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Hard magnets; dia, para and ferromagnetism

Short answer

Hard magnets retain strong magnetism due to high retentivity, coercivity, and (BH)max, resisting demagnetization. Diamagnetic materials (e.g., Cu, Bi) have negative small susceptibility, weakly repelled; paramagnetic (e.g., Al, Pt) have small positive χ, weakly attracted; ferromagnetic (e.g., Fe, Co, Ni) have very large positive χ, strong attraction, and hysteresis.

Why hard magnetic material is preferred for making permanent magnet? Explain the differences between diamagnetic, paramagnetic and ferromagnetic materials with their suitable examples.

Why hard magnetic material for permanent magnets

A permanent magnet must keep strong magnetism for years without a supply. Hard materials (Alnico, NdFeB, SmCo, hard ferrite) have:

  • High retentivity BrB_r: strong field left after magnetizing.
  • High coercivity HcH_c: stray fields, vibration and temperature changes cannot easily demagnetize them, because domain walls are pinned by fine precipitates and defects.
  • Large (BH)max(BH)_{max}: more energy stored per unit volume, so a smaller magnet does the job.

Diamagnetic vs paramagnetic vs ferromagnetic

PropertyDiamagneticParamagneticFerromagnetic
Permanent atomic momentNone (paired electrons)Present, randomPresent, aligned in domains
Susceptibility χ\chiSmall, negative (~−10−5-10^{-5})Small, positive (10−510^{-5}–10−310^{-3})Very large, positive (10210^2–10510^5)
Relative permeability μr\mu_rSlightly < 1Slightly > 1Very large (100–100000)
Behaviour in fieldWeakly repelledWeakly attractedStrongly attracted
Field linesPushed outSlightly drawn inStrongly concentrated
Effect of temperatureIndependentχ=C/T\chi = C/T (Curie law)χ=C/(T−TC)\chi = C/(T - T_C) above TCT_C
HysteresisNoNoYes
Magnetization after field removedNoneNoneRemains (retentivity)
ExamplesCu, Bi, Ag, Au, water, graphiteAl, Pt, Mn, O₂, CuSO₄Fe, Co, Ni, Gd
  Dia: lines bend away      Para/Ferro: lines drawn in
  --- \   /---              ---\     /---
  ---- ( ) ----             ----(===)----
  --- /   \---              ---/     \---

Mechanisms in brief

  • Diamagnetism: the field alters electron orbits (Lenz's law) and induces a moment opposite to the field; present in all materials but masked if others exist.
  • Paramagnetism: unpaired spins partially align with the field against thermal agitation.
  • Ferromagnetism: exchange interaction aligns spins parallel within domains; the field just rotates/grows domains, giving very high magnetization.

✓ Answer checked by Claude Opus 5.5 (high effort)

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