Magnetic domain walls are useful functional elements that exhibit a magnetoresistive response in transport. This includes anisotropic magnetoresistance effects and in particular intrinsic domain wall magnetoresistance effects due to the changing magnetization direction. The intrinsic contribution is theoretically predicted to scale with the wall width, providing a way to tailor the contribution for devices. In a collaboration as part of the Transregional Collaborative Research Center (SFB/TRR) 173 “Spin+X – Spin in its collective environment” we have investigated these effects in a combined theory and experimental approach. Experimentally the magnetization gradient in the wall is controlled by tailoring the wall confinement in electromigrated Permalloy nanocontacts, revealing a positive intrinsic domain wall magnetoresistance that scales with the domain wall width. The observed scaling is supported by quantum-mechanical transport simulations based on ab-initio band-structure calculations which provide excellent agreement with the experimental results. The work is published in Physical Review B: https://journals.aps.org/prb/abstract/10.1103/PhysRevB.99.214437
Franziska Martin, together with research colleagues from Chile, Spain and the UK, won the IEEE Magnetics Society Summer School Project Award with their project “A non-collinear antiferromagnet for energy reclamation and memory applications”. Within this future collaborative work, the non-collinear antiferromagnet Mn3SnN will be investigated with regard to the anomalous Hall effect, anomalous Nernst effect and magneto-optical Kerr effect.
Computing with diffusion: Our work shows how the diffusion of magnetic nanostructures, so-called skyrmions, can be employed in so-called probabilistic computing.
Probabilistic computing is a novel approach for information technology applications. It relies on random sequences of the fundamental data bits 1 and 0, with the relative probabilities of the two being used to represent rational numbers. However, until now, a key element for the realization of probabilistic computing was missing: The bit sequences has to be randomized – reshuffled – to decorrelate the information strings and allow for the probabilistic element of the approach to kick in. This missing key element could be realized by utilizing skyrmions in a spintronic device that are able to diffuse freely if no or only weak drives are applied. Such a system was now reported in our work in Nature Nanotechnology (http://dx.doi.org/10.1038/s41565-019-0436-8) and directly applied to a reshuffling device. The random thermally activated diffusion fulfills the requirements for probabilistic computing and thus allows for the construction of an application relevant device. The work resulted from a collaboration with theory colleagues in Mainz and Konstanz.
More information at: http://www.uni-mainz.de/presse/aktuell/8323_DEU_HTML.php
The group held its regular group retreat to identify new research directions and enhance collaborations at Oppdal. We have been fortunate to be this year invited by the Centre for Quantum Spintronics at NTNU in Trondheim (https://www.ntnu.edu/web/quspin/) for a joint retreat to Norway. With in-depth discussions many new ideas for collaborative projects were born and we exchanged ideas to identify exciting topics for future work. In addition to the intense scientific work during the seminars, we have also had a great time hitting the slopes and various winter sports activities.
Two new relevant videos have been released with contributions from our group. The IEEE Magnetics Society has on the IEEE.TV a new movie on Magnetism and Magnetics Technology in the 21st Century. This general outreach video includes footage from our lab and introduces modern spintronics concepts as well as magnetism applications from memory to medical imaging.
Have a look here: https://ieeetv.ieee.org/technology/magnetics-technology-21st-century?rf=channels|70&.
Furthermore our recent work on a staggered domain wall memory concept (Physical Review Applied 11, 024034 (2019) https://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.11.024023) is also now featured as a video. The work explores a new idea to selectively position domain walls as required for a memory device based on walls moving along a wire. Using staggered wire segments well defined pinning positions for domain walls can be fabricated, which is shown in this combined theoretical and experimental study.
For the video, have a look here: https://www.youtube.com/watch?v=gulUD96qznM.
The talk “Electrical reading-out and writing of antiferromagnetic insulators” by Lorenzo Baldrati was selected for the “Roberta Ciprian” best oral presentation award at the conference MAGNET 2019. The work demonstrates by electrical measurements and direct x-ray imaging that electrical currents can switch antiferromagnetic materials and proposes a model for the switching (https://arxiv.org/abs/1810.11326 ). L. Baldrati is currently a postdoctoral Marie Skłodowska-Curie fellow in charge of the project AntifeRromagnetic spin Transport and Switching (Twitter: #ARTES_H2020).
