Classification of magnetic materials by magnetization

Magnetic materials are classified by how they magnetize in an applied field $H$, i.e. by the magnetization $M = \chi H$ and the arrangement of atomic magnetic moments.

1. Diamagnetism

• Atoms have no net permanent moment (all electrons paired).
• An applied field changes electron orbital motion (Lenz's law) and induces a small moment opposing the field, so $\chi < 0$ and $\mu_r$ slightly less than 1.
• Independent of temperature. Diamagnetic materials are repelled by a magnet. Superconductors are perfect diamagnets ($\chi = -1$).

2. Paramagnetism

• Atoms have permanent moments (unpaired electrons) but they do not interact; thermal motion keeps them random.
• A field partly aligns them, giving a small positive $M$.
• Obeys Curie's law: $\chi = C/T$. Weakly attracted by a magnet.

3. Ferromagnetism

• Strong exchange interaction aligns neighbouring moments parallel, forming domains with spontaneous magnetization.
• Very large $\chi$, hysteresis, saturation, retentivity.
• Above the Curie temperature $T_C$ it becomes paramagnetic: $\chi = \dfrac{C}{T - T_C}$. Fe: $T_C = 770$ °C.

4. Antiferromagnetism

• Exchange interaction aligns neighbouring moments antiparallel and of equal size, so net $M \approx 0$.
• $\chi$ is small and rises with $T$ up to the Néel temperature $T_N$, then falls as paramagnetic: $\chi = \dfrac{C}{T + \theta}$.

5. Ferrimagnetism

• Two sublattices with antiparallel but unequal moments, so a net spontaneous magnetization remains.
• Domains and hysteresis like ferromagnets, but with high electrical resistivity (they are ceramics), giving low eddy-current loss.
• Examples: magnetite Fe₃O₄, MnZn and NiZn ferrites; used in high-frequency transformer cores.

 Para:   ^ / -> v \   random
 Ferro:  ^ ^ ^ ^ ^    parallel
 Anti:   ^ v ^ v ^    antiparallel equal
 Ferri:  ^ v ^ v ^    antiparallel unequal
         (big ^, small v)