Though essentially present in all substances, diamagnetism is a much weaker effect and therefore, often suppressed by relatively strong opposite effects, like paramagnetism. This effect is exclusively observed in those solids which do not carry any permanent, intrinsic magnetic (dipole) moment i.e., does not contain unpaired valence electrons. The existence of a very small non-zero magnetic moments for these materials is attributed to the orbital motion of electrons.
Why is then that in absence of an external magnetic field, the macroscopic magnetization is zero? How do we understand this from classical Langevin theory and quantum theory?