Eugen Weschke

Institut für Experimentalphysik

Freie Universität Berlin

The investigation of periodic magnetic structures is a classical application of neutron diffraction. With the advent of high-intensity synchrotron x-ray sources, magnetic x-ray diffraction became a competitive complementary tool for such studies.

Magnetic x-ray diffraction is characterized by a small scattering amplitude from spin and orbital momentum densities, usually by orders of magnitude smaller than the Thomson scattering from the charge densities. A separation can be readily achieved in periodically modulated magnetic structures, which are characterized by well-resolved magnetic and charge contributions in momentum space; polarization analysis provides an additional tool to identify the magnetic part of the signal.

In finite systems, such as thin films, however, diffraction features are substantially broadened, leading to an increasing overlap between charge and magnetic diffraction signals with decreasing number of scatterers, aggravating a separation of the two contributions. These difficulties can be overcome by tuning the photon energy to an absorption resonance, which may result in a considerable enhancement of the magnetic scattering amplitude compared to charge scattering. This effect is particularly strong at the L_2,3 edges of the 3d-transition metals and the M_4,5 edges of the lanthanides, since they involve excitations of core electrons into the magnetically active 3d and 4f states, respectively. It will be demonstrated that in case of the lanthanides, the magnetic diffraction signal is increased by a factor of the order of 10⁶.

While resonant magnetic x-ray diffraction is well established in the hard x-ray region, the exploitation of the soft x-ray resonances represents a rather new development. Due to the strong absorption in air, experiments have to be carried out in vacuum. Furthermore, with a comparably large x-ray wave length of the order of 1-2 nm, the method cannot probe crystalline structures. It is, however, well adapted to the dimensions of periodic nanostructures.

The talk will present some of the experiments carried out in the field, discussing possible future applications and also technical problems such as the strong absorption of the x-rays at resonance. For the prototypical antiferromagnet Ho metal, well-suited for an exploratory investigation of the potential of resonant magnetic x-ray diffraction, it will be demonstrated that the huge enhancement of the magnetic scattering amplitude at soft x-ray resonances increases the sensitivity of the method to allow investigations of films down to 10 atomic layers or even less.