Magnetism in Solids: Five Types Explained in Simple Words

Every material responds to a magnetic field, but not in the same way — and the differences reveal what its electrons are doing. Physicists sort magnetic behaviour into five types: diamagnetism, paramagnetism, ferromagnetism, antiferromagnetism and ferrimagnetism. The first two are weak and universal-ish; the last three involve the permanent atomic magnetic moments lining up in different patterns, producing everything from fridge magnets to the read heads of hard disks.

This classification matters because magnetic technology is built on choosing the right type: soft ferromagnets for transformer cores, hard ones for permanent magnets, ferrimagnetic oxides for high-frequency inductors, and antiferromagnets for the newest spintronic memory. One wrong choice and a transformer would waste energy as heat or a compass would not point north.

Key Concepts

• Diamagnetism: weak repulsion (χ ≈ −10⁻⁶ to −10⁻⁵). Orbital electrons adjust to oppose the field. Present in all materials; visible only when stronger types are absent. Examples: copper, water, bismuth.
• Paramagnetism: weak attraction (χ ≈ 10⁻⁴ to 10⁻³) from unpaired electrons aligning with the field; destroyed by thermal jiggling, following Curie's law χ = C/T. Examples: aluminium, oxygen.
• Ferromagnetism: spontaneous alignment of moments below the Curie temperature T_c, giving permanent magnets. Examples: iron (T_c = 1043 K), cobalt, nickel.
• Antiferromagnetism: neighbouring moments point in opposite directions and cancel exactly, so there is no net magnetisation below the Néel temperature T_N. Example: MnO (T_N = 116 K).
• Ferrimagnetism: opposite alignment but with unequal moments, leaving a net magnetisation. Example: magnetite Fe₃O₄, the original lodestone.

Formulas

• Magnetic susceptibility: χ = M / H (dimensionless in SI), where M is the magnetisation (A/m) and H the applied field (A/m). Its sign and size identify the type.
• Curie law (paramagnets): χ = C / T, with C the Curie constant. Susceptibility falls as temperature rises because heat randomises the moments.
• Curie–Weiss law: χ = C / (T − T_c) for ferromagnets above T_c; the divergence at T_c marks the onset of spontaneous order.

Worked Example

A paramagnetic salt has χ = 2.0×10⁻⁴ at room temperature. It sits in a field H = 8.0×10⁵ A/m (about 1 tesla). Find its magnetisation.

1. M = χ·H = 2.0×10⁻⁴ × 8.0×10⁵ = 160 A/m.
2. For comparison, saturated iron reaches about 1.7×10⁶ A/m — ten thousand times larger — showing how feeble paramagnetism is.
3. Cooling to half the temperature would double χ (Curie law) and give M = 320 A/m, still tiny next to a ferromagnet.

Common Misconceptions

• "Only iron is magnetic." All matter is at least diamagnetic; iron is just the famous strong case. Even water is repelled (very weakly) by magnets.
• "Antiferromagnets are non-magnetic." Their atomic moments are large and perfectly ordered — they simply cancel out, and they still show a sharp susceptibility kink at T_N.
• "Heating a magnet destroys it forever." Heating past T_c only disorders the moments; cooling back down restores the magnetism (though the domain pattern may differ).

Real-World Applications

• Permanent magnets: hard ferromagnets like neodymium-iron-boron drive motors, headphones and wind turbines.
• Transformer cores: soft ferromagnets (silicon steel) magnetise and demagnetise easily, minimising energy loss at 50 Hz.
• Spintronics: antiferromagnetic and ferrimagnetic layers pin the reference direction in hard-disk read heads and MRAM bits.

The five magnetic types run from universal weak diamagnetism, through Curie's-law paramagnetism, to the three ordered states — ferromagnetic, antiferromagnetic and ferrimagnetic — distinguished by how atomic moments align. Susceptibility χ = M/H and its temperature dependence identify each, guiding the choice of material for every magnetic device.