2023 Abstracts AEO

SPICE Workshop on Altermagnetism: Emerging Opportunities in a New Magnetic Phase, May 9th - 11th 2023

Stefan Blügel

It has become common practice to study the stability, dynamics, thermodynamics and phase diagrams of magnets using classical spin-lattice models with pairwise exchange interactions of the Heisenberg type. In recent years, however, the focus has been on increasingly complex magnetic structures including noncollinear spin textures such as skyrmions, many of which arise from a competition of different spin interactions. On the other hand, we studied magnets with increasingly involved and complex electronic structures. As a consequence, we witnessed additional interactions had to be added to the established spin-models more and more often to explain the magnetic order (four-spin-three-site interaction [1,2], four-spin-four- site interaction [3], chiral-biquadratic interaction [4-7], topological chiral-chiral and spin-chiral interaction [8], etc.). This sounds like a very spontaneous unsatisfactory case-by-case procedure and it seems totally unclear all important interactions are captured or important ones are still missing. In this lecture, I present an attempt to derive systematically the spin Hamiltonian of all exchange interactions from the very general principle of indistinguishability of electrons in a many-electron system. This provides an rigorous ansatz for reasonable spin- Hamiltonians.

Work was carried out in collaboration with Hiroshi Katsumoto, Fabian Lux and Yuriy Mokrousov.

Funding from Deutsche Forschungsgemeinschaft (DFG) through SPP-2137 (project BL444/16-2) and SFB-1238 (project C1) is greatly acknowledged.

[1] M. Hoffmann, S. Blügel, PRB 10, 024418 (2019)
[2] A. Krönlein, M. Schmitt, M. Hoffmann, J. Kemmer, N. Seubert, M. Vogt, J. Küspert, M. Böhme, B. Alonazi, J.Kügel, H. A. Albrithen, M. Bode, G. Bihlmayer, and S. Blügel, PRL 120, 207202 (2018)
[3] D. J. Thouless, Proc. Phys. Soc. 86, 893 (1965)
[4] A. Lászlóffy, L. Rósa, K. Palotás, L. Udvardi, and L. Szunyogh, Phys. Rev. B 99, 184430 (2019)
[5] S. Brinker, M. d. S. Dias, and S. Lounis, New Journal of Physics 21, 083015 (2019)
[6] S. Mankovsky, S. Polesya, and H. Ebert, Phys. Rev. B 101, 174401 (2020)
[7] M. dos Santos Dias, S. Brinker, A. Lászlóffy, B. Ny á ri, S. Blügel, L. Szunyogh, and S. Lounis, Phys. Rev. B103, L140408 (2021)
[8] S. Grytsiuk, J.-P. Hanke, M. Hoffmann, J. Bouaziz, O. Gomonay, G. Bihlmayer, S. Lounis, Y. Mokrousov, S. Blügel, Nat. Commun. 11, 511 (2020)

SPICE Workshop on Altermagnetism: Emerging Opportunities in a New Magnetic Phase, May 9th - 11th 2023

Aleksei V. Kimel

Light-spin interactions in altermagnets are very interesting and offer intriguing opportunities for unprecedentedly fast magnetic data storage. In my talk I will start with introduction of symmetry analysis of light-spin interaction in antiferromagnetic materials with a special attention to the materials with the point group hosting altermagnetism. It will be shown, for instance, that in these materials the magnetic circular birefringence scales linearly with the antiferromagnetic Neel vector. Using similar formalism, it will be also shown how light can control spin in antiferromagnetic materials. I will also review experimental works supporting the conclusions of the symmetry analysis. In my talk, I will also discuss magneto-optical effects, which are driven by the exchange interaction and show that these effects are not necessarily observed in altermagnets.

SPICE Workshop on Altermagnetism: Emerging Opportunities in a New Magnetic Phase, May 9th - 11th 2023

Qihang Liu

Symmetry formulated by group theory plays an essential role with respect to the laws of nature, from fundamental particles to condensed matter systems. In this talk, we elucidate that the crystallographic symmetries of a vast number of magnetic materials with light elements, in which the neglect of relativistic spin-orbit coupling is an appropriate approximation, are considerably larger than the conventional magnetic groups [1]. Thus, a symmetry description that involves partially-decoupled spin and spatial rotations, dubbed as spin group, is required. We then derive the classifications of spin “point groups” describing coplanar and collinear magnetic structures, and the irreducible co-representations of spin “space groups” illustrating more energy degeneracies that are disallowed by magnetic groups. These new symmetries directly give rise to further discoveries without any relativistic origins, including spin splitting, Z2 topological classification and new quasiparticles [1,2]. Using angle-resolved photoemission spectroscopy measurements and density functional theory calculations, we demonstrate the existence of such spin splitting effect in a noncoplanar antiferromagnet MnTe2 [3], and the spectral evidence of chiral Dirac fermions in a collinear antiferromagnet CoNb3S6 [4].

[1] Liu et al. Phys. Rev. X 12, 021016 (2022)
[2] Liu et al. The Innovation 3, 100343 (2022)
[3] Zhu et al. arXiv:2303.04549
[4] Zhang et al. arXiv:2301.12201

SPICE Workshop on Altermagnetism: Emerging Opportunities in a New Magnetic Phase, May 9th - 11th 2023

Sang-Wook Cheong

Ferromagnetism is associated with various characteristic phenomena, including non-zero magnetization (inducing magnetic attraction/repulsion), diagonal piezomagnetism, odd-order (including linear) anomalous Hall effect, nonreciprocal circular dichroism (such as Faraday effect), and magneto-optical Kerr effect. By identifying the broken symmetries linked with each of these phenomena and their relevant magnetic point groups (MPGs), we can gain a better understanding of materials exhibiting these phenomena. Interestingly, some (seemingly) antiferromagnetic materials can exhibit ferromagnetism-like behaviors, which is referred to as Trompe L'oeil Ferromagnetism. We classify these Trompe L'oeil Ferromagnets based on their broken symmetries, MPGs, and the relevant phenomena. Since antiferromagnets with little or zero magnetization have the potential to enable high-density and ultrafast spintronic technologies, our findings can provide essential guidance for future magnetism-related research and technology.

