The Sign of the Dzyaloshinskii-Moriya Interaction in R-3c Symmetries

Abstract

The Dzyaloshinskii–Moriya (DM) interaction is a type of exchange-coupling between twospins that can have significant effects on the properties of magnetic materials. Its magnitudeis usually small, but its direction is often a decisive factor in the determination ofthe system’s chirality. A better understanding of the spin–orbit interaction and its implicationshave been a particular target of condensed matter research over the past decade:multiferroics, topological insulators, and Rashba and Dresselhaus spin–orbit coupling areall intensively studied. Recently in Nature Physics, V. Dmitrienko and colleagues havefound a way to measure the sign of the coupling vector, in order to determine the directionof the DM interaction, using sophisticated techniques based on synchrotron spectroscopy.In this Master thesis, the sign of the DM interaction is determined in hematite (alpha-Fe2O3)and rhodochrosite (MnCO3) single crystals with R-3c symmetry by means of polarizedneutron diffraction (PND).The theoretical basis for the DM interaction, based on a symmetry analysis in both compounds,is introduced. The polarized single crystal diffraction theory and its methodsare briefly presented. A dedicated PND setup, using a new symmetric-field high Tc superconductingmagnet with a maximal field of 2.2T in combination with a 3He polarizerand Mezei-type flipper, has been developed. The corresponding numerical simulationsand optimization for each component are presented and the complete setup is successfullytested and calibrated.This new PND setup is used to collected flipping-ratio (FR) data as function of the appliedmagnetic field and temperature for both compounds. The measured data were evaluatedaccording to the theoretical basis provided in the first part of this thesis. In addition, anadvanced approach for the reconstruction of maximum entropy spin density maps fromFR data is presented and appropriate software tools developed. Using these softwaretools, 3D spin density maps are build for the paramagnetic and antiferromangetic phase,in both compounds for the first time, revealing new features compared to the results fromconventional maximum entropy software.The analysis of the obtained spin density distribution maps showed clearly on one sidethat the origin of the magnetic scattering is not the localized moments at the atomic positions,but rather magnetic fields of the displaced orbitals. On the other side, 3D mapsallowed the extraction of the sign of the DM interaction