Magnetostriction

Magnetostriction is a property of ferromagnetic materials that causes them to change their shape when subjected to a magnetic field. The effect was first identified in 1842 by James Joule when observing a sample of nickel. (Compare with electrostriction)

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This effect can cause losses due to frictional heating in susceptible ferromagnetic cores.

Explanation

Internally, ferromagnetic materials have a structure that is divided into domains, each of which is a region of uniform magnetic polarization. When a magnetic field is applied, the boundaries between the domains shift and the domains rotate, both these effects causing a change in the material's dimensions. The reciprocal effect, the change of the susceptibility 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 40000 A/m (500 oersteds). On magnetization a magnetic material under goes changes in volume which are small - of the order 10^-6.

Magnetostrictive materials

Magnetostrictive materials can convert magnetic energy into kinetic energy, or the reverse, and are used to build actuators and sensors. The property can be quantified by the magnetostrictive coefficient, L, which is the fractional change in length as the magnetization of the material increases from zero to the saturation value. The effect is responsible for the familiar "electric hum" (Listen (help·info)) which can be heard near transformers and high power electrical devices (depending on country, either 100 or 120 hertz, plus harmonics).

Cobalt exhibits the largest room temperature magnetostriction of a pure element at 60 microstrain. Among alloys, the highest known magnetostriction is exhibited by Terfenol-D, (Ter for terbium, Fe for iron, NOL for Naval Ordnance Laboratory, and D for dysprosium). Terfenol-D, Tb_xDy_1-xFe₂, exhibits about 2000 microstrains in a field of 2 kOe (160 kA/m) at room temperature and is the most commonly used engineering magnetostrictive material [1].

See also

• Electrostriction
• Piezoelectricity