In the 2021 Funding Atlas of the German Research Foundation (DFG), Johannes Gutenberg University Mainz (JGU) garners first place in the field of Physics by a clear margin. In this discipline alone, Mainz University received some EUR 45 million in funding from the DFG in the years 2017 to 2019, more than any other university in Germany.
More information can be found here: https://www.uni-mainz.de/presse/aktuell/14290_ENG_HTML.php.
At the Trends in Magnetism 2021 Conference Isabella Boventer was selected as a finalist for the young scientist award. Isabella carried out her PhD in our group on Cavity Magnon-Polariton Spectroscopy and then she moved on to a Postdoc position in Paris. Congratulations to Isabella!
At the same conference Mathias Kläui was invited for a distinguished lecture on Antiferromagnetic Insulatronics.
In this year's Shanghai ranking we have again been ranked in top 5 in Germany and top 75 worldwide. Johannes Gutenberg University Mainz (JGU) has consistently achieved a top spot for physics in the major rankings in the recent past (#1 in the German Research Foundation's recent ranking, top 5 or top 10 in Shanghai and Nature Index ranking, etc.).
With a large number of national collaborative research centres, the Max Planck Graduate Centre and the Cluster of Excellence PRISMA physics hosts major research activities.
In her PhD thesis entitled “Electric field-induced strain control of magnetism in in-plane and out-of-plane magnetized thin films”, Mariia demonstrated that electric field-induced strain is a promising tool for energy efficient manipulations of nanoscale magnetic structures. It ensures reliable control of not only the static magnetic configuration, but also the magnetodynamic response of the system. This approach can potentially be combined with other means, for example spin torques, and implemented in more advanced device architectures.
Mariia and four other awardees received their certificates from JGU President Prof. Dr. Georg Krausch at this year’s corona-related small-scale award ceremony on June 30th.
More information can be found here (in German): https://www.gnk.uni-mainz.de/dies-academicus/preisverleihung-2021.
JGU President Prof. Dr. Georg Krausch and Mariia Filianina. Photo by “Agentur Sämmer”
Antiferromagnets are particularly promising for next generation magnetic devices as their dynamics is orders of magnitude faster than for conventional ferromagnets. Furthermore they are largely immune to stray fields thus being promising for ultra-stable memory devices. To understand the dynamics, in a collaboration led by former Mainz PhD student I. Boventer with colleagues from the Centre of Excellence for Quantum Spintronics, we studied the dynamics of the antiferromagnet hematite, the main component of "rust". In the work "Room-Temperature Antiferromagnetic Resonance and Inverse Spin-Hall Voltage in Canted Antiferromagnets", we could demonstrate that we can generate large electrical signal resulting from the antiferromagnetic spin dynamics thus allowing for electrical detection of the dynamics. The electrical signal furthermore allows us to identify the mode handedness of the dynamic eigenmode.
The full text of the publication in Physical Review Letters is accessible at: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.126.187201.
Understanding magnetoresistance effects at THz frequencies is key to using magneto-resistivie devices at ultra-high frequencies. In a collaboration led by the research group of Tobias Kampfrath (Freie Universität Berlin/Fritz Haber Institute of the Max Planck Society, Berlin), we have studied the anisotropic magnetoresistance at frequencies from the DC regime to tens of THz. We find that we can explain the frequency dependence in Ni and Co we identify both intrinsic as well as extrinsic contributions and we find a sizeable magnetoresistance even at the highest frequencies. This demonstrates that magneto-resistive devices can operate at tens of THz frequencies orders of magnitude faster than current microelectronics devices.
The full text of the publication in Physical Review X is accessible at: https://journals.aps.org/prx/abstract/10.1103/PhysRevX.11.021030.
Movement of skyrmions is heavily impacted by geometric confinement of skyrmion positions. Better understanding this impact is required for applications such as Brownian computing and skyrmion-based information storage. In our recent work in collaboration with the theory group of PD Dr. Peter Virnau, titled “Commensurability between Element Symmetry and the Number of Skyrmions Governing Skyrmion Diffusion in Confined”, we performed experiments and computer simulations of skyrmion movement in different geometric confinements. For example, using a triangular confinement, both in experiments and simulations skyrmions diffused less for commensurate states of 3, 6 or 10 skyrmions which were regularly ordered compared to different skyrmion numbers that do not fit into the triangle in an ordered manner.
