Physics:Altermagnetism

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In condensed matter physics, altermagnetism is a type of persistent magnetic state in ideal crystals.[1][2][3][4][5] Altermagnetic structures are collinear and crystal-symmetry compensated, resulting in zero net magnetisation.[1][5][6][7] Unlike in an ordinary collinear antiferromagnet, another magnetic state with zero net magnetization, the electronic bands in an altermagnet are not Kramers degenerate, but instead depend on the wavevector in a spin-dependent way due to the intrinsic crystal symmetry connecting different magnetic sublattices.[1][8] Related to this feature, key experimental observations were published in 2024.[9] It has been speculated that altermagnetism may have applications in the field of spintronics.[6][10]

Crystal structure and symmetry

In altermagnetic materials, atoms form a regular pattern with alternating spin and spatial orientation at adjacent magnetic sites in the crystal.[5][7]

Atoms with opposite magnetic moment are in altermagnets coupled by crystal rotation or mirror symmetry.[1][5][6][7][9][11][8] The spatial orientation of magnetic atoms may originate from the surrounding cages of non-magnetic atoms.[7][12] The opposite spin sublattices in altermagnetic manganese telluride (MnTe) are related by spin rotation combined with six-fold crystal rotation and half-unit cell translation.[7][9] ruthenium dioxide (RuO2) was claimed to be an altermagnet,[7][11] but it was later confirmed in two independent studies that it is completely non-magnetic[13][14].

Alternating magnetic and crystal pattern in altermagnetic manganese telluride (MnTe, left) and suggested magnetic structure for ruthenium dioxide (RuO2, right), which however later turned out to be non-magnetic.

Electronic structure

One of the distinctive features of altermagnets is a specifically spin-split band structure[7] which was first experimentally observed in work that was published in 2024.[9] Altermagnetic band structure breaks time-reversal symmetry,[7][12] Eks=E−ks (E is energy, k wavevector and s spin) as in ferromagnets, however unlike in ferromagnets, it does not generate net magnetization. The altermagnetic spin polarisation alternates in wavevector space and forms characteristic 2, 4, or 6 spin-degenerate nodes, respectively, which correspond to d-, g, or i-wave order parameters.[7] A d-wave altermagnet can be regarded as the magnetic counterpart of a d-wave superconductor.[15]

  • Fermi surface of an altermagnetic metal. The blue and red colors correspond to the up and down polarization of the spin.
    Fermi surface of an altermagnetic metal. The blue and red colors correspond to the up and down polarization of the spin.
  • The band structure of an altermagnet.
    The band structure of an altermagnet.

The altermagnetic spin polarization in band structure (energy–wavevector diagram) is collinear and does not break inversion symmetry.[7] The altermagnetic spin splitting is even in wavector, i.e. (kx2−ky2)sz.[7][9] It is thus also distinct from noncollinear Rashba or Dresselhaus spin texture which break inversion symmetry in noncentrosymmetric nonmagnetic or antiferromagnetic materials due to the spin-orbit coupling.

Materials

Direct experimental evidence of altermagnetic band structure in semiconducting MnTe was first published in 2024.[9] Many more materials are predicted to be altermagnets – ranging from insulators, semiconductors, and metals to superconductors.[6][7] Altermagnetism was predicted in 3D and 2D materials[3][6][8] with both light as well as heavy elements and can be found in nonrelativistic as well as relativistic band structures.[7][9][12]

Properties

Altermagnets exhibit an unusual combination of ferromagnetic and antiferromagnetic properties, which remarkably more closely resemble those of ferromagnets.[1][5][6][7][8] Hallmarks of altermagnetic materials such as the anomalous Hall effect[12] have been observed before[16] (but this effect occurs also in other magnetically compensated systems such as non-collinear antiferromagnets[17]). Altermagnets also exhibit unique properties such as unconventional piezomagnetism [8] anomalous and noncollinear spin currents [8] that can change sign as the crystal rotates.[18]

