Karin Leistner and her co-worker seek to discover novel electrochemical approaches to tune functional properties of nanoscale materials. Aspects of magnetism, electrochemistry and materials science are strongly cross-linked and advanced in situ measurement techniques are developed. One focus lies on magneto-ionic effects which are based on reversible ionic motion and electrochemical reactions. These approaches enable a non-volatile low-voltage control of magnetism up to ON/OFF switching at room temperature and are therefore highly attractive for energy-efficient magnetic actuation, spintronics, and magnetophoretic devices. The materials are based on Fe, Co, FeNi and FePt nanostructures as well as Co/Ni multilayers polarized in liquid electrolyte. In situ methods based on the anomalous Hall effect, ferromagnetic resonance and Kerr microscopy measurements allow us to probe nanomagnetism in electrochemical environment. A second focus lies on the nanoelectrodeposition of magnetic elements and alloys (Fe, Co, FePt) and the understanding of electrode processes during the nucleation stage and epitaxial growth mechanisms. For example, epitaxial Fe nanocuboids are achieved, which are ideal to study the shape- and size-dependent evolution of magnetism in reduced dimension. Collaborators include the Simon Fraser University (Canada), the Massachusetts Institute of Technology (USA), the University of Kassel, and the TU Vienna.
The group of Tom Woodcock focusses on fundamental and applied aspects of novel magnetic materials, which are needed for application in highly efficient electric motors and generators. Our current interest is in rare-earth-free permanent magnets based on Mn-Al-C alloys. The performance of magnetic materials depends not only on their intrinsic magnetic properties but also on the microstructure over length scales from pm to cm. Understanding and controlling the complex interactions which arise is highly challenging and requires both high quality materials synthesis and state-of-the-art materials characterisation. We synthesize nano- and microcrystalline materials using a variety of melting, powder metallurgy and deformation techniques. We carry out materials characterisation using scanning and transmission electron microscopy (SEM and TEM), electron backscatter diffraction (EBSD), x-ray diffraction and a variety of physical properties measurement techniques including at applied magnetic fields of up to 14 T.
F. Bittner et al., Journal of Alloys and Compounds 727 (2017) 1095-1099
As τ-MnAl is a thermodynamically metastable phase, it tends to decompose into the equilibrium phases at elevated temperatures. This restricts the kind of processing which can be carried out. Preventing the decomposition of τ is therefore a critical factor in developing MnAl magnets. Here, the preferential nucleation of the equilibrium phases at general grain boundaries rather than other interfacial types is shown using electron backscatter diffraction measurements. This explains the higher resistance to decomposition of materials which contain low fractions of general grain boundaries.
A. Chirkova et al., Acta Materialia 131 (2017) 31-38
FeRh alloys undergo a magnetic transition from the antiferromagnetic state to the ferromagnetic state. The transition temperature has been shown to vary with prior heat treatment but the reason for this was unknown. In this paper, microstructural investigations showed that heat treaments led to different number density, size, shape and distribution of the secondary fcc phase. Finite element models indicated that stress fields from the secondary phase grains could overlap, thus influencing the transition temperature of the main phase through the well-known effect of pressure.
T. Mix et al., Acta Materialia 128 (2017) 160-165
Two different L10 phases can be made to coexist in alloys of the form Mn55Al45-xGax with 5 < x < 9. One appears to be thermodynamically stable, like binary MnGa, and the other is metastable, like binary MnAl, but in the ternary alloys, both phases contain only a few atomic percent of Ga. The thermodynamically stable L10 phase does not undergo a phase transformation at temperatures up to at least 700°C. These results enable longer processing times at higher temperatures thus facilitating the development of rare earth free MnAl-based magnets which are capable of providing a sustainable alternative to certain types of Nd-Fe-B.
The research of Rudolf Schäfer's group touches the rich world of magnetic microstructure or magnetic domains, extending from the nano-world to visible dimensions. The subject, which might be called „mesomagnetism“, forms the link between atomic foundations and technical applications of magnetic materials, ranging from computer storage systems to the cores of electrical machinery, and including novel research fields like spintronics and spinorbitronics. In collaboration with technology groups we are contributing to a fundamental understanding of magnetic domains and magnetization processes in all kinds of ferro-, ferri- and antiferromagnetic materials of current interest, mainly based on their experimental analysis by advanced magneto-optic imaging- and magnetometry techniques.
