Editing Ferromagnetism (section)

===Magnetic domains===
[[File:Electromagnetic dynamic magnetic domain motion of grain oriented electrical silicon steel.gif|thumb|Electromagnetic dynamic magnetic domain motion of grain-oriented electrical silicon steel]]
[[File:Weiss-Bezirke1.png|thumb|[[Kerr micrograph]] of a metal surface showing magnetic domains, with red and green stripes denoting opposite magnetization directions]]
{{Main|Magnetic domain}}

The spontaneous alignment of magnetic dipoles in ferromagnetic materials would seem to suggest that every piece of ferromagnetic material should have a strong magnetic field, since all the spins are aligned; yet iron and other ferromagnets are often found in an "unmagnetized" state. This is because a bulk piece of ferromagnetic material is divided into tiny regions called ''[[magnetic domain]]s''<ref name="Feynman">
 {{cite book
  | last = Feynman | first = Richard P.
  | author2 = Robert B. Leighton>
  | author3 = Matthew Sands
  | title = The Feynman Lectures on Physics
  | volume = I
  | publisher = California Inst. of Technology
  | year = 1963
  | location = Pasadena
  | pages = 37.5–37.6
  | url = {{google books|plainurl=y|id=bDF-uoUmttUC|page=4}}
  | isbn = 0-465-02493-9
  }}</ref> (also known as ''Weiss domains''). Within each domain, the spins are aligned, but if the bulk material is in its lowest energy configuration (i.e. "unmagnetized"), the spins of separate domains point in different directions and their magnetic fields cancel out, so the bulk material has no net large-scale magnetic field.

Ferromagnetic materials spontaneously divide into magnetic domains because the [[exchange interaction]] is a short-range force, so over long distances of many atoms, the tendency of the magnetic dipoles to reduce their energy by orienting in opposite directions wins out. If all the dipoles in a piece of ferromagnetic material are aligned parallel, it creates a large magnetic field extending into the space around it. This contains a lot of [[magnetostatics|magnetostatic]] energy. The material can reduce this energy by splitting into many domains pointing in different directions, so the magnetic field is confined to small local fields in the material, reducing the volume of the field. The domains are separated by thin [[Domain wall (magnetism)|domain walls]] a number of molecules thick, in which the direction of magnetization of the dipoles rotates smoothly from one domain's direction to the other.