Benjamin Borie carried out a PhD between the group at Johannes Gutenberg-University Mainz and Sensitec as part of the EU funded Training Network WALL. He developed sensors that allow for the detection of multiple rotations.
For his work that resulted not only in commercially relevant findings but also three first author publications in leading journals he was awarded the first prize by the Chamber of Commerce.
More information can be found (in German) here [https://www.allgemeine-zeitung.de/lokales/rheinhessen/ihk-verleiht-preise-fur-spitzen-abschlusse_19903565 ].
Chamber of Commerce Representatitves award the best PhD Prizes 2018.
Our recent publication in Physical Review B entitled “Direct observation of spin diffusion enhanced nonadiabatic spin torque effects in rare-earth-doped permalloy” has been featured in the Kaleidoscope section of the journal homepage: https://journals.aps.org/prb/kaleidoscope
The work is a collaborative project with the Technical University of Kaiserslautern as part of the Spin+X collaborative research centre ( https://www.uni-kl.de/trr173/ ), and also includes data taken at the Helmholtz-Zentrum Dresden-Rossendorf.
In the paper we investigate the relationship between two important parameters for magnetization dynamics, namely the non-adiabaticity and the damping, via direct imaging of current-induced excitations of magnetic vortices in doped Permalloy discs using scanning electron microscopy with polarization analysis. We demonstrate different contributions to the enhanced non-adiabaticity in the system and show that both parameters exhibit a similar scaling with low-level rare-earth doping, providing an avenue for tailoring the dynamic properties.
The publication is available here:
Our new perspective article,” Magnetic skyrmions—Overview of recent progress in an active research field”, has been selected as the Cover Page for the latest issue of Journal of Applied Physics. In the article we highlight some of the recent developments in the field of skyrmionics, which has developed from a niche prediction to a huge and active research field within the last decade. In particular, magnetic skyrmions have become promising candidates as information carriers for next generation spintronic devices. In the piece, we describe some of the recent progress in efficiently creating and manipulating these magnetic quasiparticles and review some of the proposed device applications where skyrmions may be employed, as well as offering up our perspectives on some of the future challenges and opportunities presented by this field.
More detailed information can be found in the publication in Journal of Applied Physics at:
Congratulations to Joel Cramer who obtained a distinction in his PhD on the "Propagation, manipulation and detection of magnonic spin currents in magnetic oxides and metals". In his studies, he investigated different aspects of magnon spintronics principles, among which the magnetization-dependent detection of spin currents in a metallic ferromagnet has been published in the scientific journal Nature Communications.
In this article we determine the energetics of different topological phases in magnetic systems. We examine current‐induced generation of skyrmions in heavy‐metal/ferromagnet multilayers and show that Joule heat pulses can drive topological transitions in magnetic textures and enable skyrmion creation on nanosecond timescales. The 3D renderings for the skyrmion and domain structure of this work is featured on the front cover of the Advanced Materials journal ( https://onlinelibrary.wiley.com/toc/15214095/2018/30/49 )
We show that the antisymmetric Dzyaloshinskii-Moriya interaction is modified by driving currents. We measure and analyze the chirality of Dzyaloshinskii-Moriya-interaction (DMI) stabilized spin textures in multilayers of Ta/Co20F60B20/MgO. The effective DMI is measured experimentally using domain wall motion measurements, both in the presence (using spin-orbit torques) and absence of driving currents (using magnetic fields). We observe that the current-induced domain wall motion yields a change in effective DMI magnitude and opposite domain wall chirality when compared to field-induced domain wall motion (without current). We explore this effect, which we refer to as current-induced DMI, by providing possible explanations for its emergence, and explore the possibility of its manifestation in the framework of recent theoretical predictions of DMI modifications due to spin currents.
This is a collaborative work with the recently started group of Prof. Y. Mokrousov who is working on the theory of topological nanoelectronics and co-funded by a joint grant.
Congratulations to Kai Litzius who obtained a distinction in his PhD on the "Spin-Orbit-Induced Dynamics of Chiral Magnetic Structures". The jury was impressed with the high quality of the scientific results. Dr. Litzius is a co-author of a publication in Nature Materials dedicated to the observation of room temperature magnetic Skyrmions. Further, he revealed the Skyrmion Hall effect as reported in his first-author publication in Nature Physics.
