Magnetic and structural properties of isolated and assembled clusters

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 size-dependent 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 magnetic properties. We describe two different approaches to produce magnetic nanoparticles: (i) cluster aggregation on reconstructed single crystal surfaces and (ii) deposition of mass-filtered clusters from the gas phase onto surfaces and into matrices. The process of cluster deposition offers the possibility of creating new materials in non-equilibrium conditions with tailored properties. A theoretical description of the evolution of cluster magnetism is given with respect to contributions from magnetic anisotropy effects and the local atomic environment. Especially small clusters show size-dependent magnetic orbital and spin moments that can experimentally be accessed by the element- specific technique of X-ray magnetic circular dichroism (XMCD). The magnetic properties of self-organized iron and cobalt clusters on Au(1 1 1) surfaces are discussed with respect to growth conditions. For deposited Fe clusters on surfaces increased orbital moments have been found even for large particles. Additionally, the influence of capping layers and deposition into matrices is discussed. While investigations on relative simple structures in pure 3d metal particles yield insight into the basic mechanisms of magnetism, alloy nanoparticles seem to be more promising in terms of technical application since they offer the possibility to adjust the magnetic properties by varying the stoichiometry. Alloys consisting of 3d metals (e.g. FexCo1-x alloys) have usually very high magnetic moments and are soft-magnetic. Binary clusters consisting of a 3d metal (e.g. Co) in combination with a heavy element (Sm, Ag or Pt) are candidates for materials with high magnetic anisotropies and increased blocking temperatures. The magnetic properties are directly related to their structural order. Here, we show first results for such alloy nanoparticles. (C) 2004 Elsevier B.V. All rights reserved.