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.
712. WE-Heraeus Seminar, Physikzentrum Bad Honnef, 26 Jan - 29 Jan 2020
Organized by Dr. Karin Leistner, IFW Dresden • Prof. Jordi Sort Viñas, U Barcelona, Spain • Dr. Robert Kruk, Karlsruhe Institute of Technology
The seminar aims at providing a forum for an overview of the current understanding of ionic effects in magnetoelectric materials. Fundamental ionic mechanisms and their correlation with magnetic phenomena, utilization of the spatial resolution achieved by advanced interface-sensitive measurement techniques, and routes toward the implementation of nanodevices will be covered. For more detailed information see URL.
Application deadline is 22 November 2019
by Jonas Zehner, Rico Huhnstock, Steffen Oswald, Ulrike Wolff, Ivan Soldatov, Arno Ehresmann, Kornelius Nielsch, Dennis Holzinger and Karin Leistner
Electric manipulation of exchange bias (EB) systems is highly attractive for the development of modern spintronic and magnetophoretic devices. To date, electric control of the EB has mainly been based on multiferroic or resistive switching behavior in specific antiferromagnets, which limits the material choice and accessible EB states. In addition, the effects are mostly volatile, requiring constant voltage application. The continuous and nonvolatile tuning of the EB via electrochemical manipulation of the ferromagnetic layer is presented. In FeOx/Fe/IrMn systems, large changes in the EB field of fully shifted magnetization curves are achieved at low voltage (<1 V) and room temperature. A ferromagnetic‐layer thickness change resulting from the electrochemical reduction of iron oxide to iron is proposed as the underlying mechanism and is consistent with a simple model for the EB and surface analysis. Nonvolatility is achieved as the reduction proceeds at the buried FeOx/Fe interface, leaving the remaining oxide as a protective layer. A lateral voltage‐controlled patterning of the EB fields and magnetic domain state is demonstrated. This versatile redox‐based electric control of the EB paves a new route for the design of EB systems in general and for the development of future electrically controlled EB devices.
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.
M. Gusenbauer et al., Journal of Applied Physics 129, 093902 (2021)
MnAl-C is a prominent candidate for the replacement of rare earth magnets with a moderate energy density product. Crystallographic defects have a strong effect on magnetization properties. In this work, we show the influence of twinning defects in the nanometer regime on the quality of the magnet. Standard micromagnetic simulations and computations of the saddle point configuration for magnetization reversal highlight the importance of optimizing the fraction of and reducing the width of crystallographic twin defects. Switching field distributions and the maximum possible coercive field for ideal microstructures without defects are estimated using a reduced order micromagnetic model.
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.
Stefan Schwabe, Robert Niemann, Anja Backen, Daniel Wolf, Christine Damm, Tina Walter, Hanuš Seiner, Oleg Heczko, Kornelius Nielsch, and Sebastian Fähler
Adv. Funct. Mater. 2005715 (2020)
Martensitic materials show a complex, hierarchical microstructure containing structural domains separated by various types of twin boundaries. Several concepts exist to describe this microstructure on each length scale, however, there is no comprehensive approach bridging the whole range from the nano- up to the macroscopic scale. Here, it is described for a Ni-Mn-based Heusler alloy how this hierarchical microstructure is built from scratch with just one key parameter: the tetragonal distortion of the basic building block at the atomic level. Based on this initial block, five successive levels of nested building blocks are introduced. At each level, a larger building block is formed by twinning the preceding one to minimize the relevant energy contributions locally. This naturally explains the coexistence of different types of twin boundaries. The scale-bridging approach of nested building blocks is compared with experiments in real and reciprocal space. The approach of nested building blocks is versatile as it can be applied to the broad class of functional materials exhibiting diffusionless transformations.
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 Bulletin43(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.
Benjamin Schleicher, Robert Niemann, Stefan Schwabe, Ruben Hühne, Ludwig Schultz, Kornelius Nielsch & Sebastian Fähler
Sci. Rep.7, page 14462 (2017)
Tuning functional properties of thin caloric films by mechanical stress is currently of high interest. In particular, a controllable magnetisation or transition temperature is desired for improved usability in magnetocaloric devices. Here, we present results of epitaxial magnetocaloric Ni-Mn-Ga-Co thin films on ferroelectric Pb(Mg1/3Nb2/3)0.72Ti0.28O3 (PMN-PT) substrates. Utilizing X-ray diffraction measurements, we demonstrate that the strain induced in the substrate by application of an electric field can be transferred to the thin film, resulting in a change of the lattice parameters. We examined the consequences of this strain on the magnetic properties of the thin film by temperature- and electric field-dependent measurements. We did not observe a change of martensitic transformation temperature but a reversible change of magnetisation within the austenitic state, which we attribute to the intrinsic magnetic instability of this metamagnetic Heusler alloy. We demonstrate an electric field-controlled entropy change of about 31 % of the magnetocaloric effect - without any hysteresis.
Coupling Effects in Re-Programmable Micro-Matter
Sebastian Fähler (DFG, SPP 2206), Manfred Kohl (KIT), Frank Wendler (FAU), 2020-2023
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