Additional information can be found in the press release on the JGU website: https://www.uni-mainz.de/presse/aktuell/13181_ENG_HTML.php (English) or https://www.uni-mainz.de/presse/aktuell/13181_DEU_HTML.php (Deutsch).
The full text of the publication in Advanced Functional Materials is accessible at: https://onlinelibrary.wiley.com/doi/10.1002/adfm.202010739.
ZEIT Campus published an article about the top research of the JGU Mainz, highlighting projects like TopDyn. In this initiative we are striving to find sustainable chemical solutions for material development, raw material supply, energy conversion and production processes.
Congratulations to Andrew Ross, who obtained a distinction in his PhD on "Probing Magnetostatic and Magnetotransport Properties of the Antiferromagnetic Iron Oxide Hematite". In his studies, he showed the extraction of magnetic anisotropies for insulating antiferromagnets from surface sensitive measurements and pioneered the long-distance transport of antiferromagnetic magnons. The latter results led to first-author publications in Nature and Nano Letters and a co-authored publication in Nature Communications.
The poster “Orbital decomposition of Yu-Shiba-Rusinov resonances from magnetic impurities in multiband superconducting Pb” by Tom G. Saunderson was selected for a best poster award at the “736. Wilhelm und Else Heraeus-Seminar on Magnetism at the Nanoscale: Imaging - Fabrication – Physics 2021”. The work focuses on Tom’s PhD at the University of Bristol, completed in October 2020, which investigates the effects of magnetic impurities on bulk and surface superconducting Pb from first principles. The orbital character of the ensuing Yu-Shiba-Rusinov bound states are assessed, making contact to experimental observations. This research was conducted under the supervision of Dr. Martin Gradhand and funded by the UKRI engineering and physical research council (https://www.epsrc.ukri.org) through the centre for doctoral training in condensed matter physics (https://www.cdt-cmp.ac.uk).
Congratulations to Mariia Filianina, who obtained a distinction in her PhD on the "Electric field-induced strain control of magnetism in in-plane and out-of-plane magnetized thin films". In her studies, she investigated the impact of energy-efficient strain on various magnetic phenomena, such as magnetic vortex dynamics and spin-orbit torques. The latter results led to a first author publication in Physical Review Letters.
A joint research project of Johannes Gutenberg University Mainz (JGU), the University of Siegen, Forschungszentrum Jülich, and the Elettra Synchrotron Trieste has achieved a new milestone for the ultra-fast control of magnetism. The international team has been working on magnetization configurations that exhibit chiral twisting. Magnetization configurations with a fixed chirality are currently investigated intensively due to their fascinating properties such as enhanced stability and efficient manipulation by current. These magnetic textures thus promise applications in the field of ultrafast chiral spintronics, for example in ultrafast writing and controlling of chiral topological magnetic objects such as magnetic skyrmions, i.e., specially twisted magnetization configurations with exciting properties.
The new insights published in Nature Communications shed light on the ultrafast dynamics after optical excitation of chiral spin structures compared to collinear spin structures. According to the researchers' findings, the chiral order restores faster compared to the collinear order after excitation by an infrared laser.
The research team performed small angle x-ray scattering experiments on magnetic thin film samples stabilizing chiral magnetic configurations at the free electron laser (FEL) facility FERMI in Trieste in Italy. The facility provides the unique possibility to study the magnetization dynamics with femtosecond time resolution by using circular left polarized or right polarized light. The results indicate a faster recovery of chiral order compared to collinear magnetic order dynamics, which means that twists are more stable than straight magnetic configurations.
Additional information can be found in the press release on the JGU website:
The full text of the publication in Physical Review Letters is accessible at: https://www.nature.com/articles/s41467-020-19613-z
Figure: Incoming circular left polarized (CL) and right polarized (CR) x-ray pulses scatter differently on chiral magnetic domain walls, leading to an asymmetry observed in the difference signal (CL-CR).