Experimental observations

In December 2024, researchers from the University of Nottingham provided the first experimental imaging of altermagnetism, confirming its unique spin-symmetry properties. Using Nitrogen-vacancy center microscopy and X-ray magnetic linear dichroism (XMLD), they visualized spin-polarized currents arising from the crystal-symmetry-protected altermagnetic order. This order featured antiparallel spin alignment within distinct crystal sublattices, creating a compensating spin polarization without macroscopic magnetization.[19] These findings validated theoretical predictions and demonstrated the potential of altermagnetic materials in high-speed, low-energy spintronic devices.[20]

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 Mazin, Igor (2022-12-08). "Altermagnetism—A New Punch Line of Fundamental Magnetism" (in en). Physical Review X 12 (4). doi:10.1103/physrevx.12.040002. Bibcode: 2022PhRvX..12d0002M. 
  2. ↑ Mazin, Igor (2024-01-08). "Altermagnetism Then and Now" (in en). Physical Review X 17: 4. doi:10.1103/PhysRevX.12.031042. Bibcode: 2022PhRvX..12c1042S. https://physics.aps.org/articles/v17/4. 
  3. ↑ 3.0 3.1 Mazin, Igor; González-Hernández, Rafael; Šmejkal, Libor (2023-09-05), Induced Monolayer Altermagnetism in MnP(S,Se)$_3$ and FeSe 
  4. ↑ Wilkins, Alex (14 February 2024). "The existence of a new kind of magnetism has been confirmed" (in en-US). https://www.newscientist.com/article/2417255-the-existence-of-a-new-kind-of-magnetism-has-been-confirmed/. 
  5. ↑ 5.0 5.1 5.2 5.3 5.4 Savitsky, Zack (2024). "Researchers discover new kind of magnetism". Science 383 (6683): 574–575. doi:10.1126/science.ado5309. PMID 38330121. Bibcode: 2024Sci...383..574S. https://www.science.org/content/article/researchers-discover-new-kind-magnetism. Retrieved 16 February 2024. 
  6. ↑ 6.0 6.1 6.2 6.3 6.4 6.5 Šmejkal, Libor; Sinova, Jairo; Jungwirth, Tomas (2022-12-08). "Emerging Research Landscape of Altermagnetism". Physical Review X 12 (4). doi:10.1103/PhysRevX.12.040501. Bibcode: 2022PhRvX..12d0501S. https://link.aps.org/doi/10.1103/PhysRevX.12.040501. 
  7. ↑ 7.00 7.01 7.02 7.03 7.04 7.05 7.06 7.07 7.08 7.09 7.10 7.11 7.12 7.13 Šmejkal, Libor; Sinova, Jairo; Jungwirth, Tomas (2022-09-23). "Altermagnetism: spin-momentum locked phase protected by non-relativistic symmetries". Physical Review X 12 (3). doi:10.1103/PhysRevX.12.031042. ISSN 2160-3308. Bibcode: 2022PhRvX..12c1042S. 
  8. ↑ 8.0 8.1 8.2 8.3 8.4 8.5 Ma, Hai-Yang; Hu, Mengli; Li, Nana; Liu, Jianpeng; Yao, Wang; Jia, Jin-Feng; Liu, Junwei (2021-05-14). "Multifunctional antiferromagnetic materials with giant piezomagnetism and noncollinear spin current" (in en). Nature Communications 12 (1): 2846. doi:10.1038/s41467-021-23127-7. ISSN 2041-1723. https://www.nature.com/articles/s41467-021-23127-7. 
  9. ↑ 9.0 9.1 9.2 9.3 9.4 9.5 9.6 Krempaský, J.; Šmejkal, L.; D'Souza, S. W.; Hajlaoui, M.; Springholz, G.; Uhlířová, K.; Alarab, F.; Constantinou, P. C. et al. (February 2024). "Altermagnetic lifting of Kramers spin degeneracy" (in en). Nature 626 (7999): 517–522. doi:10.1038/s41586-023-06907-7. ISSN 1476-4687. PMID 38356066. Bibcode: 2024Natur.626..517K. 
