Permanent magnets are a cornerstone of green energy generation and an essential component of numerous modern technologies. They are characterized by spontaneous magnetization and exhibit the highest possible remanence and coercivity. Our research focuses on improving existing magnets in this regard—that is, broadening the hysteresis loop to increase energy density—or discovering new magnets, both with and without critical so-called rare earth elements.
In both cases, a solid understanding of the elementary magnetic properties of atoms, the microstructure of the materials, and their interactions is crucial. Specific research questions relate to the elementary magnetic moments and their interactions, the formation of magnetic anisotropy in the crystal lattice, and domain wall motion across phase boundaries and defects. In our group, we rely on advanced techniques such as X-ray absorption (XAS, XMCD, EXAFS) and tomographic methods to extract a wide range of information from the material and thus address both fundamental and applied questions.
This research topic is included in CRC/SFB 270 as project A03.
The study of out-of-equilibrium states in magnetic and functional materials can give rise to interesting phenomena, like charge transfer dynamics between constituents, ultrafast demagnetization or the emergence of collective orders. We employ time resolved Xray absorption spectroscopy to investigate the excitation and relaxation of optically excited states with temporal resolutions from a tens of picoseconds down to a few tens of femtoseconds. Crucial to disentangle the contributions of different elements is the element selectivity of X-ray techniques in alloys or other multi component materials.
The works provide fundamental contributions to the field ultrafast switching of magnetic states or more general spin- and orbitronics. Instead of transmitting informations with electron motions these aim to utilize the magnetic properties of the materials as the transfer medium for signals.
This research topic is included in CRC/SFB 1242 as project A07.
Sketch of an experiment. The pump (red arrow) is a laser pulse, exciting the material. After excitation, the x-ray probe (blue energy) can measure these excitations. In this case, the measurement takes place at the TM L-edge, probing the valence 3d states by exciting electrons from 2p to 3d. Adapted with AI from: https://www.uni-due.de/sfb1242/overview_a7.php
Due to the growing global demand for energy for cooling, there is considerable interest in the development and research of alternative cooling concepts. Materials with a magnetocaloric effect represent a promising class of solid-state cooling materials and are already being used successfully in cryogenic applications (including here at KIP!), but they may also offer an alternative to conventional gas compressor cooling.
When the applied magnetic field changes, magnetocaloric materials exhibit a reversible change in temperature under adiabatic conditions due to an isothermal change in the entropy of the magnetic lattice. This effect is significantly enhanced when 1) the temperature falls below the material’s Curie temperature and 2) this spontaneous magnetization occurs within a narrow temperature range. In this case of a first-order phase transition at the Curie temperature, the change in entropy is maximized even with only small applied magnetic fields. In our group, these processes are investigated using, among other methods, X-ray absorption techniques. This allows us to determine the element-specific contributions to the magnetic, structural, and vibrational properties. On this basis, materials can be specifically optimized with regard to their magnetocaloric properties and their suitability for solid-state cooling.
Sketch of a magnetocaloric cooling cycle. Adapted using AI from: O. Tegus, E. Brück, K. H. J. Buschow, and F. R. de Boer, Nature 415, 150 (2002).
Artificial spin-ice systems are custom-designed arrangements of nanoscale magnets that can exhibit a number of fascinating magnetic phenomena, such as magnetic frustration, magnetic monopoles, or collective excitations.
A particular focus is on how such states and excitations form, change, and can be dynamically controlled. Spectroscopic methods, particularly time- and space-resolved X-ray spectroscopy, provide element- and spin-specific information about the magnetic structure and its dynamics. As a result, artificial spin-ice systems open up new perspectives for research into frustrated magnetism as well as for future applications in computer technology.
This project is funded via GEPRIS.
Measurement of the magentization of the nanomagnets of an artificial spin-ice system in a regular grid. Copyright to Patrick Steinkraus
