Level 3 · Magnetism in layered materials

Ferromagnetism (FM)

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All spins lined up the same way. A weak applied field locks the material into this 'all-aligned' mood.

Build on:Spin

All arrows aligned

A material is ferromagnetic (FM) when its local spins spontaneously line up parallel. The classic example is iron at room temperature; the example for this project is what you get above the spin-flip transition in CrSBr.

AFM 0Mixed 0FM 0

Try the FM preset to see all spins aligned. The whole strip turns red because every site is classified as FM by the 3-cell rule.

Why optics cares

In CrSBr the FM state shifts the exciton to a slightly higher energy than the AFM ground state. That tiny shift is enough to move the dip in R(E) by a few meV, which is what the simulator displays.

Key takeaways

FM = all spins parallel.
In CrSBr the FM phase sits energetically above AFM by ~17 meV per layer — a small but visible shift in the optics.

Up next

Antiferromagnetism (AFM)
Neighboring layers cancel each other out. CrSBr's natural ground state — calm, with no net magnetization.
Spin-flip transition
Above a critical magnetic field, AFM layers snap into the FM configuration. This is what re-paints the spin pattern in the simulator.
Magnetic domains
Patches of the sample where the spins all share the same pattern. Pushing the material with a field grows or shrinks these patches.