Magnetism and Spintronics

We study the ordering of spins, magnetization dynamics and spin transport in magnetic materials to understand the formation of complex spin textures, their high-frequency response and the transport of angular momentum. We fabricate complex magnetic heterostructures and nanostructures required for advanced data storage and the next-generation spintronic devices.
Recent projects
Rudolf Gross, Stefan Filipp, Frank Deppe, Hans Huebl, Matthias Althammer, Qi-Ming Chen, Frank Deppe, Kirill Fedorov, Florian Fesquet, Kedar Honasoge, Achim Marx, Yuki Nojiri, Michael Renger, Nadezhda Kukharchyk, Stephan Geprägs, Thomas Luschmann, Ana Strinic
Matthias Althammer, Rudolf Gross, Janine Gückelhorn
Recent publications
Tobias Wimmer, Janine Gückelhorn, Sebastian Wimmer, Sergiy Mankovsky, Hubert Ebert, Matthias Opel, Stephan Geprägs, Rudolf Gross, Hans Huebl, Matthias Althammer
Research Article | arXiv:2103.12697  (2021)
Manuel Müller, Lukas Liensberger, Luis Flacke, Hans Huebl, Akashdeep Kamra, Wolfgang Belzig, Rudolf Gross, Mathias Weiler, Matthias Althammer
Research Article | Physical Review Letters 126, 087201  (2021)
Jasmin Graf, Sanchar Sharma, Hans Huebl, and Silvia Viola Kusminskiy
Research Article | Physical Review Research 3, 013277  (2021)
R. Ramazashvili, P. D. Grigoriev, T. Helm, F. Kollmannsberger, M. Kunz, W. Biberacher, E. Kampert, H. Fujiwara, A. Erb, J. Wosnitza, R. Gross & M. V. Kartsovnik
Research Article | npj Quantum Materials 6, 11  (2021)
Stephan Geprägs, Christoph Klewe, Sibylle Meyer, Dominik Graulich, Felix Schade, Marc Schneider, Sonia Francoual, Stephen P. Collins, Katharina Ollefs, Fabrice Wilhelm, Andrei Rogalev, Yves Joly, Sebastian T.B. Goennenwein, Matthias Opel, Timo Kuschel, Rudolf Gross
Research Article | Physical Review B 102, 214438  (2020)

The ordering of spins in magnetic materials is determined by the subtle interplay between various interactions ranging from exchange and dipolar interactions to more exotic ones such as the Dzyaloshinskii-Moriya interaction (DMI). These interactions may result in simple parallel and anti-parallel spin configurations such as in ferromagnetic and antiferromagnetic materials, but also in more complex spin configurations such as magnetic skyrmions and other topological spin textures. In our research, we aim at identifying the static and dynamic properties of spin structures in magnetic materials and clarifying the underlying physical mechanisms in both bulk materials and magnetic heterostructures. This is of key relevance for their applications in magnetic data storage or in spintronics devices.

In spintronics  – one of the emerging fields for the next-generation nanoelectronic devices – the transport of spin-polarized charge carriers or even the transport of pure angular momentum (pure spin currents) without any charge transport is in the focus of present research. We particularly focus on pure spin currents in electrically insulating magnetic materials carried by quantized spin waves (magnons), as devices based on such currents may have reduced power consumption and allow for an increase in memory and processing capabilities.

We fabricate insulating and electrically conducting magnetic materials as well as complex heterostructures consisting of magnetic and non-magnetic materials. The combination of magnetically ordered materials with non-magnetic metals with strong spin-orbit interactions allows us to generate and detect pure spin currents by the direct and inverse spin Hall effect (SHE). Such structures lead to important discoveries of the spin Hall magnetoresistance (SMR) and the spin Nernst effect (SNE) at WMI. Recently, we also could demonstrate spin transport in a magnetic insulator with zero effective damping and the magnonic analogue of the electronic Hanle effect and the Datta-Das transistor.