Conventional devices using current CMOS based technologies have the unwelcome side effects of getting too hot and being limited in their speed, operating at GHz frequencies. Eventually, this is slowing down the progress of information technology. In the last years, the emerging field of “magnon spintronics” aimed at using insulating magnets capable of carrying magnetic waves, known as magnons, to solve these problems. Magnons are able to carry information at increased speeds without the production of excess heat. However, experimental observations had so far been limited to ferromagnetic materials. In collaboration with the Quantum Spintronics at NTNU and Utrecht University, our group has demonstrated that magnons can also efficiently carry spin information in antiferromagnets, the largest group of magnetic materials. This class of material has several crucial advantages over ferromagnetic components as they are stable and unaffected by external magnetic fields, a key requirement for future data storage. Additionally, antiferromagnet based devices can be potentially operated thousands of times faster than current technologies, as their intrinsic dynamics are in the THz range. As a result, antiferromagnetic magnons could thus be used in future ultra-fast and low power technological devices.
An electrical current in a platinum wire (left) creates a magnetic wave in the antiferromagnetic iron oxide (red and blue waves). This is measured as a voltage in a second platinum wire (right). The red and blue arrows represent the antiferromagnetic order of the iron oxide (© Joel Cramer)
More information can be found at: https://www.nature.com/articles/s41586-018-0490-7
Magnetic imaging allows the direct investigation of magnetic states. Especially the switching behavior of nanoscale devices is of interest because of potential future application as storage media. To image the magnetization dynamics, time-resolved imaging techniques with high spatial resolution are needed. We have accordingly developed a laboratory-based technique in cooperation with the spin-off company Surface Concept GmbH based on a commercial scanning electron microscope with polarization analysis (SEMPA). The inherent spatial resolution is <20 nm while the time resolution of the new system is better than 2 ns. To overcome the challenge of the low signal-to-noise ratio of this technique in addition a phase-sensitive detection mode was implemented for measurements with periodic excitations, where the magnetization is changing proportionally to the excitation. The research is part of the collaborative research center SPIN+X, spin in its collective environment, project B02. More information can be found at: Review of Scientific Instruments 89, 083703 (2018)
Spintronics concepts developed in modern magnetism research prevalently rely on the efficient transfer of angular momentum between interfacing subsystems. In particular, the spin information exchange between magnetically ordered insulators and conductive systems, both magnetic and non-magnetic, has been studied extensively in recent years to obtain a comprehensive understanding of the processes involved. Among others, the time scale, on which the transfer of angular momentum occurs, is decisive for the achievable data processing rates in prospective application schemes. An international team of physicists, including researchers at the Fritz Haber Institute in Berlin and the Johannes Gutenberg University Mainz, now succeeded in determining a lower time limit of the spin exchange across a magnetic insulator/metal interface. In a bilayer comprising the yttrium iron garnet Y3Fe5O12 and the heavy metal platinum, ultrafast spin current flow is generated by means of a pulsed laser heating technique. Evaluating the rise time of the spin current from detected THz responses reveal a spin correlation time of ~4 fs, implying an almost instantaneous transfer of spin informationema across the interface.
More information can be found at: Nature Communications 9, 2899 (2018)
Electric devices based on antiferromagnetic materials entail the potential to overcome the performance of conventional electronic devices in terms of speed, bit density and robustness against external magnetic fields, thanks to the particular properties of this class of materials. Such devices might be used in the future as building blocks for credit cards and devices than cannot be wiped by magnetic fields, high-speed memory and logic devices and high-capacity memory storage drives. However, at present, reading and writing bits of information electrically in antiferromagnets are challenging operations, especially in antiferromagnetic insulators, where a charge current cannot flow. In the publication “Full angular dependence of the spin Hall and ordinary magnetoresistance in epitaxial antiferromagnetic NiO(001)/Pt thin films” our group, in collaboration with the groups of J. Sinova at the JGU Mainz, E. Saitoh at the Tohoku University (Japan) and A. Kleibert at the Paul Scherrer Institute (Switzerland), has shown that it is possible to electrically read information in bilayers of antiferromagnetic NiO(001) thin films, the most stable NiO orientation, and the heavy metal Pt, by a mechanism called spin Hall magnetoresistance. We also show that strong magnetic fields (several Teslas) induce redistribution of the antiferromagnetic domains in NiO, which can be read electrically. Our results are a step toward the reliable and efficient electrical read-out of information in devices based on antiferromagnetic insulators.Additional details can be found in the publication in Physical review B, accessible at: https://link.aps.org/doi/10.1103/PhysRevB.98.024422
The poster “Current-Induced Skyrmion Generation Through Morphological Phase Transitions in Chiral Ferromagnetic Heterostructures” by Nico Kerber was selected for a best poster award at the “IEEE Magnetics Society Summer School 2018”.The work demonstrates by using high resolution x-ray microscopy that in thin films transient Joule heating can drive morphological phase transitions.
