Magnetic properties of small deposited 4d and 4f transition metal clusters

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Small clusters are intriguing systems because their physical and chemical properties lie somewhere in between the properties of their atomic or molecular constituents and the respective bulk solids. For larger clusters specific physical quantities typically emerge as a smooth function of the size of the clusters. This evolution is often a result of the changes in the average coordination of the constituents as a function of cluster size. If the cluster size gets comparable to or smaller than the wavelength of the electrons which determine the physical properties we enter the regime where quantum size effects become important. In this size regime the evolution of physical quantities becomes less predictable and typically adding or removing a single atom or molecule from a cluster may result in very significant changes. However, small cluster have the advantage that they present tractable systems for sophisticated high-level theoretical treatments. Hence, small clusters are an ideal playground for the test of theoretical models against experimental results.The aim of the proposed project in the framework of the Schwerpunktprogramm is to study the size dependent magnetic properties of small supported metal clusters. We plan to investigate small 4d transition metal clusters (e.g. Mo, Ru, Rh, Pd) where the 4d electrons are delocalized and which show no magnetism in the bulk or as ultrathin thin films as well as 4f rare earth metal clusters (e.g. Gd) where the magnetic properties are governed by the localized, atomic like 4f electrons.The clusters will be size selected prior to deposition; cluster sizes will be in the range from 2 to 20 atoms. We will perform x-ray magnetic circular dichroism experiments (XMCD) in the soft X-ray regime using circular polarized synchrotron radiation. The goal is to study the size dependent magnetic structure of these clusters and in combination with theoretical studies to get detailed information of the magnetic coupling within the clusters and relative to the substrates. In particular, the XMCD studies will allow us to extract spin and orbital magnetic moments for the atoms in the clusters.