Magnetic moment of a bar magnet is defined as the product of the pole strength and the effective length of the magnet. If ‘m’ is the pole strength and ‘2l’ is the effective length, the magnetic moment (M) is M = m × 2l.
On the basis of total magnetic moment of materials, materials are classified as dia-magnetic materials, para-magnetic materials and ferro-magnetic materials. The descriptions of the materials mentioned above are given below:
Dia-magnetic materials are those materials which in their normal states have no magnetic moment at all. When such materials are kept in external magnetic field, they acquire induced dipole moment in the direction opposite to the field. For example, bismuth, copper, water, alcohol, mercury, silver, gold, hydrogen etc.
Properties of Dia-magnetic materials
The properties of dia-magnetic materials are given below:
- They are feebly repelled by a magnet, i.e. they tend to move from stronger to weaker field.
- When a dia-magnetic bar is suspended freely in a magnetic field, it moves slowly and sets up in the direction perpendicular to the magnetic field.
- These materials have a small and negative value of susceptibility and the susceptibility is independent of temperature.
- Dia-magnetic properties do not depend upon temperature.
- The relative permittivity of dia-magnetic materials is slightly less than one.
- Dia-magnetic materials do not exhibit hysteresis.
- These materials do not obey Curie’s law.
- Dia-magnetic liquid depresses at middle when kept over closely spaced poles of magnets.
- Diamagnetic liquids rise up in the middle when kept over more spaced pole pieces.
- They fall down in U-tube when placed between two pole pieces of magnet.
These are the materials which in their normal state have small non-zero magnetic moment and get weakly magnetized in the direction of field. For example, aluminium, chromium, oxygen, platinum, solution of salt of nickel and iron etc.
Properties of Para-magnetic materials
The properties of para-magnetic materials are given below:
- They are feebly attracted by a magnet, i.e. they tend to move from weaker to stronger field.
- When a para-magnetic bar is suspended freely in a magnetic field, it moves slowly and sets up in the direction of field.
- These materials have a small and positive value of magnetic susceptibility. The susceptibility is inversely proportional to the absolute temperature, i.e.
χ ∝ 1/T
or, χ = c/T, where c is Curie’s constant and this law is called Curie’s law.
- Para-magnetic properties decrease with the rise in temperature.
- The relative permittivity of para-magnetic materials is slightly greater than one.
- These materials do no exhibit hysteresis.
- Para-magnetic liquids, when placed in a watch glass is placed over two closely spaced pole-pieces of magnet, rise at the middle.
- When a para-magnetic liquid is placed in a U-tube and one of the limbs of the tube is placed between two strong pole pieces of magnet, the liquid rises at that limb.
These are the materials which in their normal states have magnetic moment of high degree and get strongly magnetized in the direction of field. For example, iron, cobalt, nickel and their alloys.
Properties of Ferro-magnetic materials
The properties of ferro-magnetic materials are listed below:
- They are strongly attracted by a magnet, i.e. they tend to move from weaker to stronger field.
- When a ferro-magnetic bar is suspended freely in the magnetic field, it moves quickly and sets up in the direction of field.
- These materials have high and positive value of magnetic susceptibility which is given by the formula:
χ = c/T – Tc
The susceptibility of ferro-magnetic substance decreases with the rise in temperature. And at a temperature called Curie’s temperature (Tc), the ferro-magnetic material turns into para-magnetic.
- Ferro-magnetic properties decrease with the rise in temperature.
- The relative permittivity of ferro-magnetic material is very high.
- They exhibit hysteresis.