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Altermagnetism

Type of magnetic state From Wikipedia, the free encyclopedia

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][10] It has been speculated that altermagnetism may have applications in the field of spintronics.[6][11]

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An example of an altermagnetic ordering, with the direction of the spins and the spatial orientation of the atoms alternating on the neighbouring sites in the crystal.
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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][10][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] In altermagnetic ruthenium dioxide (RuO2), the opposite spin sublattices are related by four-fold crystal rotation.[7][10]

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Alternating magnetic and crystal pattern in altermagnetic manganese telluride (MnTe, left) and ruthenium dioxide (RuO2, right).
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Electronic structure

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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=Eks (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.[13]

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. (kx2ky2)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. Unconventional time-reversal symmetry breaking, giant ~1eV spin splitting and anomalous Hall effect was first theoretically predicted[12] and experimentally confirmed[14] in RuO2.

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Materials

Direct experimental evidence of altermagnetic band structure in semiconducting MnTe and metallic RuO2 was first published in 2024.[9][10] 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[14][15] (but this effect occurs also in other magnetically compensated systems such as non-collinear antiferromagnets[16]). Altermagnets also exhibit unique properties such as unconventional piezomagnetism [8] anomalous and noncollinear spin currents [8] that can change sign as the crystal rotates.[17]

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Experimental observations

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In February 2024, researchers at Johannes Gutenberg University Mainz (JGU) achieved a significant milestone by experimentally demonstrating altermagnetism. They utilized a specially adapted momentum microscope to expose a thin layer of ruthenium dioxide to X-rays, resulting in the emission of electrons. By analyzing the velocity distribution of these electrons, the researchers determined their velocities and inferred their spin directions using circularly polarized X-rays. This experiment provided direct evidence of altermagnetic behavior, confirming the existence of this third class of magnetism.[18]

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]

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References

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