Editing Magnetostriction (section)

== Explanation ==
Internally, ferromagnetic materials have a structure that is divided into ''[[magnetic domain|domains]]'', each of which is a region of uniform magnetization.  When a magnetic field is applied, the boundaries between the domains shift and the domains rotate; both of these effects cause a change in the material's dimensions.  The reason that a change in the magnetic domains of a material results in a change in the material's dimensions is a consequence of [[magnetocrystalline anisotropy]]; it takes more energy to magnetize a crystalline material in one direction than in another.  If a magnetic field is applied to the material at an angle to an easy axis of magnetization, the material will tend to rearrange its structure so that an easy axis is aligned with the field to minimize the [[Thermodynamic free energy|free energy]] of the system.  Since different crystal directions are associated with different lengths, this effect induces a [[Deformation (mechanics)|strain]] in the material.<ref>{{cite journal|last1=James|first1=R. D.|last2=Wuttig|first2=Manfred|title=Magnetostriction of martensite|journal=Philosophical Magazine A|date=12 August 2009|volume=77|issue=5|pages=1273–1299|doi=10.1080/01418619808214252}}</ref>

The reciprocal effect, the change of the magnetic susceptibility (response to an applied field) of a material when subjected to a mechanical stress, is called the [[Villari effect]]. Two other effects are related to magnetostriction: the [[Matteucci effect]] is the creation of a helical anisotropy of the susceptibility of a magnetostrictive material when subjected to a [[torque]] and the [[Wiedemann effect]] is the twisting of these materials when a helical magnetic field is applied to them.

The Villari reversal is the change in sign of the magnetostriction of [[iron]] from positive to negative when exposed to magnetic fields of approximately 40&nbsp;[[Amperes_per_meter|kA/m]].

On magnetization, a magnetic material undergoes changes in volume which are small: of the order 10<sup>−6</sup>.