Magnetostriction

Animation of magnetostriction

Magnetostriction (cf. electrostriction) is a property of ferromagnetic materials that causes them to change their shape or dimensions during the process of magnetization. The variation of material's 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 nickel.^[1]

This effect causes 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 of these effects cause a change in the material's dimensions.

The reciprocal effect, the change of the susceptibility [response to an applied field] 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 (500 oersteds).

On magnetization, a magnetic material undergoes changes in volume which are small: of the order 10⁻⁶.

Magnetostrictive materials

Cut-away of a transducer comprising: magnetostrictive material (inside), magnetising coil, and magnetic enclosure completing the magnetic circuit (outside)

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 (=2·50) or 120 (=2·60) hertz, plus harmonics).

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, (Ter for terbium, Fe for iron, NOL for Naval Ordnance Laboratory, and D for dysprosium). Terfenol-D, Tb_xDy_1-xFe₂, exhibits about 2,000 microstrains in a field of 2 kOe (160 kA/m) at room temperature and is the most commonly used engineering magnetostrictive material ^[2]

Another very common magnetostrictive composite is the amorphous alloy Fe₈₁Si_3.5B_13.5C₂ 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, H_A, of less than 1 kA/m (to reach magnetic saturation). 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.

See also

• Inverse magnetostrictive effect
• Electrostriction
• Piezoelectricity
• Piezomagnetism
• SoundBug
• FeONIC Developer of Audio products using Magnetostriction
• Terfenol-D
• Galfenol

References

1. ^ Joule, James (1842), Sturgeon's Annals of Electricity 8: 219 
2. ^ Magnetostriction and Magnetostrictive Materials - Magnetostrictive Composites - Active Material Laboratory

External links

• Magnetostriction
• Magnetostriction(2)
• Magnetostriction(3)
• Why do transformers hum?
• Invisible Speakers from FeONIC using Magnetostriction