Using graphene as a light-sensitive material for light detectors can offer significant improvements with respect to materials being used nowadays. For example, graphene can detect light of almost any colour, and it gives an extremely fast electronic response within one millionth of a millionth of a second.
Published recently in Science Advances, the work gives a thorough explanation of why, in some cases, the graphene conductivity increases after light absorption and in other cases, it decreases. The researchers show that this behaviour correlates with the way in which energy from absorbed light flows to the graphene electrons: After light is absorbed by the graphene, the processes through which graphene electrons heat up happen extremely fast and with a very high efficiency.
For highly doped graphene, ultrafast electron heating leads to carriers with elevated energy – hot carriers – which, in turn, leads to a decrease in conductivity. Interestingly enough, for weakly doped graphene, electron heating leads to the creation of additional free electrons, and therefore an increase in conductivity.
More detailed information can be found in the publication in Science Advances accessible at: doi:10.1126/sciadv.aar5313
The group has received a 10.000 Euro donation by Mercedes - Benz to support work on the conversion of waste heat into useful electric energy. Compared to conventional thermoelectric cells, spin-caloritronic approaches enable possibly a simpler approach that allows for the generation of a sizeable voltage from a temperature gradient in a monolithic cell. Due to the fact that the vast majority of the energy in a conventional combustion engine is converted not into mechanical power but is lost as waste heat, such approaches are heavily investigated for industrial applications. However also the fundamental mechanisms that generate spin currents from heat currents are under heavy investigation with the important effects of the interaction of magnons and phonons being studied in detail in order to maximize the conversion efficiency.
The Marie Skłodowska-Curie individual fellowship is a competitive research grant awarded to excellent internationally-mobile young scientists by the European Union. Each fellow receives funding for 24 months, to work on his/her own research proposal. This year Dr. Lorenzo Baldrati was awarded with such a Marie Skłodowska-Curie fellowship in the Kläui lab. His fellowship adds to the ones of Dr. Romain Lebrun and Dr. Kyujoon Lee, who are already members of the same group.
The research projects of the three fellows focus on different aspects of spintronics, aiming to advance information technology by taking advantage of a property of matter related to magnetism known as “spin”. In 2016, Dr. Kyujoon Lee started his project on spin-orbit torques and Dzyaloshinskii-Moriya interactions. The following year Dr. Romain Lebrun joined the group, initiating a project on magnetization dynamics and transport in antiferromagnetic insulators. Starting this year, Dr. Lorenzo Baldrati will be working on the transport of spin currents and voltage control of antiferromagnetic materials. We are grateful to the EU for funding our Marie Skłodowska-Curie fellows in the research area of spintronics for three consecutive years.
From left to right: Dr. Romain Lebrun, Dr. Kyujoon Lee, Dr. Lorenzo Baldrati
The implementation of logic operations and thus information processing by means of spin wave (magnon) spin currents is a fundamental goal of the emerging research field of magnon spintronics. In contrast to electrical currents, on which todays information technology is based, magnon spin currents do not conduct electrical charges but magnetic momenta. So far, experimental demonstrations of magnon logic, e.g. a magnon transistor or majority gate, were based on either the manipulation or superposition of spin waves during the propagation phase. In a collaboration with the group of Prof. Ulrich Nowak from the University of Konstanz and Prof. Eiji Saitoh from the Tohoku University in Sendai, Japan, our group was now able to add a further element to the construction set of magnon logic directly at the detection site of magnon spin currents. In a ferroic spin valve structure including insulating as well as metallic ferromagnets and antiferromagnets, it was possible to demonstrate a magnon detection efficiency which depends on the magnetic configuration of the spin valve. In that manner, the suppression or transmission of the incoming magnon signal can be controlled.
