Q: Which of the following statement is not correct?

Explanation:
Option A: Paramagnetic substances lose their magnetism in the absence of a magnetic field.
This statement is correct. Paramagnetic materials are attracted to magnetic fields when they are present, but they do not retain any magnetism once the external field is removed.

Option B: Diamagnetic substances are weakly magnetised in a magnetic field in the opposite direction.
This statement is correct. Diamagnetic materials create an induced magnetic field in a direction opposite to an externally applied magnetic field, resulting in a weak repulsion.

Option C: Ferromagnetic substances become paramagnetic on heating.
This statement is not correct. Ferromagnetic materials retain their magnetic properties up to a certain temperature known as the Curie temperature. When heated above the Curie temperature, they lose their ferromagnetic properties and become paramagnetic. However, the statement as presented implies that all heating causes ferromagnetic substances to become paramagnetic, which is not accurate. They only undergo this change when heated beyond their specific Curie temperature.

Option D: In antiferromagnetism, domains are oppositely oriented and cancel out each other’s magnetic moment.
This statement is correct. In antiferromagnetic materials, adjacent atomic magnetic moments align in opposite directions, effectively canceling each other out and resulting in no net macroscopic magnetization.

Therefore, Option C is the incorrect statement among the given choices.