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NanoMagnetism

Focus

The central research direction of the NanoMagnetism group is focused towards studying the impact of internal interfaces and the consequences of crossing length scales on the magnetic properties of nanocrystalline magnetic materials. These are polycrystalline bulk materials with an average crystallite size D  in the range of a few 10 nm. In these systems, nanocrystallinity affects magnetism essentially through the following two scenarios:

  • the downscaling of the average grain size D  of a polycrystalline solid to the nanometer regime eventually leads to the situation that crossing length scales appear in the system, i.e., D  becomes comparable to or even smaller than a characteristic intrinsic magnetic length scale such as the single-domain size or the domain-wall width.
  • additionally, one has to take into account that the reduction of D  is inevitably coupled to an increase in the volume fraction of internal interfaces (grain or phase boundaries), which is proportional to 1/ D .

Another focus of our research is micromagnetic modeling of magnetic neutron scattering. In a collaboration with the group of Dmitri Berkov ( INNOVENT Technology Development , Jena, Germany) we use numerical micromagnetic theory for the calculation of the magnetic small-angle neutron scattering (SANS) cross section of nanocomposites. This approach allows us to study the field dependence of the individual contributions to the total magnetic scattering. Such a micromagnetic tool ideally complements neutron experiments in which one generally measures only a weighted sum of the Fourier components of the magnetization. The procedure furnishes unique and fundamental information regarding the magnetic microstructure and corresponding SANS from nanomagnets.

Materials

The materials under study include

  • rare-earth metals such as Gd, Tb, Dy, Ho or Er
  • nanorod ferrofluids
  • the technologically relevant class of NdFeB-based permanent magnets
  • as well as soft magnetic Fe-based nanocomposites from the Finemet or Nanoperm family of alloys.

Methods

The main methods which we employ for microstructure determination are magnetometry, x-ray diffraction, electron microscopy, micromagnetic modeling and, in particular, neutron scattering. Most of our experiments are therefore conducted at large-scale neutron facilities such as FRM-II , HZB , PSI , ILL , NIST and ANSTO .