Head of department: Sabine Wurmehl
Our research is aiming at a fundamental understanding of functional properties of novel materials and their potential for applications. The basis of this research is high quality samples; hence, our work is dedicated to synthesis and crystal growth, full characterization and investigation of materials properties. This strategy involves four different, synergistic aspects: (i) synthesis and crystal growth of high quality samples (ii) advanced characterization of composition, microstructure and crystal structure (iii) study of local magnetic, electronic and structural order using nuclear magnetic resonance spectroscopy (iv) measurement and analysis of macroscopic physical properties, which is done in close collaboration with in-house partners as well as with national and international collaborators, including numerous neutron and X-ray synchrotron experiments. Our work currently focuses on different classes of emergent materials: intermetallics showing high spin polarization, itinerant antiferromagnetism, first order phase transitions, thermoelectric and hard-magnetic properties, unconventional superconductors, magnetically frustrated materials and materials with strong spin-orbital coupling.
Intermetallic materials exhibit different electronic ground states such as spin density wave ordering, thermoelectricity, topological insulating properties, first-order phase transitions, metamagnetic transitions, high magnetic anisotropy, high magnetic moments and Curie temperature, as well as high spin polarization. This diversity renders intermetallics attractive materials to explore fundamental aspects of matter but also raises interest for the technological exploitation of their functionalities, e.g., in spintronic or ferroic cooling devices, in sensors and actuators or as permanent magnets. Typical representatives of functional intermetallics are Heusler compounds.
A key tool in the rational design of this class of materials is the knowledge and control of the properties and whether they are dominated by intrinsic factors (e.g. electronic structure and magnetic anisotropy) or by extrinsic factors (structural degree of order, segregation, phase dynamics). We address these issues by combining crystal growth techniques with NMR investigations. Related publications
Our research currently focuses on the crystal growth of pnictides representing the newest members of the class of unconventional superconductors. We explore growth routes towards new and emergent iron-pnictides where a reliable synthesis route guaranteeing high quality samples is yet unknown. An important aspect is the systematic investigation of their electronic phase diagrams by systematic substitution and doping within new and emergent 1111, 122 and 111 series. Related publications
Magnetic frustration is closely connected to the structural, geometric arrangement of magnetic moments and, importantly, is also inherently sensitive to all kinds of defects. These generic aspects of magnetic frustration call for compounds with special structural motifs and for samples of the highest quality.
Iridates with their very anisotropic exchange interaction are considered as the paradigm materials of magnetically frustrated model systems giving rise to novel quantum spin ground states and excitations. Besides, their energy scales of strong spin-orbit coupling (SOC), Coulomb interaction (U), and crystal fields (CF) are comparable. In addition to the exploration of Ir-oxides, we investigate selected 3d transition-metal oxides such as spin-chain systems, triangular, and Kagome lattices with geometrically induced magnetic frustration. Related publications
We prepare polycrystalline samples as well as precursor materials for subsequent growth experiments, including the rods for the floating-zone experiments, by state-of-the-art solid-state reactions and melting techniques (arc-melting and casting).
We use the Bridgman technique, flux growth methods and the floating zone technique with both inductive and optical heating. For the latter, we use a unique mirror furnace which was developed by the IFW research technology department. This setup allows the growth of crystals under pressure of up to 150 bar.
For the systematic investigations of growth conditions, we use a home-made in-situ high-temperature optical microscope, which allows us to monitor a given melting process. As a complementary method, we use differential (scanning) thermoanalysis (DTA,DSC) to explore the growth relevant thermodynamic phase diagrams.
All samples and crystals are comprehensively characterized regarding their structure and phase purity (powder and single-crystal x-ray diffraction), and their composition (scanning electron microscopy with energy and wavelength dispersive x-ray analysis). Furthermore sophisticated sample characterization involves in-house cooperation: (i) The oxygen content in oxide samples is measured using the carrier gas-hot-extraction analysis combined with simultaneous COx detection with an in-house collaboration. (ii) The global composition whenever light elements are involved is analyzed using inductive couples plasma mass and emission spectrometry also with an in-house collaboration.
NMR on ferromagnets
Nuclear magnetic resonance spectroscopy (NMR) is an ideal tool for studying composition and local (magnetic and electronic) structure of bulk materials and thin films but also of interfaces between different thin magnetic layers. Related publications