Mini-Symposium on Magnetism

15:15 – 15:20 Opening

15:20 – 15:40 Erik Folven, NTNU, Norway ‘‘Adding Memory to the Artificial Spin Ice Computer’’

15:40 – 16:00 Oscar Grånäs, Uppsala University, Sweden ‘‘Laser-induced magnetization dynamics from first-principles – Prospects of optical control and observability on sub 100-fs time scales’’

16:00 – 16:15 Fika break

16:15 – 16:35 Dominik Kriegner, Academy of Sciences of the Czech Republic ‘‘Altermagnetism and its manifestation in
MnTe’’

16:35 – 16:55 Amalio Fernández-Pacheco, TU Wien, Austria ‘‘Chiral nanomagnetism induced by 3D nanopatterning’’

16:55 – 17:00 Closing remark

Laser-induced magnetization dynamics from first-principles - Prospects of optical control and observability on sub 100-fs time scales

Oscar Grånäs, Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden

The direct manipulation of spins via light may provide a path toward ultrafast energy-efficient devices. However, distinguishing and controlling the microscopic processes that can occur during laser excitation in magnetic alloys is challenging. In this talk I give an overview of a computational framework to simulate out-of-equilibrium conditions arising in ultrafast laser experiments, and how transient response functions may be extracted to facilitate comparison to experimental observables relating to magnetism and topological band features. Further, the method is applied to study laser-induced magnetization dynamics in the Heusler compound Co₂MnGa, a ferro-magnetic half-metal. By combining theory and experiment, we disentangle the competition between three ultrafast light-induced processes that occur in Co₂MnGa: same-site Co-Co spin transfer, intersite Co-Mn spin transfer, and ultrafast spin flips mediated by spin-orbit coupling. By measuring and simulating the dynamic magnetic asymmetry across the entire M-edges of the two magnetic sublattices involved, we uncover the relative dominance of these processes at different probe energy regions and times during the laser pulse. This combined approach enables a comprehensive microscopic interpretation of laser-induced magnetization dynamics on time scales shorter than 100 femtoseconds.

Altermagnetism and its manifestation in MnTe

Dominik Kriegner, Department of Spintronics and Nanoelectronics, Institute of Physics, Academy of Sciences of the Czech Republic, Prague, Czech Republic

Altermagnets emerge as a novel type of compensated collinear magnets that complement the conventional classes of ferro- and antiferromagnets [1,2]. In contrast to antiferromagnets, for which the opposite sublattices are connected by simple lattice translation or inversion, in altermagnets, they are connected by a lattice rotational symmetry (symmorphic/non-symmoprhic, proper/improper) [1,2]. This causes time reversal symmetry breaking in the band structure and – in contrast to ferromagnets – a spin splitting that alternates in sign within the Brillouin zone motivating the term altermagnetism. The magnetization, however, integrates to zero over the entire Brillouin zone. Here we show experimental verification of the spin-splitting by angle resolved photo emission spectroscopy investigations in MnTe [3]. The characteristic spin splitting further enables linear responses such as the anomalous Hall effect [4-7], and X-ray magnetic circular dichroism [8]. A combination of X-ray magnetic circular and linear dichroism allows for the nanoscale imaging of domain patterns in a photoemission electron microscope [9].

References

1) L. Smejkal, J. Sinova, and T. Jungwirth, Phys. Rev. X 12, 031042 (2022)
2) L. Šmejkal, J. Sinova, and T. Jungwirth, Phys. Rev. X 12, 040501 (2022)
3) J. Krempaský, et al., Nature 626, 517-522 (2024)
4) R. D. Gonzalez Betancourt, et al., Phys. Rev. Lett. 130, 036702 (2023)
5) R. D. Gonzalez Betancourt, et al., npj Spintronics 2, 45 (2024)
6) T. Tschirner, et al. APL Mater. 11, 101103 (2023)
7) H. Reichlova, et al. Nat. Comm. 15, 4961 (2024)
8) A. Hariki, et al, Phys. Rev. Lett. 132, 176701 (2024)
9) O. J. Amin, et al., Nature 636, 348–353 (2024)

Chiral nanomagnetism induced by 3D nanopatterning

Amalio Fernández-Pachecoa, Institute of Applied Physics, TU Wien, Vienna, Austria

Chiral nanomagnetism – a key concept in the design of next-generation spintronic systems – can arise from a range of mechanisms, including interfacial and bulk Dzyaloshinskii–Moriya interactions. In this work, we exploit geometrically induced magnetochirality in 3D nanostructures, using artificial double-helix ferromagnetic nanowires. These nanowires combine structural chirality with competing exchange and dipolar interstrand interactions [1]. They also integrate chirality interfaces that connect spin states of opposite handedness. We have previously demonstrated that these systems provide a tuneable platform for exploring 3D chiral spin textures [1] and topologically structured stray fields [2]. Here, we will show that such systems also support the formation of stable fractional skyrmion tubes [3] and Bloch point singularities [4].

The nanowires are fabricated using focused electron beam induced deposition (FEBID) [5] and imaged via high-resolution XMCD-ptychography at the SOLEIL synchrotron. Under various field protocols, we observe spin configurations such as vortex tubes and anti-parallel states, where the interplay between geometrical and magnetic chirality plays a key role. Specific field sequences can break this coupling, leading to the emergence of fractional skyrmion tubes, which we analyse using micromagnetic simulations to map their topological properties. We will also show that these nanowires enable the controlled nucleation of Bloch point singularities with well-defined spin circulation. These singularities are created through field-driven processes in the helical geometry and are directly visualized using transmission electron microscope magnetic holography and X-ray magnetic tomography. These results show how 3D nano-patterning leads to advanced forms of chiral nanomagnetism.

[1] Sanz-Hernández et al, ACS Nano 14 (2020), 8084–8092.
[2] Donnelly et al, Nature Nanotechnology 17 (2022), 136–142.
[3] Fullerton et al, Adv. Funct. Mater. 2501615 (2025).
[4] Leo et al, in preparation.
[5] Skoric et al, Nano Lett. 20 (2020), 184–191.