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

by Mr Henrik Thoma (TUM)

Monday 25 Jun 2018, 14:30 → 15:30 Europe/Berlin

HS 3 (Physics Department)

The Dzyaloshinskii–Moriya (DM) interaction is a type of exchange-coupling between two spins that can have significant effects on the properties of magnetic materials. Its magnitude is usually small, but its direction is often a decisive factor in the determination of the system's chirality. A better understanding of the spin–orbit interaction and its implications have been a particular target of condensed matter research over the past decade: multiferroics, topological insulators, and Rashba and Dresselhaus spin–orbit coupling are all intensively studied. Recently in Nature Physics, V. Dmitrienko and colleagues have found a way to measure the sign of the coupling vector, in order to determine the direction of the DM interaction, using sophisticated techniques based on synchrotron spectroscopy. In the presented work, the sign of the DM interaction is determined in hematite (α-Fe2O3) and rhodochrosite (MnCO3) single crystals with R-3c symmetry by means of polarized neutron diffraction (PND).

 

The theoretical basis for the DM interaction, based on a symmetry analysis in both compounds, is briefly introduced. A dedicated PND setup, using a new symmetric-field high Tc superconducting magnet with a maximal field of 2.2 T in combination with a ³He polarizer and Mezei-type flipper, has been developed. Each component is optimized by numerical simulations and the complete setup is successfully tested and calibrated.

 

This new PND setup is used to collected flipping-ratio (FR) data as function of the applied magnetic field and temperature for both compounds. In addition, an advanced approach for the reconstruction of maximum entropy spin density maps from FR data is presented. Using this approach, 3D spin density maps are built for the paramagnetic and antiferromagnetic phase, in both compounds for the first time, revealing new features compared to the results from conventional maximum entropy software.

 

The analysis of the obtained spin density distribution maps showed clearly on one side that 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 maps allowed the extraction of the sign of the DM interaction. It has been determined to be the same for both compounds, and in agreement with recently published data for MnCO3 from synchrotron.