A skyrmion is a tiny whirl in the magnetisation of a material: the magnetic direction turns smoothly from pointing down at its centre to pointing up at its rim. It behaves like a particle – a small current can move it, and it is hard to destroy, because undoing the whirl would mean flipping a whole region at once – which makes skyrmions candidates for dense, low-power magnetic memory. Most are held together by the Dzyaloshinskii–Moriya interaction, a twisting force between neighbouring magnetic atoms that appears only where a is missing.
Going deeper
In a skyrmion the magnetisation turns from down at the core to up at the rim, covering every direction once, so it cannot be smoothed away. The Dzyaloshinskii–Moriya interaction, present only where a mirror symmetry is missing, sets neighbouring moments at an angle and so supplies the twist. Small currents move skyrmions along a strip like bits on a track.
A whirl that cannot be undone
In a all the magnetic moments point the same way. In a skyrmion they twist: pointing down at the core, they turn steadily outwards until they point up at the edge, matching the surroundings. The turn passes through every direction exactly once, and that count – the charge – cannot change through any smooth rearrangement. Removing a skyrmion would require the moments at its core to flip abruptly, which costs energy, so skyrmions survive as stable objects a few to a few hundred nanometres across.
They are named after Tony Skyrme, who used such textures in the early 1960s as a model of nuclear particles, and were first seen in a magnet in 2009, as a lattice of whirls in MnSi found by .
The twist from broken symmetry
Ordinary exchange between neighbouring moments makes them parallel or antiparallel and has no preferred sense of rotation. The Dzyaloshinskii–Moriya interaction does: it favours neighbouring moments set at an angle to each other, always turning the same way. Dzyaloshinskii derived it from symmetry in 1958, and Moriya traced it to in 1960. It vanishes where the crystal or interface has a centre of symmetry, so it appears in crystals and at interfaces between different materials, where up and down are not equivalent.
Its competition with ordinary exchange sets a preferred twist, and in a magnetic field the twisted state breaks up into skyrmions. How the moments turn on the way out from the core depends on the symmetry: like the wall of a Bloch domain in bulk crystals, like a Néel wall at interfaces.
Skyrmions in 2D magnets
Most magnets have a centre of symmetry and so no Dzyaloshinskii–Moriya interaction of their own. In Fe3GeTe2, magnetic imaging found skyrmion bubbles held together by the magnetic field of the itself. Stacking can break the symmetry where the crystal does not: at a WTe2/Fe3GeTe2 interface, strong spin–orbit coupling in the neighbouring layer induces a Dzyaloshinskii–Moriya interaction and Néel-type skyrmions. layers and polar stacking are being explored for the same purpose.
The interest is practical too. Skyrmions can be moved by currents far smaller than those needed to push domain walls, and their stability suggests racetrack memories that store bits as skyrmions. Electrical detection often relies on a topological contribution to the – a signal easily mimicked by two overlapping anomalous Hall signals, so it needs imaging to confirm.
For specialists
A particle-like whose magnetisation wraps the unit sphere once, giving it an integer topological charge. Bloch-type skyrmions form in bulk chiral magnets (MnSi, 2009) and Néel-type ones at interfaces and in polar crystals. They are stabilised by the antisymmetric Dzyaloshinskii–Moriya interaction – proportional to the cross product of neighbouring , arising from spin–orbit coupling where inversion symmetry is broken – competing with exchange, and the applied field; dipolar-stabilised skyrmion bubbles form without it. Among 2D magnets, Fe3GeTe2 hosts bubbles and, in WTe2/Fe3GeTe2 , interface-induced Néel skyrmions. Their emergent magnetic field can add a topological contribution to the Hall effect, and their current-driven motion by underlies racetrack-memory proposals.