Magnetostriction: Features and Applications | Material Engineering

In this article we will discuss about:- 1. Meaning and Types of Magnetostriction 2. Salient Features of Magnetostriction 3. Materials 4. Applications.

Meaning and Types of Magnetostriction:

When a ferromagnetic material is magnetized, changes in physical dimensions in general occur. This phenomenon is known as magnetostriction.

There are three type of magnatostriction:

i. Longitudinal:

When change in the dimension is in the direction of applied field.

ii. Transverse:

When change in the dimension is perpendicular to applied field.

iii. Volume:

When change in the dimension is perpendicular as well as parallel to applied field.

iii. It has been observed experimentally that Ni and Co crystals contract when magnetised in either of three directions of crystal and an iron crystal expands when magnetised in the easy direction of magnetisation and contracts when magnetised in the hard direction. So Ni and Co crystals possess negative magnetostriction.

Y = Stress/Strain

Normal and Inverse Spinel:

The spinel can be of two types, (i) Normal spinel (ii) Inverse spinel. Zinc ferrite (ZnFe₂O₄) is a normal spinel while magnetite (Fe₃Q₄) is an inverse spinel. The curie temperature of spinel ferrites ranges between 700 K to 860 K. Their conductivity varies between 10² ohm-m to 10^-11 ohm-cm.

Garnets:

This is the name for a class of compounds crystallizing in a certain crystal structure. Example is yttrium-iron garnet (Y₃Fe₅O₁₂), which is ferromagnetic for a curious reason. The spin of the yttrium atoms is opposite to the spin of the iron atoms so the magnetic moments line up alternately, if the orbital magnetic moments are small.

But for yttrium the orbital magnetic moment is larger than the spin, and is in the opposite direction. Hence the total magnetic moment of yttrium atom is in the same direction as that of iron, making the compound ferromagnetic.

Ferrites in Memory Devices:

For a typical ferrite the properties are as follows:

i. Coefficient of linear thermal expansion ≈ 10^-5/°C

ii. Thermal conductivity ≈ 5 × 10² W/m°C

iii. Thermal conductivity ≈ 0.17 kcal/kg°C

iv. Density ≈ 3000 to 5000 kg/m³

v. Ferrites are suitable for use in those devices which require:

(a) High speed response,

(b) Very low power loss, and

(c) High frequency

Salient Features of Magnetostriction:

i. This effect is analogous to electrostriction that occurs in polarized dielectrics.

ii. It has an inverse effect also, if the physical dimensions of magnetostrictive materials are changed by application of external force, a change in magnetization also occurs.

iii. Also a very small change in volume occurs at ordinary temperatures, if a very strong magnetic field is applied on the magnetic material.

iv. The change in dimension takes place in the direction of magnetization.

Magnetostrictive Materials:

The property magnetostriction is found in iron, nickel, cobalt, and gadolinium. It is also observed in alloys of iron with chromium, cobalt and aluminium. Pure nickel and nickel-iron alloys. Pure iron shows either a positive or negative magnetostriction on magnetization, which on adding nickel possess only positive magnetostriction for all the field intensities. Longitudinal magnetostriction diminishes to zero on adding 30% Ni in iron.

Volume magnetostriction becomes zero by adding 36% Ni in iron. (A 36% Ni-iron alloy is called INVAR and possesses almost zero value of coefficient of thermal expansion). It is due to the volume contraction because of loss of magnetization on heating upto Curie temperature, compensates for very small value of thermal expansion. Metglass, an amorphous alloy, having a composition of Fe₈₁B_13.5 Si_3.5 C₂. It is available in the form of thin ribbon of around 25um thickness. Terfenol, an alloy of terbium and dysprosium with steel, having a composition of Tb_0.3Dy_0.7Fe₂.

Applications of Magnetostriction:

i. Audio-frequency oscillators at sonic and ultrasonic frequencies.

ii. Magnetostrictive transducers.