The breaking of inversion symmetry at interfaces in heterostructures of magnetic and non-magnetic materials with strong spin-orbit coupling allows one to stabilize skyrmion states in a wide temperature and magnetic field range. We systematically study metallic multilayers nanostructures patterned into these layers to evaluate their suitability for hosting ferromagnetic and antiferromagnetic skyrmions.

Subtopics
Hybrid Systems
Hybrid systems aim to couple two properties to gain novel functionalities. They are the basis of transducers and are presently considered for various quantum applications. Our research focusses on achieving strong coupling between various subsystems, such as magnons and photons, magnons and phonons, photons and phonons, as well as coupled excitations of the same nature. The dynamics of the hybrid system can vastly differ from from the behavior of their constituents, which is highly interesting from fundamental science perspective and is of key importance for applications based on hybrid concepts.

At the Walther-Meissner-Institute we pioneered research in the direction of strong magnon-photon interaction, which is key to investigate magnons on the quantum level, but also gives deep insight in the hybridization of magonic and phonic states, an aspect which we have intensely researched by combining this hybrid system with electical readout techniques. 

Spin ensembles based on paramagnetic centers are complementary to their exchange coupled counterpart. Although, the coupling is less intense, they still can be operated in the strong coupling regime. Due to their extreme coherence times, these spin systems are discussed for quantum memory or quantum transduction applications. 

In addition, we study magnon-phonon hybrids based on acoustic resonators and magnetic thin films, as well as coupled nano-string resonator networks. 

Recent publications
Jasmin Graf, Sanchar Sharma, Hans Huebl, and Silvia Viola Kusminskiy
Research Article | Physical Review Research 3, 013277  (2021)
Stefan Weichselbaumer, Christoph W. Zollitsch, Martin S. Brandt, Rudolf Gross, Hans Huebl
Research Article | Physical Review Letters 125, 137701  (2020)
Hans Huebl, Christoph Zollitsch, Johannes Lotze, Fredrik Hocke, Moritz Greifenstein, Achim Marx, Rudolf Gross, Sebastian T. B. Goennenwein
Research Article | Physical Review Letters 111, 127003  (2013)
Quantum Materials
The properties of a large class of materials are determined by quantum effects. In such quantum materials, interesting novel phases emerge due to a subtle interplay between different microscopic degrees of freedom.

Like magnetism and superconductivity, those quantum phases are not only of high interest from the fundamental science perspective, but also have potential applications in quantum science and technology. We fabricate thin film and multilayer quantum materials, including magnetically ordered insulators (e.g. Y3Fe5O12 or α-Fe2O3), spin-orbit driven materials (e.g. Sr2IrO4), or Dzyaloshinskii-Moriya-active interfaces...

Recent projects
Rudolf Gross, Stefan Filipp, Frank Deppe, Hans Huebl, Matthias Althammer, Qi-Ming Chen, Frank Deppe, Kirill Fedorov, Florian Fesquet, Kedar Honasoge, Achim Marx, Yuki Nojiri, Michael Renger, Nadezhda Kukharchyk, Stephan Geprägs, Thomas Luschmann, Ana Strinic
Recent publications
Manuel Müller, Lukas Liensberger, Luis Flacke, Hans Huebl, Akashdeep Kamra, Wolfgang Belzig, Rudolf Gross, Mathias Weiler, Matthias Althammer
Research Article | Physical Review Letters 126, 087201  (2021)
Stephan Geprägs, Christoph Klewe, Sibylle Meyer, Dominik Graulich, Felix Schade, Marc Schneider, Sonia Francoual, Stephen P. Collins, Katharina Ollefs, Fabrice Wilhelm, Andrei Rogalev, Yves Joly, Sebastian T.B. Goennenwein, Matthias Opel, Timo Kuschel, Rudolf Gross
Research Article | Physical Review B 102, 214438  (2020)
Spin Currents and Magnon Transport
Pure spin currents represent the charge-less transport of angular momentum. Our present research is focused on the injection, transport, and detection of magnon-based angular momentum transport in insulating, long-range ordered magnetic materials. We aim at the detailed understanding of spin transport and its precise control as the key ingredient for novel spin current-based devices.

