Magnetic properties of materials

Magnetic properties of materials

The study of magnetic properties of materials assumes significance since these properties decide whether the material is suitable for permanent magnets or electromagnets or cores of transformers etc. Before classifying the materials depending on their magnetic behaviour, the following important terms are defined.

(i) Magnetising field or magnetic intensity

The magnetic field used to magnetise a material is called the magnetising field. It is denoted by H and its unit is A m^?1.

(Note : Since the origin of magnetism is linked to the current, the magnetising field is usually defined in terms of ampere turn which is out of our purview here.)

(ii) Magnetic permeability

Magnetic permeability is the ability of the material to allow the passage of magnetic lines of force through it.

Relative permeability ?_r of a material is defined as the ratio of number of magnetic lines of force per unit area B inside the material to the number of lines of force per unit area in vacuum B_o produced by the same magnetising field.

Relative permeability ?r  = B/B₀

?_r = ?H / ?₀ ? = ?/ ?₀

(since ?_r is the ratio of two identical quantities, it has no unit.)

The magnetic permeability of the medium ? = ?_o?_r where ?_o is the permeability of free space.

Magnetic permeability ? of a medium is also defined as the ratio of magnetic induction B inside the medium to the magnetising field H inside the same medium.

? =B/H

(iii) Intensity of magnetisation

Intensity of magnetisation represents the extent to which a material has been magnetised under the influence of magnetising field H.

Intensity of magnetisation of a magnetic material is defined as the magnetic moment per unit volume of the material.

I = M/V

Its unit is A m^-1.

For a specimen of length 2l, area A and pole strength m,

I = 2lm/2lA

I=m/A

Hence, intensity of magnetisation is also defined as the pole strength per unit area of the cross section of the material.

(iv) Magnetic induction

When a soft iron bar is placed in a uniform magnetising field H, the magnetic induction inside the specimen B is equal to the sum of the magnetic induction B_o produced in vacuum due to the magnetising field and the magnetic induction B_m due to the induced magnetisation of the specimen.

B = B_o + B_m

But      B_o= ?_oH and B_m = ?_oI

B = ?_oH + ?_oI

B = ?_o (H + I)

(v) Magnetic susceptibility

Magnetic susceptibility χ_m is a property which determines how easily and how strongly a specimen can be magnetised.

Susceptibility of a magnetic material is defined as the ratio of intensity of magnetisation I induced in the material to the magnetising field H in which the material is placed.

Thus χ_m = 1/H

Since I and H are of the same dimensions, χ_m has no unit and is dimensionless.

Relation between χ_m and ?_r

χ_m = 1/H

I= χ_mH

We know B = ?_o (H + I)

B = ?_o (H + χ_mH)

B = ?_oH (1 + χ_m)

If ? is the permeability, we know that B = ?H.

∴ ?_r = 1 + χ_m