SPICE Workshop on Altermagnetism: Emerging Opportunities in a New Magnetic Phase, May 9th - 11th 2023

Dominik Kriegner

The anomalous Hall effect, commonly observed in metallic magnets, has been established to originate from the time-reversal symmetry breaking by an internal macroscopic magnetization in ferromagnets or by a noncollinear magnetic order. Here we observe a spontaneous anomalous Hall signal in the absence of an external magnetic field in MnTe, which is a semiconductor with a collinear antiparallel magnetic ordering of Mn moments, and altermagnetic symmetry due to the non-magnetic Te atoms.

SPICE Workshop on Altermagnetism: Emerging Opportunities in a New Magnetic Phase, May 9th - 11th 2023

Arnab Bose

Recently a new type of magnetic material is theoretically proposed, referred to as “altermagnet” which is a collinear antiferromagnet in real space although hosting the spin-split bands in the momentum space that allows it to exhibit the properties of ferromagnet depending upon the direction of the current flow with respect to the crystal axis [1,2]. We report the first experimental evidence of strongly crystal axis-dependent unconventional transverse spin-current generation by the altermagnet RuO2 [3] arising from the novel spin-split bands as theoretically predicted [1]. This unconventional tilted spin-current is the key to the implementation of high-density nonvolatile magnetic memories.

Reference
[1] R. González-Hernández, L. Šmejkal, K. Výborný, Y. Yahagi, J. Sinova, T. Jungwirth, and J. Železný, Efficient Electrical Spin Splitter Based on Nonrelativistic Collinear Antiferromagnetism, Phys. Rev. Lett. 126, 127701 (2021).
[2] L. Šmejkal, J. Sinova, and T. Jungwirth, Emerging Research Landscape of Altermagnetism, ArXiv: 2204.10844, 1 (2022).
[3] A. Bose, N. J. Schreiber, R. Jain, D.-F. Shao, H. P. Nair, J. Sun, X. S. Zhang, D. A. Muller, E. Y. Tsymbal, D. G. Schlom, and D. C. Ralph, Tilted Spin Current Generated by the Collinear Antiferromagnet Ruthenium Dioxide, Nat. Electron. 5, 267 (2022).

SPICE Workshop on Altermagnetism: Emerging Opportunities in a New Magnetic Phase, May 9th - 11th 2023

Davide Bossini

The wildly growing field of antiferromagnetic spintronics is currently addressing several fundamental questions. A major topic of investigation concerns the generation and manipulation of coherent magnons on the ultrafast timescale. The development of novel pulsed-laser sources has enabled scientists to address the following scientific question: which magnetic excited state can be induced by resonantly drive coherent magnons throughout the Brillouin zone? In my talk I will outline our approach to this open issue, which relies on the resonant drive of pairs of high-energy magnons in the weak ferromagnet α-Fe2O3, with wavevector near the edges of the Brillouin zone. This unprecedented concept results in strongly perturbing the entire magnetic system of the material, in particular: i) magnon modes with different wavevectors are excited and amplified; ii) the eigenfrequencies of magnons are modified; iii) a coupling between magnon modes is observed. Our results effectively report a femtosecond modification of the magnonic dispersion relation of α-Fe2O3 which, de facto, means transforming the system into a different magnetic material in the transient state, in which the magnetic interactions are changed by several percent of their ground-state values. These groundbreaking observations are rationalised in view of a resonant impulsive stimulated Raman scattering mechanism. The perspective of our results in terms of femtosecond coherent magnonics will be discussed.

SPICE Workshop on Altermagnetism: Emerging Opportunities in a New Magnetic Phase, May 9th - 11th 2023

Cheng Song

The altermagnetic spin splitting torque (SST) was theoretically proposed to combine advantages of conventional spin transfer torque (STT) and spin-orbit torque (SOT) as well as enable controllable spin current [1]. We provide the experimental evidence of SST in collinear antiferromagnet RuO₂(100) films. The spin current direction is found to be correlated to the crystal orientation of RuO₂ and the spin polarization direction is dependent on (parallel to) the Néel vector [2]. We also observe spin-to-charge conversion arising from the inverse effect of the altermagnetic spin splitting effect in RuO₂(101)/Py and RuO₂(101)/[Co/Pt] bilayers, based on both spin Seebeck effect. The spin Seebeck voltage can be detected even when the injected spin current is polarized along the directions of either the voltage channel or the thermal gradient, indicating the successful conversion of x- and z-spin polarizations into the charge current. The crystal axes-dependent conversion efficiency further demonstrates that the non-trivial spin-to-charge conversion in RuO₂ is ascribed to inverse altermagnetic spin splitting effect, which is distinct from the magnetic/antiferromagnetic inverse spin Hall effects [3]. Also, THz emission signals are greatly enhanced once the inverse altermagnetic spin splitting effect emerges [4]. These findings not only present a new member for the spin torques besides traditional STT and SOT, but also propose RuO₂ for both promising spin source and spin sink for spintronics.

[1] R. González-Hernández, et al., Phys. Rev. Lett. 126, 127701 (2021).
[2] H. Bai, C. Song, et al. Phys. Rev. Lett. 128, 197202 (2022).
[3] H. Bai, C. Song, et al. Phys. Rev. Lett. (under review)
[4] Y. Liu, C. Song, et al. Adv. Opt. Mater. (under review)