced ferromagnetic Co core

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 C-60 supports, were studied by x-ray photoemission spectroscopy, x-ray absorption spectroscopy, and magnetic linear dichroism. Clusters deposited onto a C-60 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 C-60 is seen. The magnetic dichroism signal of the exposed clusters increases sharply with the film thickness because of the increased cluster interactions

Structure and magnetic properties of Fe nanoparticles embedded in a Cr matrix

The structure of 2 nm diameter (340 atoms) Fe nanoparticles embedded in a Cr matrix was determined using X-ray Absorption Fine Structure (EXAFS) and the magnetic properties studied by Superconducting Quantum Interference Device (SQUID) magnetometry. The thin films were produced by the co-deposition of pre-formed gas-phase Fe clusters synthesised by a gas aggregation source with an atomic vapour of Cr produced by an MBE source. The behaviour was studied as a function of Fe nanoparticle volume fraction in the range 5-20% and was compared to previous results on ferromagnetic nanoparticles in antiferromagnetic matrices. EXAFS showed that the atomic structure in the Cr-embedded Fe nanoparticles is the same as the bulk bcc structure. Whereas alloying between the nanoparticles and matrix material has previously been shown to be very pronounced for Co nanoparticles in antiferromagnetic Mn, it was found that any alloying between Fe nanoparticles and Cr matrix material is limited. For dilute samples of Fe nanoparticles in Cr the measured saturation magnetisation (M) was 1μ/Fe atom, which is significantly less than the bulk M value of 2.22μ/Fe atom indicating that the surface of Fe nanoparticles is either antiferromagnetic or non-magnetic. An increase in the volume fraction produces an increase in the value of M and at a volume fraction of 20%, M exceeds the value of bulk Fe showing that some Cr spins provide a ferromagnetic contribution. After field cooling below 30 K, all films show Exchange Bias (EB) and an increase of coercivity, which are both much larger for the most concentrated sample. The Cr spins at the surface of the Fe particles play a key role in determining the overall magnetic behaviour

A novel approach towards the production of luminescent silicon nanoparticles: sputtering, gas aggregation and co-deposition with H2O

Silicon clusters were produced by sputtering of a p-doped Si target and aggregation of the Si atoms in an argon gas atmosphere. The clusters were deposited in ultra high vacuum onto either (i) carbon transmission electron microscope (TEM) grids or (ii) a liquid nitrogen cooled finger on which a thick layer of ice was co-deposited during the exposure to the cluster beam. The ice layer containing the clusters was melted to form a liquid sample which showed luminescence peaking at 421 nm when excited at 307.5 nm. The luminescence is attributed to electron-hole recombination in oxygen deficient defects in the Si–SiO2 interface region. TEM images of the nanoparticles deposited on the carbon grids show spherical particles with diameters ranging from 4 to 50 nm, flake-like structures or nanotube-like shapes. Grids with higher deposited densities reveal clusters that are agglomerated into chains, TEM images of the dried liquid sample show a network of fibres indicating that growth into fibres is further promoted when the clusters gain mobility in the melted ice

Magnetic force microscope tip-induced remagnetization of CoPt nanodisks with perpendicular anisotropy

We report on the results of a magnetic force microscopy investigation of remagnetization processes in arrays of CoPt nanodisks with diameters of 35 and 200 nm and a thickness of 9.8 nm fabricated by e-beam lithography and ion etching. The controllable magnetization reversal of individual CoPt nanodisks by the magnetic force microscope (MFM) tip-induced magnetic field was demonstrated. We observed experimentally two essentially different processes of tip-induced remagnetization. Magnetization reversal of 200 nm disks was observed when the probe moved across the particle while in case of 35 nm nanodisks one-touch remagnetization was realized. Micromagnetic modeling based on the Landau-Lifshitz-Gilbert (LLG) equation demonstrated that the tip-induced magnetization reversal occurs through the essentially inhomogeneous states. Computer simulations confirmed that in case of 200 nm disks the mechanism of embryo nucleation with reversed magnetization and further dynamic propagation following the probe moving across the particle was realized. On the other hand one-touch remagnetization of 35 nm disks occurs through the inhomogeneous vortexlike state. Micromagnetic LLG simulations showed that magnetization reversal in an inhomogeneous MFM probe field has a lower energy barrier in comparison with the mechanism of coherent rotation, which takes place in a homogeneous external magnetic field