C2: Unconventional dynamics at the mesoscale in constrained spin systems
Christian Back, Felix Büttner, István Kézsmárki
In this project we aim to confirm the recent and exciting prediction that magnetic Weyl semimetals offer exotic and highly efficient mechanisms of driving mesoscale spin textures owing due to spin-momentum locking. Such unconventional mechanisms may include exceptionally large non-adiabatic spin-transfer torques, the ability to drive domain walls by electric fields without a net charge current, and a modification of the fundamental speed of spin relaxation. We will quantify such effects in materials such as Fe2Sn3 and explore their potential in driving transitions between otherwise energetically protected phases.
Publications
2025 |
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Kovács, A.; Weber, J. T.; Charilaou, M.; Kong, D.; Prodan, L.; Tsurkan, V.; Schröder, A.; Kiselev, N. S.; Kézsmárki, I.; Dunin-Borkowski, R. E.; Tavabi, A. H.; Schäfer, S. All-optical stochastic switching of magnetisation textures in Fe3Sn2 Journal Article Commun. Mater. 6, 223 (2025). @article{kovacs_all-optical_2025,The all-optical control of magnetisation at room temperature broadens the scope of applications of spin degrees-of-freedom in data storage, spintronics, and quantum computing. Topological magnetic spin structures, such as skyrmions, are of particular interest due to their particle-like properties, small size and inherent stability. Controlling skyrmion states without strong magnetic fields or large current densities would create new possibilities for their application. In this work, we utilize femtosecond optical pulses to alter the helicity of the spin configuration in dipolar skyrmions formed in the kagome magnet Fe3Sn2 in the absence of an external magnetic field and at room temperature. In situ Lorentz transmission electron microscopy is used to visualise the light-induced stochastic switching process of chiral Néel caps, while the internal Bloch component of the dipolar skyrmions remains unchanged. In addition to this switching process, we observe the interconversion between type I skyrmionic and type II bubble configurations depending on the external magnetic field and illumination conditions. To corroborate the spin states and the light-induced magnetisation dynamics, micromagnetic modelling and simulations of the resulting electron phase shift maps are conducted to elucidate the spin rearrangement induced by individual femtosecond optical pulses. |  |
Ghara, S.; Winkler, M.; Schmid, S. W.; Prodan, L.; Geirhos, K.; Tsurkan, V.; Ge, Wenbo; Wu, Weida; Halbritter, A.; Krohns, S.; Kézsmárki, I. Nonvolatile electric control of antiferromagnetic states on nanosecond timescales Journal Article Phys. Rev. Lett. 135, 126704 (2025). @article{ghara_nonvolatile_2025,Electrical manipulation of antiferromagnetic (AFM) states, a cornerstone of AFM spintronics, is a great challenge, requiring novel material platforms. Here we report the full control over AFM states by voltage pulses in the insulating Co3O4 spinel well below its Néel temperature. We show that the strong linear magnetoelectric effect is fully governed by the orientation of the Néel vector. As a unique feature of Co3O4, the magnetoelectric energy can easily overcome the weak magnetocrystalline anisotropy; thus, the Néel vector can be manipulated on demand, either rotated smoothly or reversed suddenly, by combined electric and magnetic fields. We achieve the nonvolatile switching within a few tens of nanoseconds between time-reversed AFM states in macroscopic volumes by voltage pulses. These observations render quasicubic antiferromagnets, like Co3O4, an ideal platform for the ultrafast (picosecond to nanosecond) manipulation of microscopic AFM domains and may pave the way for the realization of AFM spintronic devices. |  |
Szaller, D.; Prodan, L.; Geirhos, K.; Felea, V.; Skourski, Y.; Gorbunov, D.; Förster, T.; Helm, T.; Nomura, T.; Miyata, A.; Zherlitsyn, S.; Wosnitza, J.; Tsirlin, A. A.; Tsurkan, V.; Kézsmárki, I. Coexistence of antiferromagnetism and ferrimagnetism in adjacent honeycomb layers Journal Article Phys. Rev. B 111, 184404 (2025). @article{szaller_coexistence_2025,Ferro-/ferri- and antiferromagnetically ordered phases are typically exclusive in nature, thus, their coexistence in atomic-scale proximity is expected only in heterostructures. Breaking this paradigm and broadening the range of unconventional magnetic states, we report here on the observation of a new, atomic-scale hybrid spin state. This ordering is stabilized in three-dimensional crystals of the polar antiferromagnet Co2Mo3O8 by magnetic fields applied perpendicular to the Co honeycomb layers and possesses a spontaneous in-plane ferromagnetic moment. Our microscopic spin model, capturing the observed field dependence of the longitudinal and transverse magnetization as well as the magnetoelectric/elastic properties, reveals that this novel spin state is composed of an alternating stacking of antiferromagnetic and ferrimagnetic honeycomb layers. The strong intralayer and the weak interlayer exchange couplings together with competing anisotropies at octahedral and tetrahedral Co sites are identified as the key ingredients to stabilize antiferromagnetic and ferrimagnetic layers in such close proximity. We show that the proper balance of magnetic interactions can extend the stability range of this hybrid phase down to zero magnetic field. The possibility to realize a layer-by-layer stacking of such distinct spin orders via suitable combinations of microscopic interactions opens a new dimension toward the nanoscale engineering of magnetic states. |  |
