Theory of nanostructured magnetic materials

Nanostructured magnetic materials in the form of ultrathin films, multilayers, nanowires and arrays of clusters ("quantum dots") represent a major thrust in the development of electronic devices and data storage media. The upcoming concept of "spintronics", based an the fact that electrons carry not only charge, but also spin, requires the fabrication of nanosized magnetic materials with tailor-made magnetic properties. The modelling of nanostructured magnets also constitutes a major challenge to condensed-matter theory and computational materials science. In nanoscale dimensions, magnetic materials often assume properties that are drastically different from those of the bulk. Local information such as for example the magnetic profile of an ultrathin film or the exchange-coupling between nanowires forming a regular array are often not accessible to laboratory experiments, but can be calculated with excellent accuracy using modern firstprinciples local-spin-density techniques. In the recent past, these techniques contributed to our knowledge of ultrathin films and multilayers, but many questions still remain unanswered. The purpose of the current project is to expand these studies: we want to leam more about the factors governing the fabrication of ultrathin films and nanowires by studying the atomic mobility, to explore the factors determining their magnetic structure and anisotropy, to study the magnetic exchange coupling, magnetic fluctuations and excitations, and electronic transport processes. The final aim is to contribute to the computer-aided design of nanostructured magnetic components.

The investigation of the physical properties of nanostructured magnetic materials (ultrathin Films, nanowires, clusters) is at the basis of the further development of materials for magnetic and magneto-electronic devices and for magnetic information storage media. In the present project modern quantuin-mechanical techniques, based an the density-functional theory pioneered by Walter Kohn, have been applied to the investigation of those systems. The computational approach to the physical properties, as well as the numerical simulation of processes play a very important role, because in many cases laboratory experiments cannot provide atomically resolved information required for a fundametal understanding. A very important point is that an the nanoscale many materials display properties differing radically from those observed at a macroscopic scale. The investigation of those specific properties of nanomaterials was at the focus of this project. Important results are: (i) Small clusters of transition- metal atonrs are offen magnetic, even if the metal itself is nonmagnetic - examples are Pd, Pt, and Rh. The geometric and magnetic structures of the cluster were found to be strongly interdependent. (ii) Nanowires consisting of monoatomic chains er nanostripes can be formed at the step-edges of crystalline substrates. The one- dimensional structure and the strain in the wires imposed by the Substrate determine the magnetic properties, the inter-wire coupling is mediated by the electrons of the substrate. (iii) The epitaxial relation between film and substrate can stabilize a film-structure differing from that of the bulk material, a modest mismatch can lead to the formation of complex superstructures. (iv) The reduced dimensionality and the interaction with the Substrate can lead to highly unexpected magnetic properties: the ferromagnetic metals iron and cobalt can be antiferromagnetic in ultrathin films er nanowires, the antiferromagnetic metal rnanganese can become ferromagnetic.