From University of Stuttgart

In collaboration with international partners, researchers at the University of Stuttgart have experimentally demonstrated a previously unknown form of magnetism in atomically thin material layers.

New type of magnetism discovered in 2D materials
[Picture: University of Stuttgart / Ludmilla Parsyak]

The discovery is highly relevant for future magnetic data storage technologies and advances the fundamental understanding of magnetic interactions in two-dimensional systems. The results have now been published in the renowned journal Nature Nanotechnology. 

Storing ever-growing volumes of data in ever-smaller spaces
“As data volumes continue to grow, future magnetic storage media must be able to store information reliably at ever higher densities,” says Professor Jörg Wrachtrup, head, Center for Applied Quantum Technologies (ZAQuant), University of Stuttgart, whose group led the project. “Our results are therefore directly relevant for next-generation data storage technologies. At the same time, they are of fundamental importance, as they provide new insights into magnetic interactions in atomically thin materials.”

Quantum optical experiments at ZAQuant:
They can make new states of matter visible in order to verify and expand theoretical models.
(Image: University of Stuttgart / Ludmilla Parsyak)

The international research team discovered a new magnetic state that emerges in a system consisting of 4 atomic layers of chromium iodide. “We can selectively control this magnetism by tuning the interactions between electrons in the individual layers,” explains Dr. Ruoming Peng, postdoctoral researcher, 3^rd Physics Institute, University of Stuttgart, who carried out the experiments at ZAQuant together with doctoral researcher King Cho Wong. “What is particularly remarkable is that the observed magnetic properties are robust against environmental perturbations.”

Unusual magnetic behavior in two-dimensional materials
The chromium iodide investigated in the study belongs to the class of two-dimensional (2D) materials — systems composed of only a few atomic layers arranged in a crystalline lattice. It has long been known that 2D materials can exhibit properties that differ fundamentally from those of their three-dimensional bulk counterparts.

By slightly twisting two bilayers of chromium iodide with respect to each other, the Stuttgart researchers created a novel magnetic state. “In contrast, an untwisted bilayer does not exhibit a net external magnetic field, as shown in earlier studies,” says Peng.

The twisting gives rise to so-called skyrmions — nanoscale, topologically protected magnetic structures that are among the smallest and most stable information carriers known in magnetic systems. For the 1st time, the team succeeded in creating and directly detecting skyrmions in a twisted two-dimensional magnetic material.

Quantum sensing reveals weak magnetic signals
Detecting the new magnetic state posed a major experimental challenge, as the associated signals are extremely weak. To overcome this, the researchers employed a highly specialized microscope based on quantum sensing techniques. The method exploits nitrogen-vacancy (NV) centers in diamond, whose physical principles have been extensively developed and refined at the Center for Applied Quantum Technologies over the past 2 decades.

Nitrogen doping by ion implantation:
This is how commercially available diamonds are turned into quantum sensors.
They enable the discovery of new forms of magnetism.
(Image: University of Stuttgart / Ludmilla Parsyak)

Theory must be refined
Beyond their technological relevance, the findings significantly advance the theoretical understanding of collective electron behavior in atomically thin magnetic systems. “Our experimental results indicate that existing theoretical models need to be refined to fully capture the observed phenomena,” says Wrachtrup.

In addition to the University of Stuttgart, research institutions from the United Kingdom, Japan, the United States, and Canada were involved in the project. The theoretical modeling and numerical simulations were led by researchers at the University of Edinburgh. (see below)

About ZAQuant

Research and teaching at the Center for Applied Quantum Technologies (ZAQuant) focus on solid-state quantum technology, with applications ranging from nanoscale quantum sensing to quantum networks. The institute’s infrastructure is a world-wide unique combination of precision as well as quantum optics laboratories and state-of-the-art cleanroom facilities.

Article: Super-moiré spin textures in twisted two-dimensional antiferromagnets

Nature Nanotechnology has published an article written by King Cho Wong, Ruoming Peng, 3rd Physikalisches Institut, University of Stuttgart, Stuttgart, Germany, Eric Anderson, Department of Physics, University of Washington, Seattle, WA, USA, Jackson Ross, Institute for Condensed Matter Physics and Complex Systems, The University of Edinburgh, Edinburgh, UK , Bowen Yang, Institute for Quantum Computing, University of Waterloo, Walterloo, Ontario, Canada, and Department of Physics and Astronomy, University of Waterloo, Waterloo, Ontario, Canada, Meixin Cheng, Institute for Quantum Computing, University of Waterloo, Walterloo, Ontario, Canada, and Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada, Sreehari Jayaram, Malik Lenger, Xuankai Zhou, Yan Tung Kong, 3rd Physikalisches Institut, University of Stuttgart, Stuttgart, Germany, Takashi Taniguchi, Research Center for Materials Nanoarchitectonics, National Institute for Materials Science, Tsukuba, Japan, Kenji Watanabe, Research Center for Electronic and Optical Materials, National Institute for Materials Science, Tsukuba, Japan, Michael A. McGuire, Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA, Rainer Stöhr, 3rd Physikalisches Institut, University of Stuttgart, Stuttgart, Germany, Adam W. Tsen, Institute for Quantum Computing, University of Waterloo, Walterloo, Ontario, Canada, and Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada, Elton J. G. Santos, Institute for Condensed Matter Physics and Complex Systems, The University of Edinburgh, Edinburgh, UK, Higgs Centre for Theoretical Physics, University of Edinburgh, Edinburgh, UK, and Donostia International Physics Center, Donostia-San Sebastián, Spain, Xiaodong Xu, Department of Physics, University of Washington, Seattle, WA, USA, and Department of Materials Science and Engineering, University of Washington, Seattle, WA, USA, and Jörg Wrachtrup, 3rd Physikalisches Institut, University of Stuttgart, Stuttgart, Germany, and Max Planck Institute for Solid State Research, Stuttgart, Germany.

Abstract: “Stacking two-dimensional layered materials offers a platform to engineer electronic and magnetic states. In general, the resulting states—such as moiré magnetism—have a periodicity at the length scale of the moiré unit cell. Here we study magnetic order in twisted double-bilayer chromium triiodide by means of scanning nitrogen-vacancy microscopy. We observe long-range magnetic textures extending beyond the single moiré unit cell, which we dub a super-moiré magnetic state. At small twist angles, the size of the spontaneous magnetic texture increases with twist angle, opposite to the underlying moiré wavelength. The spin-texture size reaches a maximum of about 300 nm in 1.1° twisted devices, an order of magnitude larger than the underlying moiré wavelength, and vanishes at twist angles above 2°. The obtained magnetic field maps suggest the formation of antiferromagnetic Néel-type skyrmions spanning multiple moiré cells. The twist-angle-dependent study, combined with large-scale atomistic Monte Carlo simulations, suggests that the magnetic competition between the Dzyaloshinskii–Moriya interaction, magnetic anisotropy and exchange interactions—which all depend on the relative rotation of the layers—produces the topological textures that emerge in the super-moiré spin order.“