Magnetostriction

Magnetostriction is the magnetic deformation (particularly ferromagnetic ) material due to an applied magnetic field. The body learns at constant volume, an elastic length change ( Joule magnetostriction ).

As a special feature, such as 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.

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

Principle

If you put in a ferromagnet in an external magnetic field, then the white 'domains align the same. By the rotation of the dipoles, the length of a rod in the range of about 10 to 30 microns / m changes ( giant magnetostrictive materials to 2 mm / m). By an alternating magnetic field, each ferromagnetic material, for example, the iron core of a transformer excited to mechanical vibrations. By magnetostriction of ferromagnetic materials (eg in coil cores ) in varying magnetic fields can cause noise - similar to how it can come to a dielectric ( eg a capacitor ) to noise due to electrostriction in a rapidly changing electric field.

For inductive components magnetic materials with the lowest possible magnetostriction are desirable because on the one hand change the magnetic properties by pressure or train (eg by clamping, gluing or molding of nuclei ), and the other by the operation of transformers or reactors at the 50 - Hz mains AC hum occurs with a frequency of 100 Hz in Europe. The soft magnetic crystalline alloy permalloy, Ni81Fe19, satisfies the desired conditions. Even better are amorphous iron alloys.

Applications

Magnetostriction was used earlier, eg to generate ultrasound. In this case, a rod of material with high magnetostriction is magnetically reversed ( eg, nickel) in a coil with alternating current.

Even in modern security labels this effect is used.

A linear actuator, which operates with magnetostriction, the worm motor. A magnetic bubble memory use magnetostriction for data storage.

There are magneto- elastic sensors which the inverse magnetostriction, ie use the change of magnetization by mechanical stresses, for example, for the measurement of train and compression force and torsion.

Magnetostrictive constant

In a Joule- magnetostriction body is expanded under the influence of a magnetic field H. The ( field strength-dependent ) is defined magnetostrictive constant using the relative change in length parallel to the field:

Since the flipping of the magnetic moments is replaced by the volume, is considered perpendicular to the field ( see also Poisson's ratio ):

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