Ferromagnetism, paramagnetism and diamagnetism

Ferromagnetism

The property of some materials to show strong, spontaneous magnetization even without an external field, because the strong exchange interaction aligns atomic magnetic moments parallel within domains. They have very large positive susceptibility ($\chi \approx 10^2$–$10^5$), show hysteresis and saturation, and become paramagnetic above the Curie temperature, $\chi = C/(T - T_C)$. Examples: iron, cobalt, nickel, gadolinium, Alnico.

Paramagnetism

The property of materials whose atoms have permanent magnetic moments (unpaired electrons) that are randomly oriented and do not interact. An applied field partly aligns them, giving a weak magnetization in the field direction. Susceptibility is small and positive ($10^{-5}$–$10^{-3}$) and follows Curie's law $\chi = C/T$. They are weakly attracted by a magnet and lose magnetization when the field is removed. Examples: aluminium, platinum, manganese, oxygen, CuSO₄.

Diamagnetism

The property of materials whose atoms have no permanent magnetic moment (all electrons paired). An applied field changes the orbital motion of electrons and, by Lenz's law, induces a small moment opposing the field. Susceptibility is small and negative ($\approx -10^{-5}$), independent of temperature, and $\mu_r$ is slightly less than 1. They are weakly repelled by a magnet. Examples: copper, bismuth, silver, gold, water, graphite; superconductors are perfect diamagnets ($\chi = -1$).