Enabling magnonic devices centred on antiferromagnetic materials requires robust magnon propagation at room temperature. Previously, magnon transport has been shown in the antiferromagnetic phase of iron oxide, hematite, but at temperatures far below room temperature. These temperatures are needed to stabilise an easy-axis anisotropy which leads to circularly polarized magnons capable of transporting angular momentum. In our recent work titled “Long-distance spin-transport across the Morin phase transition up to room temperature in ultra-low damping single crystals of the antiferromagnet α-Fe2O3” we demonstrate that the propagation of magnons persists from low temperatures to room temperature, through the Morin transition to an easy-plane anisotropy where circularly polarised magnons cannot propagate. We find that a superposition of linearly polarised magnon modes approximating a net circular polarisation enables long-distance magnon transport.
Additional information can be found in the press release on the JGU website: https://www.uni-mainz.de/presse/aktuell/12744_ENG_HTML.php (English) or https://www.uni-mainz.de/presse/aktuell/12744_DEU_HTML.php(Deutsch)
The full text of the publication in Nature Communications is accessible at: https://www.nature.com/articles/s41467-020-20155-7
Presenting our recent work “Concurrent magneto optical imaging and magneto-transport readout of electrical switching of insulating antiferromagnetic thin films” [Appl. Phys. Lett. 117, 082401 (2020) https://doi.org/10.1063/5.0011852] in a 10min talk and a live Q&A session, Felix Schreiber received the Best Student Presentation Award for his talk at the MMM 2020 virtual conference. The online event featured over 1000 sessions reviewing the latest advances in both fundamental and applied magnetism.
The electrical manipulation of magnetism via current-driven spin-orbit torques (SOTs) promises efficient spintronic devices, for which one needs to realize a large SOT efficiency. Engineering the SOT efficiency is predominantly achieved by maximizing or modulating the nonequilibrium spin density that builds up at magnetic layer/heavy metal interfaces through the spin Hall and inverse spin galvanic effects. Regardless of the origin, the fundamental requirement for efficient SOTs is a net spin accumulation. In our paper entitled “Harnessing Orbital-to-Spin Conversion of Interfacial Orbital Currents for Efficient Spin-Orbit Torques”, published in Physical Review Letters, we report on the large enhancement of the SOT efficiency in thulium iron garnet (TmIG)/Pt by capping with a CuOx layer. An orbital current generated at the Pt/CuOx interface is converted to a spin current as it decays across the Pt layer due to the large spin-orbit coupling in Pt. This then exerts a “nonlocal” torque on the magnetic material TmIG. Our work offers a route to increasing the efficiency of current-driven torques while at the same time offering key insights into the underlying physics of orbital transport.
The full text of the publication in Physical Review Letters is accessible at: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.125.177201
Figure: Schematic illustration of “nonlocal” generation of SOTs in TmIG / Pt / CuOx structures. The orbital angular momentum (indicated by blue circulations) is generated at the Pt / CuOx interface and injected into the Pt. Due to the large spin-orbit coupling of the Pt, the orbital angular momentum is converted to a spin current (indicated by red arrows), which diffuses across the Pt and exerts a torque on the magnetic moments (yellow arrow) of the TmIG layer.
In the TopDyn collaboration of our experimental group, the theoretical group of Dr. Virnau and our esteemed former colleague Dr. Zázvorka, we investigated examples of two-dimensional phases and phase transitions using the model system of skyrmions nucleated in thin metal films. Magnetic skyrmions form regular lattices due to their repulsive interactions, which can be considered as a two-dimensional crystallization. In these ordered lattices skyrmions behave like disks, with the great advantage that the size and number of skyrmions can be controlled via magnetic fields. This enables the observation of different phases and phase transitions, which are not possible in three dimensions. A famous example is the Kosterlitz-Thouless transition, for the description of which the Nobel price 2016 was awarded. Another example is the hexatic phase, occurring between the liquid and solid phase of hard disks. Our system shows evidence for an incipient hexatic phase, validating the use of skyrmions as an example of hard disks in a two-dimensional phase transition. Additionally, we were able to simulate the repelling interaction of the skyrmions and more ordered lattices. Our results are now published in the journal Advanced Functional Materials with the title Skyrmion Lattice Phases in Thin Film Multilayer.
