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Altermagnets break time-reversal symmetry, and their spin-orbit coupling (SOC) allows for an anomalous Hall effect (AHE) that depends on the direction of the N´eel ordering vector. The AHE and the ferromagnetic spin moment share the same symmetry and hence are usually proportional. However, density functional theory (DFT) calculations find that the AHE exists with negligible ferromagnetic spin moment for some compounds, whereas it reaches sizable values for other altermagnets. By examining realistic minimal models for altermagnetism in which the DFT phenomenology is captured, we uncover a general SOC-enabled quasisymmetry, the uniaxial spin space group, that provides a natural explanation for the amplitude of the ferromagnetic spin moment across the vast range of different altermagnetic materials. Additionally, we derive analytic expressions for the magnetic anisotropy energy, providing a simple means of identifying the preferred N´eel vector orientation for altermagnets.
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Collinear altermagnets and their Landau theories
Altermagnets exhibit spontaneously spin-split electronic bands in the zero spin-orbit coupling (SOC) limit arising from the presence of collinear compensated magnetic order. The distinctive magnetocrystalline symmetries of altermagnets ensure that these spin splittings have a characteristic anisotropy in crystal momentum space. These systems have attracted a great deal of interest due to their potential for applications in spintronics. In this paper, we provide a general Landau theory that encompasses all three-dimensional altermagnets where the magnetic order does not enlarge the unit cell. We identify all crystal structures that admit altermagnetism and then reduce these to a relatively small set of distinct possible Landau theories governing such systems. In the zero SOC limit, we determine the possible local multipolar orders that are tied to the spin splitting of the band structure. We make precise the connection between altermagnetism as defined at zero SOC (“ideal” altermagnets) and the effects of weak SOC. In particular, we examine which response functions allowed by symmetry when SOC is present are guaranteed by the spin-orbit free theory and spell out the distinctive properties of altermagnets in comparison with conventional collinear antiferromagnets. Finally, we show how these ideas can be applied by considering a number of altermagnetic candidate materials.
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- Award ID(s):
- 2142554
- PAR ID:
- 10680829
- Publisher / Repository:
- Phys. Rev. Research
- Date Published:
- Journal Name:
- Physical Review Research
- Volume:
- 7
- Issue:
- 3
- ISSN:
- 2643-1564
- Subject(s) / Keyword(s):
- Altermagnetism Landau theory Magnetism Magnetotransport Spin-orbit coupling Spintronics 3-dimensional systems Altermagnets Antiferromagnets Crystal structures Magnetic insulators Discrete symmetries in condensed matter Symmetries in condensed matter
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1103/q44z-ynbr
