Paramagnetism

What is Paramagnetism?

Paramagnetism is a weak form of magnetism. A paramagnetic substance, when placed in an external magnetic field, becomes weakly magnetised in the same direction as the field. This means it is weakly attracted to a magnet and tends to move from a region of weak magnetic field to a region of strong magnetic field.

Why does this happen?

The key lies inside the atoms of the material.

• Permanent atomic dipoles: Each atom (or ion or molecule) of a paramagnetic material has a permanent magnetic dipole moment of its own. Think of each atom as a tiny bar magnet.
• Random thermal motion: At normal temperatures, these tiny atomic magnets are constantly jostled by thermal energy. This random motion causes them to point in all possible directions. As a result, their magnetic effects cancel out, and the material shows no net magnetisation on its own.
• Effect of an external field: When an external magnetic field $\mathbf{B}_0$ is applied, it tries to align these atomic dipoles in its own direction. This alignment is opposed by thermal agitation.
• Temperature and field strength: The alignment becomes significant only when:
  1. The external field $\mathbf{B}_0$ is strong enough.
  2. The temperature is low enough (to reduce random thermal motion).

What happens to the magnetic field inside?

When a bar of paramagnetic material is placed in an external field $\mathbf{B}_0$, the field lines get concentrated inside the material. This means the net magnetic field $\mathbf{B}$ inside the material is slightly greater than the external field $\mathbf{B}_0$.

• The enhancement is very slight — typically about one part in $10^5$ (i.e., $\mathbf{B} \approx \mathbf{B}_0 + \text{very small amount}$).

Saturation Magnetisation

As the external field is increased or the temperature is lowered, more and more atomic dipoles align with the field. The magnetisation $M$ of the material increases. This process continues until a point is reached where all the dipoles are perfectly aligned with the field. At this point, the magnetisation cannot increase any further. This maximum possible magnetisation is called the saturation magnetisation. …