A challenging aim of current research in magnetism is to explore structures of reduced dimensionality. As the dimensionality of a physical system is reduced, magnetic ordering tends to decrease as fluctuations become relatively more important. On the other hand, the reduced coordination has direct consequences for the orbital magnetic moments and the anisotropy energy. Additionally, the interaction with the substrate influences the magnetism. We were able to construct various atomic arrangements of Fe and Co atoms on Pt surfaces by self-organized growth, such as monolayers, islands, stripes and atomar chains, clusters and surface alloys. Even single atom impurities of Co or Rh could be investigated!

The magnetism of such structures was investigated mainly by X-ray magnetic circular dichroism (XMCD). Thus, not only the magnetic anisotropy could be analyzed as a function of the atomic arrangement, also information about the orbital moments could be detected element-specific and with high sensitivity. We find that the hybridization of 3d metal nanostructures with 4d metal substrate induces a seizable magnetic moment in the substrate, which couples ferromagnetically to the adstructure magnetization. This effect can be exploited to assemble magnetic nanostructures with high coercivities as in the case of Fe-Pt in the above figure (Fe atoms are yellow).

Instead of the linear atomar arrangement described above, also nanoclusters and single atoms on the surface can be investigated due to the high sensitivity of the XMCD technique! The figure on the right shows XMCD spectra, normalized to the L₂ edge, for Co clusters of different size n. The data are presented such that the height of the L₃ peak at 779eV reflects the orbital moment per atom. One observes that the orbital moment of the clusters dramatically increases with decreasing cluster size. Along with the orbital moment, also the anisotropy energy is found to be strongly enhanced. Thus, the present data imply a lower limit of 400 Co atoms per bit in potential data storage devices if the Co adatom coordination is assumed to be 2! (K = 3.3meV/atom, T = 350K)

Within the frame of the DFG Schwerpunktprogramm SPP1153 we are investigating the properties of deposited clusters formed in the gas phase. We concentrate our efforts on metal clusters of 3d and 4d elements, such as Fe, Co, Rh and Ru. Our goal is to compare the properties of deposited clusters, produced by buffer-layer assisted growth, with self-organized MBE grown clusters. The cluster properties are investigated by XMCD and magneto-optical techniques, as well as in-situ Variable Temperature Scanning Tunneling microscope (VT-STM) and tunneling spectroscopy.

One of our approaches to produce clusters is based on deposition of the ferromagnetic material on the substrate that has been covered by a Xenon buffer layer prior to the film growth at 30K. Clusters are formed during evaporation of the Xe layer upon heating up the substrate to 100K. The final cluster size depends, among other parameters, on the initial thickness of the Xe layer. In the figure below magnetization loops of 2 atomic layers of Fe are shown as a function of the Xe buffer thickness. The data show that clusters with magnetic properties different from those of an epitaxial Fe film are formed if the Xe film is sufficiently thick (>2ML).

Magnetism is inherently determined by electronic correlations of magnetic moments at different sites within a material. Both the moment of single atoms as well as their magnetic interaction depend on the electronic configuration within the matrix. Reducing the chemical coordination of magnetic atoms is therefore known to induce dramatic changes of the magnetic behavior. Understanding nanomagnetism and being able to tune the magnetic properties of low-dimensional systems is a mayor challenge both from a scientific and a technical (storage media) point of view. To examine the dependence of magnetism of 3d-metals on their electronic configuration and interatomic distance we use non-magnetic molecules as spacers between the magnetic atoms. Fe/TPA e.g. forms several highly ordered coordination structures on atomically flat Cu(100)-substrates in which the Fe-atoms are arranged differently as can be seen in the figure above. The magnetic behavior of the different phases measured by XMCD is characteristic for each phase and can clearly be distinguished.