Kern, L. -M.; Kuchkin, V. M.; Deinhart, V.; Klose, C.; Sidiropoulos, T.; Auer, M.; Gaebel, S.; Gerlinger, K.; Battistelli, R.; Wittrock, S.; Karaman, T.; Schneider, M.; Günther, C. M.; Engel, D.; Will, I.; Wintz, S.; Weigand, M.; Büttner, F.; Höflich, K.; Eisebitt, S.; Pfau, B. Controlled Formation of Skyrmion Bags Journal Article Adv. Mater. 2501250 (2025), (contributed). @article{kern_controlled_2025,Abstract Topologically non-trivial magnetic solitons are complex spin textures with a distinct single-particle nature. Although magnetic skyrmions, especially those with unity topological charge, have attracted substantial interest due to their potential applications, more complex topological textures remain largely theoretical. In this work, the stabilization of isolated higher-order skyrmion bags beyond the prototypical π-skyrmion in ferromagnetic thin films is experimentally demonstrate, which has posed considerable challenges to date. Specifically, controlled generation of skyrmionium (2π-skyrmion), target skyrmion (3π-skyrmion), and skyrmion bags (with variable topological charge) are achieved through the introduction of artificially engineered anisotropy defects via local ion irradiation. They act as preferential sites for the field- or laser-induced nucleation of skyrmion bags. Remarkably, ultrafast laser pulses achieve a substantially higher conversion rate transforming skyrmions into higher-order skyrmion bags compared to their formation driven by magnetic fields. High-resolution x-ray imaging enables direct observation of the resulting skyrmion bags. Complementary micromagnetic simulations reveal the pivotal role of defect geometry–particularly diameter–in stabilizing closed-loop domain textures. The findings not only broaden the experimental horizon for skyrmion research, but also suggest strategies for exploiting complex topological spin textures within a unified material platform for practical applications. | |
Metternich, D.; Litzius, K.; Wintz, S.; Gerlinger, K.; Petz, S.; Engel, D.; Sidiropoulos, T.; Battistelli, R.; Steinbach, F.; Weigand, M.; Wittrock, S.; Schmising, C. Korff; Büttner, F. Defects in magnetic domain walls after single-shot all-optical switching Journal Article Struct. Dyn. 12, 024504 (2025). @article{metternich_defects_2025,Helicity-independent all-optical switching (HI-AOS) is the fastest known way to switch the magnetic order parameter. While the switching process of extended areas is well understood, the formation of domain walls enclosing switched areas remains less explored. Here, we study domain walls around all-optically nucleated magnetic domains using x-ray vector spin imaging and observe a high density of vertical Bloch line defects. Surprisingly, the defect density appears to be independent of optical pulse parameters, significantly varies between materials, and is only slightly higher than in domain walls generated by field cycling. A possible explanation is given by time-resolved Kerr microscopy, which reveals that magnetic domains considerably expand after the initial AOS process. During this expansion, and likewise during field cycling, domain walls propagate at speeds above the Walker breakdown. Micromagnetic simulations suggest that at such speeds, domain walls accumulate defects when moving over magnetic pinning sites, explaining similar defect densities after two very different switching processes. The slightly larger defect density after AOS compared to field-induced switching indicates that some defects are created already when the domain wall comes into existence. Our work shows that engineered low-pinning materials are a key ingredient to uncover the intrinsic dynamics of domain wall formation during ultrafast all-optical switching. | |
Toyoda, S.; Kocsis, V.; Tokunaga, Y.; Kézsmárki, I.; Taguchi, Y.; Arima, T.; Tokura, Y.; Ogawa, N. All-optical control of antiferromagnetic domains via an inverse optical magnetoelectric effect Unpublished (2025), arXiv:2506.07051. @unpublished{toyoda_all-optical_2025,Antiferromagnets are a promising platform for next-generation spintronics due to their ultrafast spin dynamics and robustness to external fields. All-optical control of antiferromagnetic order is essential to fully exploit their potential in energy-efficient and high-speed spintronic and memory applications. However, optical writing of antiferromagnetic domains remains a fundamental challenge, as conventional magneto-optical techniques rely on net magnetization, which is absent in antiferromagnets. In certain multiferroic antiferromagnets, the magnetic toroidal moment provides an additional degree of freedom through its inherent magnetoelectric coupling. This coupling at higher frequencies results in the optical magnetoelectric effect (OME), which manifests as a directional asymmetry in light propagation and enables optical probing of antiferromagnetic states. Here, we demonstrate all-optical writing of antiferromagnetic domains using the inverse optical magnetoelectric effect (IOME) in ferrotoroidic LiNiPO4. The writing process is nonvolatile, non-thermal, and deterministic, driven solely by reversing the light propagation direction. This directional control arises from a strong coupling between the photon linear momentum and the magnetic toroidal moment, enabling the repeatable switching between time-reversed domains with arbitrary light polarization. Our findings establish IOME as a distinct mechanism for manipulating antiferromagnetic order, opening a new paradigm in opto-magnetism driven by photon momentum. | |