Ferroic materials comprise ferromagnetic, ferroelastic and ferroelectric materials. These functional materials react to external stimuli like temperature, magnetic or electric fields, and stress, which makes new functionalities possible. We cover the complete range of current scientific question, from fundamental aspects on the underlying principle, preparation of better materials to the implementation in novel devices and examine the following ferroic materials: We analyze (magnetic) shape memory alloys films, which are suitable for microactuators and use epitaxial films as a model system to understand the formation of the martensitic microstructure. To achieve a more efficient refrigeration, our research covers magnetocaloric films and multicaloric effects, which occur when straining magnetocaloric films by ferroelectric substrates. As an additional energy material, we examine thermomagnetic materials and their application in thermomagnetic generators and microsystems, which represents a promising approach for the conversion of low temperature waste heat to electricity.
Anja Waske, Daniel Dzekan, Kai Sellschopp, Dietmar Berger, Alexander Stork, Kornelius Nielsch & Sebastian Fähler
Nature Energy 4, pages 68–74 (2019)
To date, there are very few technologies available for the conversion of low-temperature waste heat into electricity. Thermomagnetic generators are one approach proposed more than a century ago. Such devices are based on a cyclic change of magnetization with temperature. This switches a magnetic flux and, according to Faraday’s law, induces a voltage. Here we demonstrate that guiding the magnetic flux with an appropriate topology of the magnetic circuit improves the performance of thermomagnetic generators by orders of magnitude. Through a combination of experiments and simulations, we show that a pretzel-like topology results in a sign reversal of the magnetic flux. This avoids the drawbacks of previous designs, namely, magnetic stray fields, hysteresis and complex geometries of the thermomagnetic material. Our demonstrator, which is based on magnetocaloric plates, illustrates that this solid-state energy conversion technology presents a key step towards becoming competitive with thermoelectrics for energy harvesting near room temperature.
Sebastian Fähler & Vitalij K. Pecharsky
MRS Bulletin 43(4), pages 264-268 (2018)
The fundamentals and applications of ferroic materials—ferromagnetic, ferroelectric, and ferroelastic—are common subjects discussed in just about every graduate course related to functional materials. Looking beyond today’s traditional uses, such as in permanent magnets, capacitors, and shape-memory alloys, there are worthwhile and interesting questions common to the caloric properties of these ferroic materials. Can ferroic materials be used in a cooling cycle? Why are these materials susceptible to external fields? Which combination of properties is required to make some of them suitable for efficient cooling and heat pumping? We address these questions in this introduction to ferroic cooling, which comprises magnetocaloric, electrocaloric, elastocaloric and barocaloric approaches and combinations thereof (i.e., multicalorics). These are addressed in greater detail in the articles in this issue.
Markus E. Gruner, Robert Niemann, Peter Entel, Rossitza Pentcheva, Ulrich K. Rössler, Kornelius Nielsch & Sebastian Fähler
Sci. Rep. 8, page 8489 (2018)
Heusler alloys exhibiting magnetic and martensitic transitions enable applications like magnetocaloric refrigeration and actuation based on the magnetic shape memory effect. Their outstanding functional properties depend on low hysteresis losses and low actuation fields. These are only achieved if the atomic positions deviate from a tetragonal lattice by periodic displacements. The origin of the so-called modulated structures is the subject of much controversy: They are either explained by phonon softening or adaptive nanotwinning. Here we used large-scale density functional theory calculations on the Ni2MnGa prototype system to demonstrate interaction energy between twin boundaries. Minimizing the interaction energy resulted in the experimentally observed ordered modulations at the atomic scale, it explained that a/b twin boundaries are stacking faults at the mesoscale, and contributed to the macroscopic hysteresis losses. Furthermore, we found that phonon softening paves the transformation path towards the nanotwinned martensite state. This unified both opposing concepts to explain modulated martensite.
Thermal Micro Energy Harvesting by Thermomagnetic Film Actuation
Manfred Kohl (KIT), Sebastian Fähler (DFG, FA 453/14), since 2013
Martensitic phase transformation and twinning in epitaxially grown Nickel Titanium films
Martin F.-X. Wagner (TU Chemnitz), Sebastian Fähler (DFG, FA 453/13), since 2017
Understanding magnetocaloric and electrocaloric effects using epitaxial films
Sebastian Fähler, Ruben Hühne (DFG FA 453/11), within SPP 1599, since 2012