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# Magnetostriction

The meaning of striction is the state of being constricted. When a magnetic field is given to a magnetic material, it experiences an alteration in shape or size or length or dimension. This property of some magnetic materials is known as magnetostriction. This change of dimension of magnetic materials during magnetization may continue till the magnetic saturation of the material is attained. The change in length of a ferromagnetic iron rod when magnetized was first discovered by James Joule in 1842. This is the Joules effect. Ferromagnetic and ferri-magnetic materials exhibit this effect. Manetostriction can be calculated by the magnetostriction coefficient or Joule’s magnetostriction coefficient. It is the fractional variation in length as the magnetization raises from 0 to saturation value. It is denoted by λ. If the length of a ferromagnetic rod is L and after magnetizing it from zeros to saturation level the change in length is δL. Then the ratio of δL to L is Joule’s magnetostriction coefficient (λ). λ = δL / L. There is a loss in energy due to friction in magnetic material cores. This effect is the reason of humming sound that we hear in transformer.

Next, we can go through some of the effects used in magnetostriction. They are

• Joules Effect: When a ferromagnetic substance is subjected to magnetic field, it will alter in shape.
• Villari Effect: When a substance is subjected to a mechanical stress, the susceptibility of that substance changes (magneto elastic effect i.e. inverse of magnetostriction)
• Matteucci Effect: When a magnetostrictive material is subjected to a torque, a helical anisotropy of the susceptibility of that material will be created.
• Wiedemann Effect: When a helical magnetic field is applied to a material, it results in twisting of that material (reverse of Matteucci effect). It is shown in figure below.

The structure of the ferromagnetic substances is alienated into domains. Each domain is a region of identical magnetic polarization. When a peripheral magnetic field is applied, the borders linking the domains move and domains revolve. As a result, there will be change in the dimension of the material. This effect is the outcome of magneto crystalline anisotropy. Thus, a strain is induced in the material. This is because dissimilar crystal directions are related with dissimilar length. Matteucci effect and Wiedemann effect are linked with magnetostriction. They are explained above. Villari effect is contrary of magnetostriction effect.

## Magnetostriction Materials

This effect is mostly seen in the magnetic materials such as iron, nickel, cobalt, iron-aluminium alloy and some other alloys like Terfenol-D, Galfenol, Metgla etc. They are mainly used in actuators and some motors.

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