No abstract

We have observed the magnetic behavior of nanostructured magnetic materials produced by co-depositing pre-formed Fe nanoclusters from a gas aggregation source and Ag vapor from a Knudsen cell. Films containing particle volume fractions from Ͻ1% ͑isolated clusters͒ to 100% ͑pure clusters with no matrix͒ have been prepared in UHV conditions and, after capping with a thin Ag layer for removal from the deposition chamber, have been studied at temperatures in the range 2-300 K by magnetometry and field-cooled/zero-field-cooled measurements. The results have been interpreted with the help of a Monte Carlo simulation of the clusterassembled films that includes exchange and dipolar interactions. At elevated temperatures (Ͼ50 K) the lowest concentration films display ideal superparamagnetism with an H/T scaling of the magnetization. With increasing cluster density the films pass through an interacting superparamagnetic phase in which the effective blocking temperature and the initial susceptibility above the blocking temperature increase, in contrast to predictions of nanoparticle systems interacting via dipolar forces only. It is concluded that the exchange interaction becomes important even at volume fractions of 10% as clusters that are in contact behave as a single larger particle. This is confirmed by the theoretical model. At high volume fractions, well above the percolation threshold, the cluster assemblies form correlated superspin glasses ͑CSSG's͒. At 2 K, the magnetization curves in all films, irrespective of cluster concentration, have a remanence of Ϸ50% and an approach to saturation that is characteristic of randomly oriented, particles with a uniaxial anisotropy, in agreement with the remanence. In the most dense Ag-capped films there appears to be a ''freezing out'' of the interparticle exchange interaction, which is attributed to temperature-dependent magnetoelastic stress induced by the capping layer. An uncapped 100% cluster film measured in UHV remains in the CSSG state at all temperatures and does not show the low-temperature decoupling of particles evident in the Ag-capped samples.

We demonstrate here a controllable variation in the Casimir force. Changes in the force of up to 20% at separations of ~100 nm between Au and AgInSbTe (AIST) surfaces were achieved upon crystallization of an amorphous sample of AIST. This material is well known for its structural transformation, which produces a significant change in the optical properties and is exploited in optical data storage systems. The finding paves the way to the control of forces in nanosystems, such as micro-or nanoswitches by stimulating the phase change transition via localized heat sources.Pacs numbers: 78.68.+m, 03.70.+k, 85.85.+j, 12.20.Fv 2 Casimir forces [1][2][3][4][5][6][7][8] arise between two surfaces due to the quantum zero-point energy of the electromagnetic field. The surfaces restrict the allowed wavelengths and thus the number of field modes within the cavity, which locally depresses the zero point energy of the electromagnetic field. The reduction depends on the separation between the plates thus there is a force between them, which for normal materials is always attractive [1].The zero point energy manifests itself as quantum fluctuations, which in the small separation limit give rise to the familiar van der Waals force. The original calculation of the Casimir force assumed two parallel plates with an infinite conductivity [1]. This was later modified to include the dielectric properties of real materials and the intervening medium [2,3], providing the first glimpse of possible methods to control the magnitude and even the direction of the force. This finding has motivated our attempts to manipulate the dielectric properties of a material and hence generate force contrast [9][10][11]. A particularly exciting possibility is to produce a 'switchable' force by employing materials whose optical properties can be changed in situ in response to a simple stimulus [9,10].So far the only significant contrast that has been demonstrated is only between different materials [11]. To obtain a large Casimir force contrast for a single material requires a large modification of its dielectric response, which has not been achieved in materials used up to now.Here we demonstrate that phase change materials (PCMs) [12][13][14][15][16][17][18][19][20][21], which are renowned to switch reporducibly between an amorphous and a crystalline phase, are very promising candidates to achieve a significant force contrast without a change of composition. These materials are already used in rewriteable optical data storage [13,14,[23][24][25], where the pronounced optical contrast between the amorphous and crystalline 3 state is employed to store information. This storage principle employs a focussed laser beam to locally heat a disk with a thin film of phase change material. Upon a variation of the power and length of the laser pulse the material can be reversibly switched between the amorphous and the crystalline phase many times. Here we will show that the pronounced contrast of optical properties enables a significant change of the Casimi...

Within the last years, a fundamental understanding of nanoscaled materials has become a tremendous challenge for any technical applications. For magnetic nanoparticles, the research is stimulated by the effort to overcome the superparamagnetic limit in magnetic storage devices. The physical properties of small particles and clusters in the gas phase, which are considered as possible building blocks for magnetic storage devices, are usually sizedependent and clearly differ from both the atom and bulk material. For any technical applications, however, the clusters must be deposited on surfaces or embedded in matrices. The contact to the environment again changes their properties significantly. Here, we will mainly focus on the fundamental electronic and magnetic properties of metal clusters deposited on surfaces and in matrices. This, of course, requires a well-defined control on the production of nanoparticles including knowledge about their structural behaviour on surfaces that is directly related to their www.elsevier.com/locate/surfrep Surface Science Reports 56 (2005) 189-275 $ This work is based on results of the EU ''AMMARE'' project within the Fifth Framework programme coordinated by Antonis N. Andriotis, Heraklion, Greece.

Geometry and confinement effects at the nanoscale can result in substantial modifications to a material's properties with significant consequences in terms of chemical reactivity, biocompatibility and toxicity. Although benefiting applications across a diverse array of environmental and technological settings, the long-term effects of these changes, for example in the reaction of metallic nanoparticles under atmospheric conditions, are not well understood. Here, we use the unprecedented resolution attainable with aberration-corrected scanning transmission electron microscopy to study the oxidation of cuboid Fe nanoparticles. Performing strain analysis at the atomic level, we reveal that strain gradients induced in the confined oxide shell by the nanoparticle geometry enhance the transport of diffusing species, ultimately driving oxide domain formation and the shape evolution of the particle. We conjecture that such a strain-gradient-enhanced mass transport mechanism may prove essential for understanding the reaction of nanoparticles with gases in general, and for providing deeper insight into ionic conductivity in strained nanostructures.