Additional information can be found in the press release on the JGU website: https://www.uni-mainz.de/presse/aktuell/12071_ENG_HTML.php
The full text of the publication in Advanced Functional Materials is accessible at: https://onlinelibrary.wiley.com/doi/full/10.1002/adfm.202004037?af=R
Figure: skyrmions forming a hexagonal oriented lattice
Magneto-optical imaging of current-induced switching of insulating antiferromagnetic thin films
Our work “Concurrent magneto-optical imaging and magneto-transport readout of electrical switching of antiferromagnetic thin films” [F. Schreiber et al., Appl. Phys. Lett. 117, 082401 (2020) https://doi.org/10.1063/5.0011852] made it as the cover page of Volume 117, Issue 8 of Applied Physics Letters.
In the paper we use a table-top approach to demonstrate stable and reversible current-induced switching of large-area antiferromagnetic domains in NiO/Pt by direct imaging in a Kerr microscope. Concurrent transport and magneto-optical imaging measurements allow us to correlate the AFM domain switching fraction and magneto-transport signal response. While the observation of magnetic domain switching indicates the presence of a magnetic SMR response, we also confirm the presence of artificial resistance changes in the Pt layer. The introduction of a simple procedure to subtract these non-magnetic contributions from the transverse resistance signal yields the disentanglement of magnetic and non-magnetic contributions. Across many different current densities, we find an accurate correlation of post-treated electrical signal and domain switching fraction, calculated from the imaging. This corroborates the validity of the subtraction procedure and allows us to distinguish the presence of a significant electrical SMR response, directly correlated with the switching of the antiferromagnetic domains.
We emphasize the potential of insulating AFMs by highlighting the possibility to quantify the antiferromagnetic domain switching fraction from simple transport measurements. These findings can both motivate and simplify additional research to enable using insulating AFMs in applications where electrical reading and writing are required.
Efficient switching of insulating antiferromagnets
Antiferromagnetic materials are magnetic materials that do not generate a magnetic field, due to the alternated magnetic moments. To make insulating antiferromagnetic materials useful as building blocks of future spintronic devices for memory and computing operations, and exploit the potentially fast operation speed, robustness against external magnetic fields and higher bit density compared to ferromagnets, one needs to realize reading and writing operations efficiently. Reading has been demonstrated by the spin Hall magnetoresistance for many compounds while electrical writing is being debated, with some authors reporting efficient switching and others reporting absence of magnetic switching. In our paper entitled “Efficient Spin Torques in Antiferromagnetic CoO/Pt Quantified by Comparing Field- and Current-Induced Switching”, published in Physical Review Letters, we demonstrate current-induced switching of an antiferromagnetic CoO thin film and we determine the switching efficiency electrically by current-induced and field-induced switching. This demonstration is a key step in view of applications.
Additional information can be found in the press release on the JGU website: https://www.uni-mainz.de/presse/aktuell/11958_ENG_HTML.php
The full text of the publication in Physical Review Letters is accessible at https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.125.077201.
Figure 1: Example of antiferromagnetic material, with antiparallel alignment of the magnetic moments.
Figure 2: Currents and fields can switch the magnetic moments in the same manner.
Making energy-efficient switching of memory cells even more efficient
Magnetic memory relies on using the magnetization direction of individual ferromagnetic cells to store binary information. Nowadays, the most efficient way to switch the magnetization direction, i.e. to write or delete a bit of information, is by using so-called spin torques. These torques arise in certain, typically, thin multilayer films when an electric current is passed through them, and the larger the torque, the less current is required for the switching, thus led energy is dissipated in the device. It is known that the magnitude of the torques varies when altering the systems constituents, for example, changing the materials or the layer thicknesses. In this case, however, the torques remain fixed once the multilayer is fabricated. In the light of developing a new generation of devices with simplified architectures and decreased power consumption, of key importance is to be able to control the spin torques “on the fly”. Our results recently published in Physical Review Letters demonstrate for the first time experimentally that the low-power control of the spin torques can be achieved by means of piezoelectric strain, which we explain by advanced theoretical calculations carried out in the collaboration with Jan-Philipp Hanke and Yuriy Mokrousov within the TopDyn – Dynamics and Topology Research Centre.
