Magnetic Properties of Solids

Magnetic behaviour depends on the presence and arrangement of unpaired electrons.

Type	Electron config	Alignment	Behaviour	Examples
Diamagnetic	All paired	None	Weakly repelled	NaCl, $C_{6}H_{6}$ , $H_{2}O$
Paramagnetic	Unpaired, random	Random (aligns with field)	Weakly attracted	$O_{2}$ , $Cu^{2+}$ , $Fe^{3+}$
Ferromagnetic	Unpaired, domain-aligned	All parallel	Strongly attracted	Fe, Co, Ni
Antiferromagnetic	Unpaired, antiparallel equal	Cancel	Weakly attracted/neutral	MnO, Mn $O_{2}$
Ferrimagnetic	Unpaired, antiparallel unequal	Net non-zero	Moderately attracted	$Fe_{3}O_{4}$ , ferrites

Ferromagnetic: Domains (regions of aligned moments) → permanent magnets. Above Curie temperature, domains disorient → becomes paramagnetic.

$Fe_{3}O_{4}$ (Magnetite):

Contains $Fe^{2+}$ (4 unpaired d-electrons) + $Fe^{3+}$ (5 unpaired d-electrons) in different sublattices
Antiparallel but unequal → net magnetic moment → ferrimagnetic
Natural permanent magnet (loadstone)

Antiferromagnetic: Adjacent domains exactly antiparallel → zero net moment. Above Néel temperature, disorder sets in → paramagnetic.

Paramagnetic: Curie law: magnetic susceptibility χ ∝ 1/T (decreases as temperature rises).