Classification of Magnetic Materials

MAGNETIC MATERIALS - Classification

Magnetic materials are classified into two categories based on existence of dipole moment and the response of magnetic material to external magnetic fields namely,

i. Diamagnetic materials - no permanent magnetic moment.

ii. Paramagnetic, ferromagnetic, antiferromagnetic and ferrimagnetic materials - having  permanent magnetic moment.

Generally, diamagnetic and paramagnetic materials are known as non-magnetic materials, due to poor response to an external magnetic field.

The ferromagnetic, antiferromagnetic and ferrimagnetic materials are known as magnetic materials. These materials strongly respond to an external magnetic field.

DIAMAGNETIC EFFECT

Diamagnetism is exhibited by all the materials.

The atoms in diamagnetic materials do not possess permanent magnetic moments.

However, when the diamagnetic material is placed in an external magnetic field, the electrons in the atomic orbits tend to counteract the external magnetic field. Hence, the atoms acquire an induced magnetic moment.

As a result, the material becomes magnetised. The direction of the induced dipole moment is opposite to that of externally applied magnetic field.

Due to this effect, the material is very weakly repelled in magnetic field. This phenomenon is known as diamagnetism.

We can understand diamagnetism by the schematic illustration in fig 2.23 (a) and (b).

When the magnetic field H is zero, the atoms possess zero magnetic moment (fig 2.23 (a)).

When a magnetic field H is applied in the direction shown (fig 2.23 (b)), the atoms acquire an induced magnetic moment in the direction opposite to that of the magnetic field.

(a) Diamagnetic material. The atoms do not possess possess magnetic moment.

(b) When a magnetic field H is applied, the atoms acquire induced magnetic moment in the direction opposite to the applied field resulting in negative susceptibility

The strength of the induced magnetic moment is proportional to the applied field and hence the magnetisation of the material varies directly with the strength of the magnetic field.

The induced dipoles and magnetization vanish as soon as the applied magnetic field is removed.

The susceptibility of the diamagnetic material is negative. Due to this, the material is weakly repelled in the magnetic field.

Diamagnetic materials

The materials which exhibit diamagnetism are called diamagnetic materials.

Properties

i. The diamagnetic materials repel the magnetic lines of force. The behaviour of a perfect diamagnetic material in the presence of magnetic field is shown in fig. 2.24.

ii. There is no permanent dipole moment. Therefore, the magnetic effects are very small in these materials.

iii. The magnetic susceptibility is negative and it does not depend on temperature and applied magnetic field strength.

Example: Gold, germanium and silicon.

PARAMAGNETIC EFFECT

In certain materials, each atom or molecule possesses a net permanent magnetic moment (due to orbital and spin magnetic moments) even in the absence of an external magnetic field.

The magnetic moments are randomly oriented in the absence of an external magnetic field as shown in fig. 2.25(a). This makes the net magnetic moment zero and hence the magnetisation of the material is zero.

But, when an external magnetic field is applied, the magnetic dipoles tend to align themselves in the direction of the magnetic field as shown in fig. 2.25(b) and the material becomes magnetized.

This effect is known as paramagnetism.

With an increase in temperature, increase in thermal agitation disturbs the alignment of the magnetic moments.

It tends to randomize the dipole direction thus leading to decrease in magnetization.

(a) Each atom possesses a permanent magnetic moment. When H = 0, all the magnetic moments are randomly oriented so M = 0.

(b) When a magnetic field H is applied, the magnetic moments tend to orient themselves in the direction of the field, resulting in positive susceptibility

This indicates that the paramagnetic susceptibility decreases with increase in temperature. It is noted that the paramagnetic susceptibility varies inversely with temperature.

This is known as Curie's law of paramagnetism. C is a constant which is called as Curie's constant.

Paramagnetic materials

The magnetic materials which exhibit paramagnetism are called as paramagnetic material.

Properties

i. The paramagnetic materials attract the magnetic lines of force.

ii. They possess permanent dipole moment.

iii. The value of susceptibility is positive and it depends on temperature. It is given by

iv. The spin alignment is shown in fig 2.26.

Example: Manganous sulphate, ferric oxide, ferrous sulphate and nickel sulphate

FERROMAGNETIC EFFECT

Certain metals like iron (Fe), cobalt (Co), nickel (Ni) and certain alloys exhibit high degree of magnetisation.

These materials show the spontaneous magnetization i.e., they have magnetisation (atomic magnetic moments are aligned) even in the absence of an external magnetic field.

This indicates that there is a strong internal field within the material which makes the atomic magnetic moments align with each other.

This phenomenon is known as ferromagnetism