More detailed information can be found in the publication in Nature Communications accessible at: doi:10.1038/s41467-018-03485-5
magnon transistors could give spintronics a boost
Focus: A Trio of Magnon Transistors
The efficient generation and detection of spin currents are in the focus of current research in the field of spintronics as they are a crucial ingredient for next-generation, energy-efficient information devices. Instead of exploiting the electron charge, information is transferred and processed by spin angular momentum. Among other techniques, the spin Hall effect and its inverse enable an effective spin-charge interconversion and furthermore the feasible integration into existing charge-based concepts. In a collaboration with the research group of Tobias Kampfrath (Freie Universität Berlin/Fritz Haber Institute of the Max Planck Society, Berlin) our group was now able to investigate the inverse spin Hall effect in copper-iridium (CuIr) alloys for various alloy compositions. We furthermore compared the spin-to-charge conversion of continuous (DC) and ultrafast (THz) spin currents. While CuIr in general reveals a complex dependence of the inverse spin Hall effect on the Ir content in the alloy, coinciding results were obtained for the DC and THz signals. The latter signifies a feasible transfer of established spintronic concepts in the ultrafast regime.
More detailed information can be found in the publication in Nano Letters at:
Felix Büttner received at the 2018 Advances in Magnetics Conference organized by the Italian Section of the IEEE the Best Young Researcher Award. He was picked from 5 finalists that were selected to present invited talks at a special Young Researchers Session of the conference based on his major achievements in the field of chiral spin structures and spin dynamics.Felix Büttner was a member of the Graduate School of Excellence Materials Science in Mainz and received his PhD at the end of 2013 on Nanomagnetism and Spin Dynamics. He is currently a postdoctoral researcher at the Massachusetts Institute of Technology.
Antiferromagnets for spin based information technology
Demonstration of technologically feasible read-out and writing of digital information in antiferromagnets –basic principle for ultrafast and stable magnetic memory
Within the emerging field of spin based electronics (spintronics), information is typically defined by the orientation of the magnetization of ferromagnets. However, recently the utilization of antiferromagnets, materials without macroscopic magnetization but with a staggered orientation of their microscopic magnetic moments (see figure), has been considered. In this framework the information is encoded in the direction of the modulation of the magnetic moments (Néel vector). In principle, antiferromagnets enable much faster information-writing and are very stable with respect to disturbing external fields. However, these advantages also imply a challenging manipulation and read-out processes of the Néel vector orientation. Up to now, this was possible using the semimetal CuMnAs only (P. Wadley et al. Science 351, 587 (2016)), a compound which features several disadvantages concerning applications such as toxic components, a relatively low magnetic ordering temperature, low conductivity and a relatively small magnetoresistance, on which the read-out of the Néel vector is based.
As published in the online science journal Nature Communications, we were now able to demonstrate current induced switching of the Néel vector also for metallic thin films of the compound Mn2Au, which order antiferromagnetically already a high temperatures. In particular, we measured a 10-fold larger magnetoresistance as observed for CuMnAs, which is explained by extrinsic scattering on excess gold atoms as deduced from calculations. By this we identified Mn2Au as a prime candidate to enable future antiferromagnetic spintronics.
More information can be found at:
Nature Communications 9, 348 (2018)
Coherent magnonic logic is an emerging field of research that allows potentially for carrying out complex computation with comparatively simple physical implementations. In contrast to incoherent logic gates, using coherent spin waves allow for using the phase degree of freedom of the spin waves. In contrast to previous approaches of exciting coherent spin waves by AC excitations, we show how the interference between superfluid spin currents that can be generated by DC excitations can endow spin circuits with coherent logic functionality. While the hydrodynamic aspects of the linear-response collective spin transport obviate interference features, we focus on the nonlinear regime, where the critical supercurrent is sensitive to the phase accumulated by the condensate in a loop geometry. We propose to control this phase by electrical gating that tunes the spin-condensate coherence length. The nonlinear aspects of the spin superfluidity thus naturally lend themselves to the construction of logic gates, uniquely exploiting the coherence of collective spin currents. Vice versa, this functionality can be used to reveal the fundamental properties of spin superfluids.
