Explain the cause of magnetism in a substance.

(N/A) The origin of magnetic properties in a substance is due to two types of motions of electrons:
$(i)$ Orbital motion around the nucleus.
$(ii)$ Spin motion around its own axis.
An electron in an atom behaves like a tiny magnet. Since an electron is a charged particle and undergoes these motions,it can be considered as a small loop of current which possesses a magnetic moment.
Each electron thus has a permanent spin and an orbital magnetic moment associated with it. The magnitude of the magnetic moment is very small and is measured in the unit called Bohr magneton,$\mu_{B}$,which is equal to $9.27 \times 10^{-24} \text{ A m}^{2}$.
On the basis of their magnetic properties,substances are classified as:
$(i)$ Paramagnetic
$(ii)$ Diamagnetic
$(iii)$ Ferromagnetic
$(iv)$ Antiferromagnetic
$(v)$ Ferrimagnetic