Magnetostriction

Magnetostriction is the deformation of magnetic (especially ferromagnetic ) substances as a result of an applied magnetic field. The body experiences an elastic change in length at constant volume ( Joule magnetostriction ).

As a special feature, e.g. B. Invar alloys, there is also the possibility of volume magnetostriction , in which the volume is variable; it is usually much smaller than the Joule magnetostriction.

The quantum mechanical cause of the phenomenon (as well as the underlying magnetic anisotropy ) is the spin-orbit interaction .

principle

If you apply an external magnetic field to ferromagnetic material, the Weiss domains align themselves in the same way. By rotating the dipoles , the length of a rod changes in the range of approx. 10 to 30 µm / m (highly magnetostrictive materials: up to 2 mm / m). Every ferromagnetic material, for example the iron core of a transformer , is excited to mechanical vibrations by an alternating magnetic field . Magnetostriction on ferromagnetic materials (e.g. in coil cores) in changing magnetic fields can lead to noises - similar to the way electrostriction in a strongly changing electrical field causes noises on a dielectric (e.g. in a capacitor ) can.

For inductive components magnetic materials are with as less desirable magnetostriction, since on the one hand, the magnetic properties (eg., By clamping, gluing or casting cores) by pressure or train change, and on the other hand, by the operation of transformers or chokes on 50- Hz network the mains hum with a frequency of 100 Hz occurs in Europe. The soft magnetic crystalline alloy Permalloy , Ni ₈₁ Fe ₁₉ , fulfills the desired conditions. Amorphous iron alloys are even better .

Applications

Magnetostriction was used before, e.g. B. to generate ultrasound . A rod made of material with high magnetostriction (e.g. nickel ) is magnetized in a coil with alternating current .

This effect is also used in modern retail security labels .

A linear actuator that works with magnetostriction is the worm motor. Magnetic bubble memories use magnetostriction for data storage.

There are magnetoelastic sensors that measure inverse magnetostriction, i.e. H. use the change in magnetization due to mechanical stresses , for example for measuring tensile and compressive force as well as torsion.

Magnetostrictive constant

In Joule magnetostriction, a body is stretched under the influence of a magnetic H field . With the help of the relative change in length parallel to the field, the (field strength-dependent) magnetostrictive constant is defined: ${\ displaystyle \ lambda _ {\ |}}$

{\ displaystyle \ lambda _ {\ |} = {\ frac {\ Delta l} {l_ {0}}}.}

Since the flipping of the magnetic moments maintains the volume, the following applies perpendicular to the field (see also Poisson's ratio ):

{\ displaystyle \ lambda _ {\ bot} \ approx - {\ frac {1} {2}} \ lambda _ {\ |}.}

Magnetostrictive constants of known materials are listed in the following table. The data comes from and.

material	${\ displaystyle \ lambda _ {\ \|}}$ [10 ⁻⁶ ]	Curie temperature T _C [° C]
Fe	−14	770
Ni	−50	358
Co	−93	1120
Tb (−196 ° C)	3000	−48
Dy (−196 ° C)	6000	−184
TbFe ₂	1753	424
Tb _0.3 Dy _0.7 Fe ₂ (Terfenol-D)	1620	380
SmFe ₂	−1560	403
Samfenol-D	−1125
CoFe ₂ O ₄ ( single crystal )	600 ... 900	520
CoFe ₂ O ₄ ( polycrystalline )	230	520
Metglass 2605SC	60	370

literature

William Fuller Brown, Jr .: Magnetoelastic Interactions. Springer, Berlin 1966.

Individual evidence

↑ H. Janocha: actuators. Springer Verlag, Berlin 2006, ISBN 3-540-54707-X .
↑ Electronics in vehicle technology, p. 175, Google books
↑ Hartmut Janocha: Unconventional Actuators: An Introduction. Oldenbourg Verlag, accessed September 13, 2012 .
↑ Dapino, MJ: On magnetostrictive materials and their use in adaptive structures. Structural Engineering and Mechanics 17 (3-4) (2004), pp. 303-329.
↑ Trémolet de Lacheisserie, E .: Magnetostriction - Theory and Applications of Magnetoelasticity. CRC Press, Boca Raton, 1993.

[1] H. Janocha: actuators. Springer Verlag, Berlin 2006, ISBN 3-540-54707-X .

[2] Electronics in vehicle technology, p. 175, Google books

[3] Hartmut Janocha: Unconventional Actuators: An Introduction. Oldenbourg Verlag, accessed September 13, 2012 .

[4] Dapino, MJ: On magnetostrictive materials and their use in adaptive structures. Structural Engineering and Mechanics 17 (3-4) (2004), pp. 303-329.

[5] Trémolet de Lacheisserie, E .: Magnetostriction - Theory and Applications of Magnetoelasticity. CRC Press, Boca Raton, 1993.