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Magnetostrictive Materials
Magnetostriction can be defined as the change in dimension of a piece of magnetic material
induced by a change in its magnetic state. Generally, a magnetostrictive material changes
its dimension when subjected to a change of the applied magnetic field. Alternatively,
it undergoes a change in its magnetic state under the influence of an externally applied
mechanical stress. By far the most common type of magnetostriction is the Joule mag
netostriction where the dimensional change is associated with a distribution of distorted
magnetic domains present in the magnetically ordered material. It is well known that fer
romagnetic and ferrimagnetic materials adopt a magnetic domain structure with zero net
magnetization in the demagnetized state in order to reduce the magnetostatic energy. In
a material showing Joule magnetostriction, each of the magnetic domains is distorted by
interatomic forces in a way so as to minimize the total energy.
Concentrating on a single of these domains, for materials with positive (negative)
magnetostriction, the dimension along the magnetization direction is increased (decreased)
while simultaneously the dimension in the direction perpendicular to the magnetization
direction is decreased (increased), keeping the volume constant. This means that for a piece
of magnetostrictive material, consisting of an assembly of many magnetostrictively distorted
domains, one expects dimensional changes when an external field causes a rotation of the
magnetization direction within a domain, and/or when the external field causes a growth of
domains, for which the magnetization direction is close to the field direction, at the cost of
domains for which the magnetization direction differs more from the field direction. We will
return to this point later.
The magnetostrictive properties will reflect the symmetry of the crystal lattice when the
piece of material is a single crystal. In this case, the length changes observed at magnetic
saturation depend on the measurement direction as well as on the initial and final direction
of the magnetization of the single crystal. As shown in more detail in several reviews
(Cullen et al., 1994; Gignoux, 1992; Andreev, 1995), frequently only two magnetostrictive
constants are required to describe the fractional length change associated with the saturation
magnetostriction in cubic materials:171