The overview classification of magnetic materials describes whether magnetization opposes, follows, or can persist without an applied field. The microscopic question is why different electronic systems produce those responses.

Diamagnetism arises when an applied magnetic field changes orbital motion so that the induced current produces a magnetic moment opposing the field. It does not require permanent atomic moments and gives a small negative susceptibility.

In a material containing localized permanent moments, an external field lowers the energy of moments aligned with it. Thermal agitation competes with this alignment. For independent moments in the classical high-temperature regime, the resulting Curie paramagnetism has approximately $$\chi=\frac{C}{T},$$ where the Curie constant $C$ depends on moment density and magnitude.

Conduction electrons provide a different quantum mechanism. In a degenerate Fermi gas, a magnetic field slightly redistributes spin-up and spin-down occupations near the Fermi surface. This Pauli paramagnetism is much less temperature-dependent than Curie behavior because only states near the Fermi energy can change occupancy.

Orbital and spin contributions can coexist, and real measured susceptibility can be their sum together with interaction effects. A positive susceptibility therefore does not by itself imply localized atomic moments.

Ferromagnetism requires an additional ingredient beyond independent response: interactions between moments or itinerant electrons can make an ordered state energetically favorable. Exchange interactions and collective spin models provide the next layer of that explanation.