NMR-Characterization of interacting nanostructures

The current project is devoted to studies of the eletronic and magnetic properties of macroscopic ensembles of interacting nanostructures by means of the local probe of nuclear magnetic resonance (NMR). Miniaturization of semiconductor as well as magnetic systems arrived during the last decade at nanometer length scales. For technical applications the structures have to be prepared in a coherent, homogeneous way across macroscopic length scales of the total system. Classical NMR is not well suited to characterize a single constituent of such a structure, because of its low sensitivity. However, NMR can be utilized to study the distribution of electronic and magnetic properties within a large, homogeneous ensemble of such entities in considerable detail. In this project NMR will be used to investigate the properties of the constituents and the interaction mechanisms between them in three different example systems: BaTiO3 -coated CoFe2 O4 -particles which show a large magneto- electric effect due to the coupling between the ferroelectric and the ferromagnet; ferromagnetic transition-metal nanowires in porous Silicon, which show a very unusual, strong negative differential magnetization at high fields; and thin ferromagnetic Co-films with lateral nanostructures, showing similar unusual characteristics of the magnetic hysterisis loop for certain geometries.

The project plan was to study the potential of nuclear magnetic resonance (NMR) as a method for investigations of possibly interacting nanostructures. Due to difficulties in the preparation of the materials the focus was shifted to a detailed characterization of precursors by means of solid state NMR. The materials investigated were as different as the classic magnetic material Magnetite (Fe3O4) doped with Cobalt, a Nickel-Boron based superconductor (La3Ni2B2N3), and metallic Cobalt-Silicon (Co2Si). The work on Magnetite focused on the relation between the environment of Cobalt in this structure and its magnetic properties. The interest in this question stems from the fact that Cobalt is well known to induce a large magnetrostriction in Magnetite that is a strong coupling between magnetic polarization and the lattice constants. If intimately connected with a piezoelectric material this large magnetostriction might be utilized to prepare a nanocomposite with a large magnetoelectric effect a material where the magnetic state can be controled by an applied voltage. NMR-Characterization of the local magnetic properties of Cobalt in chemically deposited Magnetite powders with grain diameters in the sub- micrometer range revealed that despite the good crystallografic homogeneity of the powders the magnetic properties of the Cobalt atoms were very inhomogeneous, severly limiting the overall magnetostrictive effect. A detailed analysis comparing the NMR results to predictions based on theoretical calculations for different structural environments of Cobalt in Magnetite is in progress. Our interest in the Cobalt-Silicon system is based on the relation between the standard material for semiconductor applications and a standard metallic ferromagnet, which might be essential for the preparation of magnetic nanostructures. In contrast to the complex magnetic phase diagram of Manganese-Silicon, superconductivity has been reported in the Cobalt- Silicon system. So it came as a surprise when we found in this project a phase transition to weak magnetism by NMR in a single crystal. This has been studied in detail and is now also being analysed by comparison to theoretical calculations of the properties of this structure. NMR-Characterization of the Nickel-Boron based superconductor has been completed. Two problems were addressed in this part, namely again the comparison of the NMR results probing the local electronic structure in this complex material with theoretical calculations from first principles, and a detailed analysis of the dynamic behaviour of the magnetic flux lines in the small pinning potential of this material.