Signature of defect-induced symmetry breaking in magnetic neutron scattering

Topic  31
Main supervisor
S. Mühlbauer

(y.su@fz-juelich.de)

MLZ institution TUM
Local supervisor 1 Andreas Michel
Institution Univ. of Luxembourg
Local supervisor 2 Jan-Hendrik Spilgies
Institution
Hoffmann Eitle Patent- und Rechtsanwälte PartmbB
Local supervisor 3
Institution
Local supervisor 4
Institution
Title Signature of defect-induced symmetry breaking in magnetic neutron scattering
Description The antisymmetric Dzyaloshinskii-Moriya interaction (DMI) plays a decisive role for the stabilization and control of chirality of skyrmion textures in various magnetic systems exhibiting a noncentrosymmetric crystal structure. A less studied aspect of the DMI is that this interaction is believed to be operative in the vicinity of lattice imperfections in crystalline magnetic materials, due to the local structural inversion symmetry breaking. If this scenario leads to an effect of sizable magnitude, it implies that the DMI introduces chirality into a very large class of magnetic materialsdefect-rich systems such as polycrystalline magnets. This effect, conjectured already by Arrott in 1963, will be experimentally investigated using polarized small-angle neutron scattering (SANS) on defect-rich nanocrystalline ferromagnets such as terbium and holmium. For these systems, the dominating defect is represented by the internal interfaces (grain boundaries), which separate crystallites with different crystallographic orientation. Decreasing the average grain size increases the volume fraction of interfaces. The experimental neutron data analysis takes advantage of a theoretical framework based on the continuum theory of micromagnets, which predicts an asymmetric polarization-dependent term in the magnetic SANS cross section. This project is expected to prove the generic relevance of the DMI for the magnetic microstructure of defect-rich ferromagnets.