Para, ferro, antiferro and ferrimagnetism

These four classes all have atoms with permanent magnetic moments (unpaired electron spins); they differ in how the neighbouring moments interact and line up.

 Paramagnetic      Ferromagnetic
  ^  /  ->  \       ^  ^  ^  ^
  v  <-  /  ^       ^  ^  ^  ^
 (random)          (parallel)

 Antiferromagnetic Ferrimagnetic
  ^  v  ^  v        ^  v  ^  v
  v  ^  v  ^        (big up, small down)
 (equal, opposite) (unequal, opposite)

Paramagnetism

• Moments do not interact, so thermal agitation keeps them random; $M = 0$ without a field.
• A field partly aligns them, giving small positive $\chi$ ($10^{-5}$–$10^{-3}$), $\mu_r$ slightly above 1.
• Curie law: $\chi = C/T$. Examples: Al, Pt, O₂, CuSO₄.

Ferromagnetism

• Strong positive exchange interaction makes neighbouring moments parallel; spontaneous magnetization in domains.
• Very large $\chi$ ($10^2$–$10^5$), hysteresis, saturation.
• Above Curie temperature it becomes paramagnetic: $\chi = C/(T - T_C)$. Examples: Fe, Co, Ni.

Antiferromagnetism

• Negative exchange interaction makes neighbouring moments antiparallel and equal, so net $M = 0$.
• Small positive $\chi$, which increases with $T$ up to the Néel temperature $T_N$, then falls: $\chi = C/(T + \theta)$ above $T_N$.
• Examples: MnO, FeO, NiO, Cr.

Ferrimagnetism

• Two sublattices with antiparallel but unequal moments; the difference gives a net spontaneous magnetization.
• Large $\chi$, domains and hysteresis like ferromagnets, but very high resistivity (ceramic), so low eddy-current loss.
• Examples: Fe₃O₄ (magnetite), MnZn and NiZn ferrites, YIG.