Sudden rearrangements of the atomic magnets in an as a magnetic field is turned up. In an antiferromagnet neighbouring point opposite ways, so a field along them has nothing to pull on – until, at a threshold, they give way. If they are only loosely tied to their axis, they swing round to lie across the field and tilt towards it: a spin flop. If they are firmly tied, those pointing against the field turn over all at once and the magnet becomes, in effect, a ferromagnet: a metamagnetic transition. In layered antiferromagnets the layers are only weakly coupled, so this happens in modest fields, often layer by layer.
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Kerr microscopy sees net magnetisation only in odd layer numbers; even ones need spin-flop signatures or tunnelling magnetoresistance.
A field along the moments of an antiferromagnet finds them balanced. With weak anisotropy they flop across the field and then tilt towards it; with strong anisotropy the opposed moments turn over all at once – a metamagnetic transition. Layered antiferromagnets, weakly coupled between layers, do this in modest fields.
Pushing on an antiferromagnet
A field along the moments of an antiferromagnet finds them balanced: half point along it, half against, and the net moment is zero. As the field grows, the magnet can lower its energy by turning its moments so that part of their magnetisation lines up with the field, and it does so suddenly once the gain beats the cost.
The cost depends on two things: the exchange that keeps neighbours antiparallel and the that ties each moment to its axis. With weak anisotropy the moments swing round to lie perpendicular to the field – the spin flop – and then tilt towards it as the field grows, until all point along it. With strong, anisotropy they cannot leave their axis, and the moments pointing against the field turn over all at once – a metamagnetic, or -flip, transition.
Layered antiferromagnets
Many layered magnets are ferromagnetic within each layer and antiferromagnetic from one layer to the next, with the coupling between layers far weaker than within them, so fields of a tesla or less can reorganise them. CrPS4, whose chromium moments point out of the layers, flops in an out-of-plane field below 1 T, and on powder samples followed its magnetic structure through the flop (Calder and colleagues, 2020). In MnBi2Te4 the resistance drops as the field carries the crystal across its metamagnetic transition (Yan and colleagues, 2019), and the antiferromagnet CeSiI shows metamagnetic steps near 2.5 and 4.5 T at 2 K, nearly the same in from two to fifteen layers thick.
In thin flakes the count of layers matters. An odd number leaves one layer uncompensated, so thin CrPS4 alternates between ferromagnetic odd and antiferromagnetic even layer counts (Son and colleagues, 2021), and a field turns layers over one at a time.
Reading it out
A single flake holds too little magnetic material for a , so these transitions are read through light or current. Kerr microscopy sees the net magnetisation jump; a current across a antiferromagnet changes with every layer that turns, because each layer favours electrons of one spin. In multilayer CrPS4, under in-plane and out-of-plane fields followed a spin reorientation that differs with the number of layers, matched by a model of coupled layers as a linear chain (Tian and colleagues, 2025).
For specialists
Field-driven transitions of an antiferromagnet with the field along its easy axis. When the anisotropy is weak compared with the exchange, the sublattices flop perpendicular to the field at the spin-flop field and then cant progressively to saturation; when it is strong, the antiparallel sublattice reverses abruptly – a metamagnetic, or spin-flip, transition. In layered antiferromagnets the relevant exchange is the weak : CrPS4 flops in an out-of-plane field below 1 T, MnBi2Te4 crosses a metamagnetic transition with a drop in resistance, and CeSiI shows metamagnetic steps near 2.5 and 4.5 T at 2 K in flakes of every thickness; in few-layer flakes the transitions proceed layer by layer, as tunnelling magnetoresistance shows.
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