Dia / Para / Ferromagnetic

Three classes of magnetic materials, distinguished by χ (susceptibility):

DIAMAGNETIC (χ < 0, small): Bi, Cu, Ag, Au, water, most non-metals. Repelled weakly by magnetic fields. Property of ALL matter; usually masked by para- or ferro-magnetism.

PARAMAGNETIC (χ > 0, small ~10⁻³): Al, Pt, Mn, Cr, O₂. Attracted weakly. Follows Curie's law: χ = C/T.

FERROMAGNETIC (χ huge, ~10³-10⁵): Fe, Co, Ni, Gd, and many alloys. Spontaneous magnetization persists without external field. Exists below Curie temperature T_C.

ANTIFERROMAGNETIC: adjacent spins anti-align (Cr, MnO). Net M ≈ 0 below Néel temperature. Special class.

FERRIMAGNETIC (ferrites): Fe₃O₄, MnFe₂O₄. Sublattice spins anti-align but unequal ⇒ net M ≠ 0. Used in transformers due to high resistivity (low eddy currents).

Soft magnets: easy to magnetize/demagnetize (electromagnets, transformer cores).

Hard magnets: retain magnetization (permanent magnets: alnico, neodymium NdFeB).