Ferro, ferri and antiferromagnetism

These three types all have atoms with permanent magnetic moments that interact strongly through the exchange interaction. They differ in how the magnetic moment vectors of neighbouring atoms line up.

 Ferromagnetic       ^  ^  ^  ^  ^   all parallel
 Antiferromagnetic   ^  v  ^  v  ^   antiparallel, equal
 Ferrimagnetic       ^  v  ^  v  ^   antiparallel, unequal
                    (long ^, short v)

Ferromagnetism

• Positive exchange interaction makes all neighbouring moment vectors parallel.
• Their sum gives a large spontaneous magnetization inside each domain; susceptibility $\chi \approx 10^2$–$10^5$.
• Shows hysteresis and saturation. Above the Curie temperature it becomes paramagnetic: $\chi = C/(T - T_C)$.
• Examples: Fe, Co, Ni.

Antiferromagnetism

• Negative exchange interaction makes neighbouring moment vectors antiparallel and equal in size.
• The two sublattice magnetizations cancel, so net magnetization is zero and $\chi$ is small and positive.
• $\chi$ increases with temperature up to the Néel temperature $T_N$, then decreases: $\chi = C/(T + \theta)$.
• Examples: MnO, FeO, NiO, Cr.

Ferrimagnetism

• Two sublattices A and B with moment vectors antiparallel but unequal ($M_A > M_B$).
• Net magnetization $M = M_A - M_B \ne 0$, so a spontaneous magnetization, domains and hysteresis exist, as in ferromagnets but smaller.
• They are ceramic oxides with very high resistivity, so eddy-current loss is low.
• Example: magnetite Fe₃O₄ (Fe³⁺ on A and B sites cancel; the Fe²⁺ moments remain), MnZn and NiZn ferrites; used in high-frequency cores.