A third kind of magnet, recognised only in the 2020s. As in an , its atomic magnets point in opposite directions and cancel out, so it has no overall magnetism and no stray field. But the two sets of atoms sit in surroundings turned against each other, so electrons moving in some directions behave as if the material were magnetic: their energy depends on their , as in a ferromagnet. Spin effects without a magnetic field are why altermagnets are seen as promising for fast, dense magnetic memory.
From outside an altermagnet looks like an antiferromagnet: its moments cancel and it has no stray field. Inside, its electrons behave partly like a ferromagnet’s, with bands split by spin, because its two kinds of magnetic site sit in rotated surroundings. Some materials long filed as antiferromagnets, MnTe among them, have turned out to be altermagnets.
As the site uses it
Stacking-dependent interlayer exchange, magnon topology, magnetic proximity effects in heterostructures and predicted two-dimensional altermagnets have become quantitative questions.
An altermagnet cancels its moments like an antiferromagnet, but because its two kinds of site sit in rotated surroundings, its electron bands split by spin like a ferromagnet’s – differently in different directions.
A third kind of magnetic order
In a ferromagnet all the atomic moments point the same way: there is a net magnetisation, a stray field, and the electron bands split by spin, which is what makes spin-polarised currents and magnetic memory work. In an ordinary antiferromagnet neighbouring moments point opposite ways and the two sets of atoms are copies of each other, shifted or inverted, so the moments cancel and the bands stay spin-degenerate.
In 2022 a symmetry classification singled out a third case. When the two sets of atoms are related only by a rotation or a mirror – each sits in surroundings turned against the other’s – the moments still cancel, but the bands split by spin, with the sign of the splitting alternating from one direction in the crystal to the next. The name altermagnet comes from that alternation.
Seeing the splitting
The splitting comes from the arrangement of the atoms, not from , so it can be large. In 2024 measurements resolved it in MnTe, lifting the spin degeneracy that an antiferromagnet would keep, and in thin films of CrSb. An effect without net magnetisation has been measured too. RuO2, the first widely discussed candidate, turned out to be doubtful, a reminder that the label needs the band splitting to be seen, not only predicted.
Why layered materials matter
Most 2D altermagnets so far are predictions: calculations list whose two magnetic sublattices sit in rotated surroundings, and stacking two layers of an ordinary antiferromagnet with a twist can create the same symmetry. The appeal for devices is that of antiferromagnetic – no stray fields, fast dynamics, dense packing – plus the spin-polarised currents that ordinary antiferromagnets lack.
For specialists
A collinear, compensated magnet whose opposite-spin sublattices are related by a rotation or mirror rather than by a translation or inversion. Its bands are spin-split without spin–orbit coupling, with a splitting that alternates in sign around the Brillouin zone in d-, g- or i-wave patterns and can reach hundreds of meV, while the net magnetisation vanishes. This allows an anomalous Hall effect, spin-polarised currents and without stray fields. MnTe and CrSb are confirmed by photoemission, RuO2 is disputed, and several layered and 2D altermagnets are predicted.
Stacked magnets provide a tunable route to altermagnetism, a compensated magnetic order characterized by momentum-dependent spin splitting, and to the magnetic excitations that such order can host. In bilayer CrI3, calculations and linear theory reveal…
Bilayers composed of two ferromagnetic Chern- V2WS4 monolayers coupled antiferromagnetically between the layers have recently been identified as a promising platform for realizing an altermagnetic state characterized by a d-wave . In this work, w… [Phys. Rev. B 114, 154427] Published Fri Sep 25…
A longitudinal spin current arises for an electric field along the lattice direction, while a transverse pure spin current emerges along the lattice angle bisector.