We generate and control the piezoelectric strain by low-power electric fields, which is an energy-efficient approach as it avoids using power-hungry electric currents and the associated energy losses due to heating. We find that the tensile strain enhances the amplitude of the torque in a thin perpendicularly magnetized multilayer, while the compressive strain leads to its decrease. This means that we can not only dynamically tune the torques by electrically controlled but also reach even higher torques than possible for a given system at zero strain. Using theoretical calculations, we uncover the microscopic origin of the observed behavior of the torques and reveal which phenomena are at the core and need to be considered when engineering the torques.
Our results are remarkable because they show that two energy-efficient approaches of magnetization manipulation, the electric field-induced strain and the spin torque magnetization switching, can be combined to enable novel device concepts.
Additional information can be found in the press release on the JGU website: https://www.uni-mainz.de/presse/aktuell/11684_ENG_HTML.php
The full text of the publication in Physical Review Letters is accessible at:
Johannes Gutenberg University Mainz (JGU) has achieved good results in the Nature Index ranking, particulary in the physics, and is among the ten most research-intensive German universities in physics. In the 2020 Nature Index, physics at JGU ranks 6th out of 84 in the national ranking and 84th out of 1,801 in international comparison. The geosciences (15th out of 67), chemistry (17th out of 81), and the life sciences (19th out of 92) also ranked in the top quarters of German universities.
The annual Nature Index ranking published by the Nature Publishing Group is based on publications in the most important international journals.
Skyrmions like it Hot – Investigation of the Temperature Dependence of the Skyrmion Hall Effect reveals further insights into possible new data storage devices.
The joint research project of Johannes Gutenberg University Mainz (JGU) and the Massachusetts Institute of Technology (MIT) that had previously demonstrated the use of new spin structures for future magnetic storage devices has yet achieved another milestone. The international team is working on structures, which could serve as magnetic shift register, so called Racetrack memory devices. This type of storage promises low access times, high information density and low energy consumption. Previously, the team observed billionfold reproducible motion of skyrmions, a new topologically stabilized spin structures that is a promising candidate to be used as the data bits in the racetrack device. The new insights published in the research journal Nature Electronics shed more light on the effects of temperature on the dynamics of skyrmions. The study finds that skyrmions could not just move more efficiently at higher temperatures but also that their trajectories only depend on the speed the skyrmions move with in a device. This makes a device design significantly easier.
The experiments were carried out in thin films of magnetic material that stabilize skyrmions at and above room temperature – a feature required for any application. They also showed that there are currently limits to the speed of a skyrmion caused by deformations that will need to be overcome, possibly in antiferromagnetic materials.
Additional information can be found in the press release on the JGU website: https://www.uni-mainz.de/presse/aktuell/10894_ENG_HTML.php
The full text of the publication in Nature Electronics is accessible at: https://www.nature.com/articles/s41928-019-0359-2
We have an assistant professorship with tenure track currently availble in the field of spin physics. Please see the advert for download or online at:
The legally binding version is the German advert at:
To make antiferromagnetic materials useful as building blocks of future spintronic devices for memory and computing operations, and exploit the potentially fast operation speed, robustness against external magnetic fields and higher bit density, one needs to realize reading and writing operations efficiently. Reading has ben demonstrated by the spin Hall magnetoresistance for many compounds while electrical writing has so far been realized primarily in antiferromagnets with particular crystal symmetry. Here we demonstrate current-induced switching of an antiferromagnetic NiO thin film and we determine the switching both by electrical read-out and direct dichroism-based imaging.
Additional information can be found in the press release http://www.uni-mainz.de/presse/aktuell/10211_ENG_HTML.php on the JGU website.
The full text of the publication in Physical Review Letters is accessible at https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.123.177201 .
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. </p
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.