The remanence and coercivity of an assembly of single-domain ferromagnetic particles are studied using the Monte Carlo simulation technique. The particles have random locations, possess random uniaxial anisotropy, and are coupled through dipolar interactions. The dependence of the magnetic properties on the packing density, the size of the particles, and the temperature are examined systematically. The role of the packing geometry ͑sc, fcc͒ and the sample boundaries are discussed. Dipolar interactions are shown to reduce the coercivity with respect to values for the noninteracting assembly in all cases except for strongly dipolar systems below the percolation threshold. An enhancement of the remanence is found in weakly dipolar systems and is attributed to the macroscopic Lorentz field. The fcc packing of the particles leads to more pronounced ferromagnetic behavior than the sc packing. The sample free boundaries and the corresponding demagnetizing field have a strong effect on the remanence of the assembly while they produce a minor reduction to the coercivity. The results from the simulations are compared with magnetic measurements on frozen ferrofluids and granular metal solids. ͓S0163-1829͑98͒03241-X͔

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.

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.

Depositing pre-formed gas-phase nanoparticles, whose properties can be widely varied, onto surfaces enables the production of films with designed properties. The films can be nanoporous or, if co-deposited with an atomic vapour, granular, allowing independent control over the size and volume fraction of the grains. This high degree of control over the nanostructure of the film enables the production of thin films with a wide variety of behaviour, and the technique is destined to make a significant contribution to the production of high-performance magnetic materials. Here we review the behaviour of magnetic nanoparticle assemblies on surfaces and in non-magnetic and magnetic matrices deposited from the gas phase at densities from the dilute limit to pure nanoparticle films with no matrix. At sufficiently low volume fractions (∼1%), and temperatures well above their blocking temperature, nanoparticle assemblies in non-magnetic matrices show ideal superparamagnetism. At temperatures below the blocking temperature, the magnetization behaviour of both Fe and Co particles is consistent with a uniaxial intra-particle magnetic anisotropy and an anisotropy constant several times higher than the bulk magnetocrystalline value. At relatively low volume fractions (≥5%) the effect of inter-particle interactions becomes evident, and the magnetization behaviour becomes characteristic of agglomerates of nanoparticles exchange coupled to form magnetic grains larger than a single particle that interact with each other via dipolar forces. The evolution of the magnetic behaviour with volume fraction is predicted by a Monte-Carlo model that includes exchange and dipolar couplings. Above the percolation threshold the films become magnetically softer, and films of pure clusters have a magnetic ground state that obeys the predicted magnetization behaviour of a correlated super-spin glass characteristic of random anisotropy materials. Magnetic nanoparticles in non-magnetic matrices show giant magnetoresistance behaviour, and the magnetotransport in deposited nanoparticle films is reviewed. Assembling Fe nanoparticles in Co matrices and vice versa is a promising technique for producing magnetic materials with a saturation magnetization that exceeds the Slater–Pauling limit. Structural studies reveal that the particles' atomic structure is dependent on the matrix material, and it is possible to prepare Fe nanoparticles with an fcc structure and, unusually, Co particles with a bcc structure. We also look to the future and discuss applications for materials made from more complex bi-metallic and core–shell nanoparticles.

Inorganic chemistry Z 0100Magnetic and Structural Properties of Isolated and Assembled Clusters -[227 refs.]. -(BANSMANN, J.; BAKER, S. H.; BINNS, C.; BLACKMAN, J. A.; BUCHER, J.-P.; DORANTES-DAVILA, J.; DUPUIS, V.; FAVRE, L.; KECHRAKOS, D.; KLEIBERT, A.; et al.; Surf. Sci. Rep. 56 (2005) 6-7, 189-275; Inst. Phys., Univ. Rostock, D-18051 Rostock, Germany; Eng.) -Schramke 52-215