Magnetostriction is a property of ferromagnetic materials that causes them
to change their shape or dimensions during the process of magnetization. The
variation of materials' magnetization due to the applied magnetic field
changes the magnetostrictive strain until reaching its saturation value, λ.
The effect was first identified in 1842 by James Joule when observing a sample
of iron. This effect causes energy loss due to
frictional heating in susceptible ferromagnetic cores. The effect is also
responsible for the low-pitched humming sound that can be heard coming from
transformers. 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 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 materials dimensions is a consequence of
magnetocrystalline anisotropy, that it takes more energy to magnetize a
crystalline material in one direction than 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 free energy of the system. Since
different crystal directions are associated with different lengths this
effect induces a strain in the material. 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 thus 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
40,000 A/m. On magnetization, a magnetic material
undergoes changes in volume which are small: of the order 10−6.
Magnetostrictive hysteresis loop Like flux density, the magnetostriction
also exhibit hysteresis versus strength of magnetizing field. The shape of this
hysteresis loop can be reproduced using the Jiles-Atherton model.
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, Λ, which may be positive or negative and is defined as 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" which can be heard near transformers and high power electrical
devices. Cobalt exhibits the largest
room-temperature magnetostriction of a pure element at 60 microstrains. Among
alloys, the highest known magnetostriction is exhibited by
Terfenol-D,. Terfenol-D, TbxDy1-xFe2, exhibits about 2,000 microstrains in a
field of 2 kOe at room temperature and is the most commonly used engineering
magnetostrictive material. Galfenol is a newer such material.
Another very common magnetostrictive composite is the amorphous alloy
Fe81Si3.5B13.5C2 with its trade name Metglas 2605SC. Favourable properties of
this material are its high saturation-magnetostriction constant, λ,
of about 20 microstrains and more, coupled with a low magnetic-anisotropy
field strength, HA, of less than 1 kA/m. Metglas 2605SC also exhibits a very
strong ΔE-effect with reductions in the effective Young's modulus up to about
80% in bulk. This helps build energy-efficient magnetic MEMS.
In early sonar transducers, during World War II, nickel was used as a
magnetostrictive material. To alleviate the shortage of nickel, Japanese navy
used an iron-aluminium alloy from the Alperm family.
See also Coil noise
Inverse magnetostrictive effect Electrostriction
Piezoelectricity Piezomagnetism
SoundBug FeONIC—developer of audio products using
magnetostriction Terfenol-D
Galfenol References
External links Magnetostriction
"Magnetostriction and transformer noise". Archived from the original on
2006-05-10. Invisible Speakers from Feonic that use
Magnetostriction