Successful PhD defense of Berthold Rimmler
Congratulations to Berthold Rimmler!
Title: “Non-collinear Magnetism, Hidden Symmetry Lowering and Magneto-transport Effects in Antiperovskite Nitrides"
Antiferromagnets are promising materials for next-generation spintronic data storage devices due to their negligible magnetization and associated magnetic stray fields that limit the usefulness of conventional ferro- and ferrimagnets at the nanoscale. Of particular interest are a class of antiferromagnets that have non-collinear spin textures and which can display unusual transport properties, of which particular examples are an anomalous Hall effect, a longitudinal spin polarization of charge currents, and a spin Hall effect with unusual spin polarization directions. These effects can be readily observed only if the sample is set predominantly into a single antiferromagnetic domain state. One way to enable control of the domain structure is if the antiferromagnetic order is perturbed and displays weak moments due to spin canting. In thin films of cubic non-collinear antiferromagnets this imbalance was previously assumed to require substrate strain-induced tetragonal distortions. In this work it is found that non-collinear antiferromagnets with an antiperovskite structure may exhibit non-zero transport effects in the absence of such tetragonal distortions. In this case, net moments result instead from structural symmetry lowering induced by displacements of the manganese atoms away from high-symmetry positions. These displacements preserve the bulk-like lattice constants of the films and, therefore, remain hidden in structural characterization by X-ray diffraction when only probing the lattice metric. Instead, structural refinement of a large set of scattering vectors is required to resolve the local atomic positions. In the antiperovskite Mn3SnN these displacements enable the observation of an anomalous Hall effect with an unusual temperature dependence, which is conjectured to be due to a previously unexplored temperature-dependent spin rotation. These findings challenge the long-held assumption that tetragonal distortions are required to observe non-zero transport effects in cubic non-collinear antiferromagnets. Furthermore, the experimental results and comparison with the literature suggest that manganese displacements are common in antiperovskite nitrides and that they may enable the observation of transport effects beyond the anomalous Hall effect.