From electronic structure to magnetism and skyrmions (Topical review)

Solid state theory, density functional theory and its generalizations for correlated systems together with numerical simulations on supercomputers allow nowadays to model magnetic systems realistically and in detail and can be even used to predict new materials, paving the way for more rapid material development for applications in energy storage and conversion, information technologies, sensors, actuators etc. Modelling magnets on different length scales (between a few \r{A}ngstr\"om and several micrometers) requires, however, approaches with very different mathematical formulations. Parameters defining the material in each formulation can be determined either by fitting experimental data or from theoretical calculations and there exists a well-established approach for obtaining model parameters for each length scale using the information from the smaller length scale. In this review, this approach will be explained step-by-step in textbook style with examples of successful multiscale modelling of different classes of magnetic materials from the research literature as well as based on results newly obtained for this review.Comment: 43 pages, 14 figure

Tunable topological magnetism in superlattices of nonmagnetic B20 systems

We predict topological magnetic properties of B20 systems, that are organized in atomically thin multilayers. In particular we focus on FeSi/CoSi and FeSi/FeGe superlattices with different number of layers and interface structure. We demonstrate that absence of long range magnetic order, previously observed in bulk FeSi and CoSi, is broken near the FeSi/CoSi interface, where a magnetic state with non-trivial topology appears. Using electronic structure calculations in combination with the magnetic force theorem, we calculate the Heisenberg and Dzyaloshinskii-Moriya (DM) interactions in these systems. With this information, we perform atomistic spin dynamics simulations at finite temperature and applied magnetic field for large supercells with up to $2\cdot10^6$ spins to capture the complexity of non-collinear textures induced by the DM interaction. The spin dynamics simulations predict the formation of antiskyrmions in a [001]-oriented FeSi/CoSi multilayer, intermediate skyrmions in a [111]-oriented FeSi/CoSi system and Bloch skyrmions in the FeSi/FeGe (001) system. The size of different types of skyrmions is found to vary between 7 nm and 37 nm. The varying topological magnetic texture in these systems can be attributed to the complex asymmetric structure of the DM micromagnetic matrix, which is different from previously known topological magnets. Furthermore, through structural engineering, we demonstrate that both FM and AFM skyrmions can be stabilized, where the latter are especially appealing for applications due to the zero skyrmion Hall effect. The proposed B20 multilayers show potential for further exploration and call for experimental confirmation.Comment: 13 pages, 10 figure

For optical flickering in symbiotic star MWC 560

This study is based on observations of MWC560 during the last two observational seasons (2020/2021 and 2021/2022). Other than looking for flickering we were interested in following the variability of brightness in the same period. Looking for similarities in the spectra with other types of stars is also of great interest to us because it could help clarify the stellar configuration of such objects. Our observations during the last two observational seasons of MWC560 confirm the absence of flickering. From the similarities of the gathered spectra of XX Oph and MWC560 we assume that the components in XX Oph are a red giant and a white dwarf, which are also surrounded by a common shell

Pressure induced half-collapsed-tetragonal phase in CaKFe4_4As4_4

We report the temperature-pressure phase diagram of CaKFe$_4$As$_4$ established using high pressure electrical resistivity, magnetization and high energy x-ray diffraction measurements up to 6 GPa. With increasing pressure, both resistivity and magnetization data show that the bulk superconducting transition of CaKFe$_4$As$_4$ is suppressed and then disappears at $p$ $\gtrsim$ 4 GPa. High pressure x-ray data clearly indicate a phase transition to a collapsed tetragonal phase in CaKFe$_4$As$_4$ under pressure that coincides with the abrupt loss of bulk superconductivity near 4 GPa. The x-ray data, combined with resistivity data, indicate that the collapsed tetragonal transition line is essentially vertical, occuring at 4.0(5) GPa for temperatures below 150 K. Band structure calculations also find a sudden transition to a collapsed tetragonal state near 4 GPa, as As-As bonding takes place across the Ca-layer. Bonding across the K-layer only occurs for $p$ $\geq$ 12 GPa. These findings demonstrate a new type of collapsed tetragonal phase in CaKFe$_4$As$_4$: a half-collapsed-tetragonal phase