More information can be found at doi: 10.1103/PhysRevLett.119.187705
In the annual elections of the IEEE Magnetics Society, Mathias Kläui was elected as one of the 8 new members from all over the world for the Administrative Committee. The Administrative Committee is responsible for the operations of the IEEE Magnetics Society, which is the leading international professional organization for magnetism. The term is initially for three years starting 2018. Amongst other activities, the IEEE Magnetics Society organizes leading conferences, such as Intermag and publishes a range of journals on magnetism topics.
Controlling the magnetic properties of materials is fundamental for developing memory, computing and communication devices at the nanoscale. As data storage and processing are evolving quickly, researchers are testing different new methods to modify magnetic properties of materials. One approach relies on elastic deformation (strain) of the magnetic material to tune its magnetic properties, which can be achieved by electric fields. This scientific area has attracted much interest due to its potential to write small magnetic elements with a low power electric field rather than magnetic fields that require high power charge currents. However, studies so far have mainly been done at very slow time scales (seconds to milliseconds).
One way to produce rapid (i.e. subnanosecond scale) changes of strain and, thus, induce magnetization changes is by using surface acoustic waves (SAWs), which are deformation (strain) waves. In a collaboration with groups from Berlin, Switzerland and Spain where a former PhD student from our group leads this effort, we have used a new experimental technique to quantitatively image these SAW and demonstrate that they can be used to switch the magnetization in nanoscale magnetic elements on top of the crystal. Results show that the magnetic squares changed their properties under the effect of SAWs on ultra-fast timescales growing or shrinking the magnetic domains depending on the phase of the SAW.
In a more applied project, we have studied the key domain-wall properties in segmented nanowire loop-based structures used in domain-wall-based sensors. The potential of magnetic-domain-wall-based sensors is due to their many attractive attributes compared to other technologies. Magnetic domain walls can be stable well above room temperature, making them a potential candidate to store data and to be used for nonvolatile sensing, and they can be displaced rapidly in an application’s relevant geometries. The two reasons for device failure, namely, distribution of the domain-wall propagation field (depinning) and the nucleation field were determined with magneto-optical Kerr effect and giant-magnetoresistance (GMR) measurements for thousands of elements to obtain significant statistics. Single layers of Permalloy and a complete GMR stack are deposited and industrially patterned to determine the influence of the shape anisotropy, the magnetocrystalline anisotropy, and the fabrication processes. Using the GMR effect in a substantial number of devices (3000) allows us to accurately gauge the variation between devices. This measurement scheme reveals a corrected upper limit to the nucleation fields of the sensors that can be exploited for fast characterization of the working elements.
The work was carried out in collaboration with industrial partners as part of EU funded projects. In particular a long-standing collaboration with Sensitec GmbH in Mainz, where a number of previous group members work has led to major joint activities. For the collaboration including the TU Kaiserslautern the State Innovation Prize was awarded.
The publication in Physical Review Applied is available at: doi: 10.1103/PhysRevApplied.8.024017.
In the 2017 Shanghai-Ranking the Physics Department at Johannes Gutenberg University Mainz (JGU) has continued its strong performance. The ranking at positions 4-6 in Germany and in the top 75 universities in the world is mirroring the success in the German Excellence Initiative where Physics at JGU is the only department in Germany that houses both a Cluster of Excellence and a Graduate School of Excellence. In particular the resulting increasing number of high impact publications and strong international collaborations have resulted in this excellent ranking, which is also in line with the results of other rankings. More details can be found at:
We held our regular group retreat to identify new research directions and enhance collaborations at Kuralpe Kreuzhof. We have been fortunate to have Prof. E. Saitoh and Prof. G. Bauer from Tohoku University as our external experts advising us on our research program for this retreat. In addition to intense scientific work, we have also had a great time visiting the "Felsenmeer" and various sports activities.
We are happy to welcome Prof. E. Saitoh from Tohoku University for a sabbatical hosted by our group. As part of the networking activities within the collaborative research center Spin+X and the DAAD Network MaHoJeRo we are looking forward to expanding our fruitful interactions in the field of spin current physics. Prof. Saitoh is also the 2017 IEEE Distinguished Lecturer and previously staff and student exchange supported by DAAD and japanese projects has forged strong links between the groups. For recent joint work on the magnon spin valve, see arxiv:1706.07592.