Patches of order
A magnetic domain is just a contiguous region of the sample where the spins all share the same pattern — all FM ↑, or all AFM with a particular phase. Domains are separated by domain walls.
Pushing the material with a magnetic field grows the FM domains and shrinks the AFM ones, until eventually the whole sample is FM.
In the simulator
Every contiguous block of red in the layer strip is one FM domain. The blue regions in between are AFM. Try sketching a small FM patch in the middle:
AFM 13Mixed 2FM 0
Key takeaways
- Domains are uniform patches of one magnetic phase.
- The optics "sees" the patches as bands of different refractive index.