Antiferromagnetic Spintronics

Antiferromagnetic materials promise improved performance for spintronic applications, as they are robust against external magnetic field perturbations and allow for faster magnetization dynamics compared to ferromagnets. The direct observation of the antiferromagnetic state, however, is challenging due to the absence of a macroscopic magnetization. The spin Hall magnetoresistance (SMR) effect, however, is a versatile tool to probe the antiferromagnetic spin structure via simple electrical transport experiments. We investigate ...

  • the spin Hall magnetoresistance (SMR) effect in antiferromagnetic insulators,
  • the antiferromagnetic spin texture and its evolution as a function of the external magnetic field, and
  • the spin-transport in antiferromagnetic insulators via the the magnon-mediated magnetoresistance (MMR).

Spin Transport and Magnon-Mediated Magnetoresistance

Pure spin currents represent the chargeless transport of angular momentum. They give rise to novel interface effects like the recently discovered spin Hall magnetoresistance. We investigate ...

  • the spin transport and the spin Hall magnetoresistance (SMR) effect in materials wit long-range magnetic order,
  • non-local effects and magnon-mediated magnetoresistance (MMR), and
  • the topological Hall effect (THE) and related phenomena in materials with large spin-orbit coupling.
Recent projects
Matthias Althammer, Rudolf Gross, Janine Gückelhorn
Recent publications
Tobias Wimmer, Janine Gückelhorn, Sebastian Wimmer, Sergiy Mankovsky, Hubert Ebert, Matthias Opel, Stephan Geprägs, Rudolf Gross, Hans Huebl, Matthias Althammer
Research Article | arXiv:2103.12697  (2021)
Stephan Geprägs, Christoph Klewe, Sibylle Meyer, Dominik Graulich, Felix Schade, Marc Schneider, Sonia Francoual, Stephen P. Collins, Katharina Ollefs, Fabrice Wilhelm, Andrei Rogalev, Yves Joly, Sebastian T.B. Goennenwein, Matthias Opel, Timo Kuschel, Rudolf Gross
Research Article | Physical Review B 102, 214438  (2020)
Tobias Wimmer, Akashdeep Kamra, Janine Gückelhorn, Matthias Opel, Stephan Geprägs, Rudolf Gross, Hans Huebl, Matthias Althammer
Research Article | Physical Review Letters 125, 247204  (2020)
Magnetization and Spin Dynamics
The study of the dynamic response of both interacting and non-interacting spin systems provides important input for the general understanding of magnetic materials. Moreover, this dynamic response can be harnessed for generating spin currents, manipulating magnon transport, or laying the foundation of self-oscillations in magnetic devices.

We study the dynamics of both paramagnetic and exchange-coupled spin systems by broadband magnetic resonance spectroscopy. We aim at extracting fundamental material parameters like magnetic anisotropy, exchange coupling and damping of spin dynamics in a wide range of materials, ranging from simple ferromagnets to topologically nontrivial magnets with complex spin textures. A particular focus of our research is the study and application of spin-orbit torques.

Recent projects
Rudolf Gross, Mathias Weiler, Luis Flacke
Recent publications
Manuel Müller, Lukas Liensberger, Luis Flacke, Hans Huebl, Akashdeep Kamra, Wolfgang Belzig, Rudolf Gross, Mathias Weiler, Matthias Althammer
Research Article | Physical Review Letters 126, 087201  (2021)
Lukas Liensberger, Akashdeep Kamra, Hannes Maier-Flaig, Stephan Geprägs, Andreas Erb, Sebastian T. B. Goennenwein, Rudolf Gross, Wolfgang Belzig, Hans Huebl, Mathias Weiler
Research Article | Physical Review Letters 123, 117204  (2019)
Stefan Klingler, Vivek Amin, Stephan Geprägs, Kathrin Ganzhorn, Hannes Maier-Flaig, Matthias Althammer, Hans Huebl, Rudolf Gross, Robert D. McMichael, Mark D. Stiles, Sebastian T.B. Goennenwein, and Mathias Weiler
Research Article | Physical Review Letters 120, 127201  (2018)