Research in the Complex Materials Spectroscopy Group (AG Ollefs) is based on X-ray absorption spectroscopy and imaging. Our main goal is to improve our understanding of solid state materials with respect to their electronic and geometrical structure as well as their magnetic properties. Hereby, we study a wide variety of topics, a few examples of which are described below.
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. The goal is to improve current magnets in this regard, that is, to broaden the hysteresis loop and thereby increase energy density, or to discover 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 imaging 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/TRR 270 as project A03.
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 that offer an alternative to conventional gas compressor cooling.
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).
After local optical laser excitation of a solid, the solid is in a non-equilibrium state. Various microscopic processes then cause it to return to thermal equilibrium within the first few hundred femtoseconds (fs). This process involves the electrons, their spins, and the lattice, which interact with one another.
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.
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
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.
Measurement of the magentization of the nanomagnets of an artificial spin-ice system in a regular grid. Copyright to Patrick Steinkraus
