Editing Ferromagnetism (section)

===Magnetized materials===
[[File:Moving magnetic domains by Zureks.gif|thumb|Moving domain walls in a grain of [[silicon steel]] caused by an increasing external magnetic field in the "downward" direction, observed in a Kerr microscope. White areas are domains with magnetization directed up, dark areas are domains with magnetization directed down.]]

Thus, a piece of iron in its lowest energy state ("unmagnetized") generally has little or no net magnetic field. However, the magnetic domains in a material are not fixed in place; they are simply regions where the spins of the electrons have aligned spontaneously due to their magnetic fields, and thus can be altered by an external magnetic field. If a strong-enough external magnetic field is applied to the material, the domain walls will move via a process in which the spins of the electrons in atoms near the wall in one domain turn under the influence of the external field to face in the same direction as the electrons in the other domain, thus reorienting the domains so more of the dipoles are aligned with the external field. The domains will remain aligned when the external field is removed, and sum to create a magnetic field of their own extending into the space around the material, thus creating a "permanent" magnet. The domains do not go back to their original minimum energy configuration when the field is removed because the domain walls tend to become 'pinned' or 'snagged' on defects in the crystal lattice, preserving their parallel orientation. This is shown by the [[Barkhausen effect]]: as the magnetizing field is changed, the material's magnetization changes in thousands of tiny discontinuous jumps as domain walls suddenly "snap" past defects.

This magnetization as a function of an external field is described by a [[Hysteresis loop|hysteresis curve]]. Although this state of aligned domains found in a piece of magnetized ferromagnetic material is not a minimal-energy configuration, it is [[metastable]], and can persist for long periods, as shown by samples of [[magnetite]] from the sea floor which have maintained their magnetization for millions of years.

Heating and then cooling ([[Annealing (metallurgy)|annealing]]) a magnetized material, subjecting it to vibration by hammering it, or applying a rapidly oscillating magnetic field from a [[degaussing|degaussing coil]] tends to release the domain walls from their pinned state, and the domain boundaries tend to move back to a lower energy configuration with less external magnetic field, thus [[demagnetization|demagnetizing]] the material.

Commercial [[magnet]]s are made of "hard" ferromagnetic or ferrimagnetic materials with very large magnetic anisotropy such as [[alnico]] and [[ferrite (magnet)|ferrites]], which have a very strong tendency for the magnetization to be pointed along one axis of the crystal, the "easy axis". During manufacture the materials are subjected to various metallurgical processes in a powerful magnetic field, which aligns the crystal grains so their "easy" axes of magnetization all point in the same direction. Thus, the magnetization, and the resulting magnetic field, is "built in" to the crystal structure of the material, making it very difficult to demagnetize.