Domain walls and demagnetization process

Domain walls

A domain wall (Bloch wall) is the thin transition layer, a few hundred atoms thick, between two neighbouring magnetic domains, inside which the direction of the atomic moments rotates gradually from the direction of one domain to that of the other. Its thickness is a balance between exchange energy (prefers a wide wall) and anisotropy energy (prefers a narrow wall).

 domain 1   |  wall (spins rotate)  |  domain 2
  ^ ^ ^ ^   |  ^ / -> \ v          |  v v v v

Demagnetization due to domain formation

A ferromagnetic crystal does not stay as a single domain, because a single domain produces poles at its ends and a large external field, which stores a large magnetostatic energy. The crystal lowers this energy by dividing into domains. This splitting is the reason an iron piece can exist in an unmagnetized (demagnetized) state.

 (a) single     (b) two        (c) four        (d) closure
     domain         domains        domains         domains
 +--------+   +--------+     +--+--+--+--+   +--------+
 | ---->  |   | ---->  |     |^ |v |^ |v |   |\ <--- /|
 | N    S |   | <----  |     |  |  |  |  |   | v    ^ |
 +--------+   +--------+     +--+--+--+--+   |/ ---> \|
 large field  field halved   field small     +--------+
                                             no free poles

1. (a) Single domain: all moments parallel; N and S poles at the ends; large external field and maximum magnetostatic energy.
2. (b) Two antiparallel domains: flux partly returns inside the crystal; magnetostatic energy roughly halves, at the cost of one wall.
3. (c) N domains: energy falls to about $1/N$ of (a). Splitting stops when the energy of extra walls equals the energy saved.
4. (d) Closure domains: triangular domains at the ends, magnetized at 90°, close the flux path completely inside the crystal. Magnetostatic energy becomes almost zero and net magnetization is zero.

Role of the external field

When a field is applied and then reduced in a controlled way (an AC field decreasing slowly to zero), domain walls move back and forth through smaller and smaller distances. The domains settle back into this low-energy, randomly oriented, flux-closed arrangement, and the material is demagnetized. Heating above the Curie temperature and cooling in zero field also restores random domains.