A magnetic order with cancelling atomic moments, like an antiferromagnet, but spin-split electron bands, like a ferromagnet
Micwyn· EnciclopédiaAI
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Altermagnetism is a kind of magnetic order in which neighbouring atomic magnets point in opposite directions and cancel out, as in an antiferromagnet, yet the material's electrons still split by spin, as in a ferromagnet. It was proposed as a third basic type of magnetism in 2021 and 2022.
Via Wikimedia Commons: Altermagnetism4 by Libor Šmejkal, CC BY-SA 4.0.
Each magnetic atom in a solid carries a magnetic moment, a tiny bar-magnet strength and direction. According to how these moments are arranged, physicists have traditionally divided magnetic solids into two elementary phases, ferromagnets and antiferromagnets.
In the years leading up to the proposal, theorists predicted, and some experiments reported, effects typical of ferromagnets in materials whose moments were arranged antiparallel, as in antiferromagnets. Libor Šmejkal, Jairo Sinova and Tomas Jungwirth, working at Johannes Gutenberg University Mainz and the Czech Academy of Sciences, described this as an apparent ferromagnetic-antiferromagnetic dichotomy that challenged the traditional two-way division.
Their answer, first posted in May 2021 and published in Physical Review X in September 2022, was a new classification. Using "spin groups", a way of describing a crystal's symmetries in which the directions of the spins are treated separately from the positions of the atoms, they found that collinear magnets (those whose moments all lie along one axis) fall into [three phases: conventional ferromagnets, conventional antiferromagnets, and a third phase combining zero net magnetization with an alternating spin pattern in the electrons' energy bands, which they named "altermagnetic"](arxiv.org/abs/2105.05820). They argued that the older classification tools, the relativistic magnetic groups used since the 1950s, cannot separate this phase out. A companion Perspective in the same journal in December 2022 described altermagnetism as a third distinct phase, comparable in abundance to the other two.
What sets an altermagnet apart is how its two groups of opposite-spin atoms are related. In a conventional antiferromagnet, one group can be carried onto the other by a simple shift (a translation) or by inversion through a point. In an altermagnet, the two opposite-spin sublattices are connected by a rotation of the crystal, and not by a translation or inversion; the review literature also counts mirror reflections, describing the connecting symmetries as rotation or mirror, rather than translation or inversion. The non-magnetic atoms in the crystal matter here. Šmejkal and colleagues noted that several insulating rutile-structure crystals were treated by Louis Néel and his contemporaries as classic examples of antiferromagnetism, a view built on looking at the magnetic atoms alone and omitting the role of the non-magnetic atoms.
The result is that the electrons' energy bands are split by spin, but the sign of the splitting alternates with the direction in which an electron moves through the crystal, with spin-up and spin-down states equally populated overall, so the magnetization still cancels. Along certain planes and lines of the crystal's momentum space, the bands stay unsplit, protected by symmetry. According to the pattern of this alternation, altermagnets are classed as d-wave, g-wave or i-wave.
Spin splitting has traditionally been attributed to two mechanisms: the magnetization of ferromagnets, which tends to be strong because it comes from the non-relativistic exchange interaction, and broken inversion symmetry, which tends to be weaker because it comes from relativistic spin-orbit coupling. Altermagnetic splitting is of the first, non-relativistic kind, so it can be large. In the metal ruthenium dioxide (RuO2), calculations put it at around 1 electronvolt in parts of momentum space, comparable to ferromagnets. Spin-orbit coupling, by contrast, only weakly affects the altermagnetic splitting.
The 2021 paper identified candidates ranging from insulators and metals to a parent crystal of a cuprate superconductor. A 2026 review in Science China Physics, Mechanics & Astronomy lists materials that have been explored as altermagnets, among them RuO2, MnTe, CrSb, MnF2, Mn5Si3, FeSb2 and others.
Manganese telluride (MnTe) has become the leading test case. In α-MnTe the moments on the two manganese sublattices order antiparallel below 310 kelvin (about 37 °C), and the sublattices are connected by a six-fold screw rotation, giving a g-wave altermagnet. In a paper published in Nature in February 2024, J. Krempaský and colleagues used angle-resolved photoemission spectroscopy, which measures the energy and direction of electrons knocked out of a material by light, to directly identify altermagnetic spin splitting in the band structure of MnTe, lifting the spin degeneracy that had been expected for a material with no net magnetization.
In October 2026 a team led by Daniel Jost of SLAC National Accelerator Laboratory reported in Science the first direct evidence of a chiral magnon in MnTe. A magnon is a collective wave of spins moving through a magnet, and a chiral one has a definite handedness. Using circularly polarized X-rays, the team found a mode whose spin polarization varies with direction consistently with the predicted g-wave symmetry of MnTe; they named it a "chiral altermagnon". The experiments were carried out at the European Synchrotron Radiation Facility in France, and SLAC's announcement described the result as conclusive proof of a third type of magnetism.
Ruthenium dioxide was the first workhorse of the field. Its antiparallel magnetic order at room temperature was discovered only recently, from neutron diffraction and resonant X-ray scattering, and experiments reported effects of a kind typical of ferromagnets in it, among them an anomalous Hall effect. The reported magnetic moments, however, were close to the limit of sensitivity of those methods.
In April 2024 M. Hiraishi and colleagues reported in Physical Review Letters that muon spin rotation, which uses implanted muons as tiny probes of internal magnetic fields, found no sign of the expected internal field in single-crystal RuO2 between 5 and 400 kelvin, and concluded that the reported magnetic order is unlikely to exist in the bulk crystal. The 2026 review describes the intrinsic magnetic order of RuO2 as still a subject of active debate, set between transport measurements that show altermagnetic signatures and magnetic and spectroscopic measurements that do not.
Researchers' interest in altermagnets rests partly on spintronics. Their proposers argued that altermagnets could replicate spintronic effects known from ferromagnets and add new ones linked to zero net magnetization, and that they can occur in high-temperature magnets with light elements and strong spin coherence. The SLAC team pointed to possible uses in memory, quantum devices and energy-efficient data centres, while noting that significant development is still needed.
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