Definition and Properties for Paramagnetism

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

The substances which when placed in a magnetic field are weakly magnetised in the direction of the applied field, are called paramagnetic substances.

Examples: Aluminium, $CuSO 4, MnSO 4$ , Platinum, Chromium, Oxygen, $NiSO 4$ , etc.

Explanation: The magnetic moments are randomly oriented in the absence of an external magnetic field. This makes the magnetisation of the material nearly zero, i.e., a little magnetization is present within the material.

(a) Each atom possesses a permanent magnetic moment. When $H=0$ , all the magnetic moments are randomly oriented; so M $\neq0$ .
(b) When a magnetic field $H 0$ is applied, the magnetic moments tend to orient themselves in the direction of the field, resulting in positive susceptibility.
But, when an external magnetic field is applied, the magnetic dipoles tend to align themselves in the direction of the magnetic field and the material becomes magnetized. (Fig).
This effect is known as Paramagnetism.
But the temperature opposes this alignment. Therefore, when there is an increase in temperature, the increase in thermal agitation tends to randomize the dipole direction thus leading to a decrease in magnetization.
This implies that the paramagnetic susceptibility decreases with increase in temperature. It is observed that the paramagnetic susceptibility varies inversely with temperature.

\chi \alpha \frac{1}{\mathrm{~T}} \quad \chi=\frac{\mathrm{C}}{\mathrm{T}}

This is known as the Curie Law of Paramagnetism and ‘ $C$ ‘ is a constant called Curie constant.

Properties

It possess permanent dipole moment.
It is feebly attracted by a magnet.
The magnetic lines of force pass through the material.

4. Since magnetic lines of force pass through the material, they posses permanent dipole moment which makes permeability greater than one.

5. Magnetic susceptibility is positive and it greatly depends on the temperature.

It is given by

\quad \chi=\frac{\mathrm{C}}{\mathrm{T}-\theta}

Where, C = curie constant, T = absolute temperature, θ = curie temperature

6. Magnetic susceptibility is independent of applied magnetic field.
7. When the temperature is less than curie temperature, paramagnetic material becomes diamagnetic material.
8. If a paramagnetic substance is placed in a nonuniform magnetic field, it is attracted towards the stronger magnetic field.