  10. ↑ Arrell, Miriam (February 14, 2024). "Altermagnetism proves its place on the magnetic family tree" (in en). https://www.sciencedaily.com/releases/2024/02/240214122553.htm. 
  11. ↑ 11.0 11.1 Fedchenko, Olena; Minár, Jan; Akashdeep, Akashdeep; D'Souza, Sunil Wilfred; Vasilyev, Dmitry; Tkach, Olena; Odenbreit, Lukas; Nguyen, Quynh et al. (2024-02-02). "Observation of time-reversal symmetry breaking in the band structure of altermagnetic RuO 2" (in en). Science Advances 10 (5). doi:10.1126/sciadv.adj4883. ISSN 2375-2548. PMID 38295181. Bibcode: 2024SciA...10J4883F. 
  12. ↑ 12.0 12.1 12.2 12.3 Šmejkal, Libor; González-Hernández, Rafael; Jungwirth, T.; Sinova, J. (5 June 2020). "Crystal time-reversal symmetry breaking and spontaneous Hall effect in collinear antiferromagnets". Science Advances 6 (23). doi:10.1126/sciadv.aaz8809. PMID 32548264. Bibcode: 2020SciA....6.8809S. 
  13. ↑ Hiraishi, M.; Okabe, H.; Koda, A.; Kadono, R.; Muroi, T.; Hirai, D.; Hiroi, Z.. "Nonmagnetic Ground State in RuO2 Revealed by Muon Spin Rotation". Physical Review Letters 132. doi:10.1103/PhysRevLett.132.166702. 
  14. ↑ Keßler, Philipp; Garcia-Gassull, Laura; Suter, Andreas; Prokscha, Thomas; Salman, Z.; Khalyavin, Dmitry; Manuel, Pascal; Orlandi, Fabio et al.. "Absence of magnetic order in RuO2: insights from μSR spectroscopy and neutron diffraction". npj spintronics 2. doi:10.1038/s44306-024-00055-y. 
  15. ↑ Šmejkal, Libor; Sinova, Jairo; Jungwirth, Tomas (2022-09-23). "Beyond Conventional Ferromagnetism and Antiferromagnetism: A Phase with Nonrelativistic Spin and Crystal Rotation Symmetry". Physical Review X 12 (3). doi:10.1103/PhysRevX.12.031042. Bibcode: 2022PhRvX..12c1042S. https://link.aps.org/doi/10.1103/PhysRevX.12.031042. 
  16. ↑ Gonzalez Betancourt, R. D.; Zubáč, J.; Gonzalez-Hernandez, R.; Geishendorf, K.; Šobáň, Z.; Springholz, G.; Olejník, K.; Šmejkal, L. et al. (20 January 2023). "Spontaneous Anomalous Hall Effect Arising from an Unconventional Compensated Magnetic Phase in a Semiconductor". Physical Review Letters 130 (3). doi:10.1103/PhysRevLett.130.036702. PMID 36763381. Bibcode: 2023PhRvL.130c6702G. 
  17. ↑ Nakatsuji, Satoru; Kiyohara, Naoki; Higo, Tomoya (November 2015). "Large anomalous Hall effect in a non-collinear antiferromagnet at room temperature". Nature 527 (7577): 212–215. doi:10.1038/nature15723. PMID 26524519. Bibcode: 2015Natur.527..212N. 
  18. ↑ González-Hernández, Rafael; Šmejkal, Libor; Výborný, Karel; Yahagi, Yuta; Sinova, Jairo; Jungwirth, Tomáš; Železný, Jakub (2021-03-26). "Efficient Electrical Spin Splitter Based on Nonrelativistic Collinear Antiferromagnetism" (in en). Physical Review Letters 126 (12). doi:10.1103/PhysRevLett.126.127701. ISSN 0031-9007. PMID 33834809. Bibcode: 2021PhRvL.126l7701G. 
  19. ↑ Amin, O.J. (11 December 2024). "Nanoscale imaging and control of altermagnetism in MnTe". Nature 636 (8042): 348–353. doi:10.1038/s41586-024-08234-x. PMID 39663495. Bibcode: 2024Natur.636..348A. 
  20. ↑ "New magnetic flow has potential to revolutionise electronic devices". Financial Times. 11 December 2024. https://www.ft.com/content/29d07e5c-123a-49d2-ae12-79dda9395a78?utm. 



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