Diamagnetism Paramagnetism Ferromagnetism Easy Explanation

All materials respond to a magnetic field, but in three very different ways. Diamagnetic materials are weakly repelled, paramagnetic materials are weakly attracted and align with the field only while it is present, and ferromagnetic materials are strongly attracted and can stay magnetised after the field is removed.

This classification explains everyday facts: why a fridge magnet clings to iron but not to wood, why some metals are useful for permanent magnets and transformer cores, and why MRI scanners demand careful screening of ferromagnetic objects. It all comes down to atomic magnetic moments and how they cooperate.

Key Concepts

Diamagnetism: induced atomic currents oppose the applied field; susceptibility χ is small and negative; present in all materials but masked by stronger effects. Examples: copper, silver, water, bismuth. Paramagnetism: atoms have permanent moments that partially align with the field against thermal agitation; χ is small and positive and falls with temperature (Curie's law). Examples: aluminium, platinum, oxygen. Ferromagnetism: neighbouring moments lock parallel in regions called domains; χ is very large and positive, with hysteresis and permanent magnetisation. Examples: iron, cobalt, nickel and their alloys. Saturation: field beyond which all domains are aligned and magnetisation stops growing.

How It Works

1. Start with electron motion: orbiting and spinning electrons act as tiny current loops with magnetic moments.
2. In diamagnetic atoms the moments cancel in pairs; an applied field induces a slight opposing change, so the material is pushed toward weaker field.
3. In paramagnetic atoms some moments remain unpaired; the field lines them up partially, but heat keeps disturbing them, so attraction is weak and temporary.
4. In ferromagnetic materials, exchange interaction makes whole domains of moments point together; the field grows domains that favour it, giving huge magnetisation.
5. Remove the field: diamagnetic and paramagnetic responses vanish, while ferromagnets retain magnetisation depending on their hysteresis loop — soft iron loses it easily, hard steel keeps it.

Heating a ferromagnet above its Curie temperature (for iron, about 770 °C) destroys domain order and it becomes merely paramagnetic — a direct proof that thermal motion fights magnetic alignment.

Worked Example

Three samples are hung one by one between the poles of a strong electromagnet: bismuth, aluminium and a soft-iron rod. Predict and rank the observed forces qualitatively using susceptibility signs and sizes.

1. Bismuth (diamagnetic, χ ≈ −1.7 × 10⁻⁴): moves slightly out of the strong-field region — weak repulsion.
2. Aluminium (paramagnetic, χ ≈ +2 × 10⁻⁵): drawn slightly into the field — weak attraction, smaller than bismuth's effect in magnitude of response observed.
3. Soft iron (ferromagnetic, χ of order hundreds): pulled in violently and clamps to a pole — attraction orders of magnitude larger.

The ranking is iron ≫ bismuth (repelled) and aluminium (attracted), matching the three classes and showing ferromagnetism dominating by factors of a million or more.

Common Misconceptions

• Only iron-like metals are magnetic at all: Every material shows at least diamagnetism; the effect is just very weak in most.
• Paramagnets become permanent magnets: Their alignment disappears when the field is removed because thermal motion randomises the moments.
• Alloys of non-magnetic metals cannot be ferromagnetic: Certain alloys of manganese with non-ferromagnetic elements are ferromagnetic; behaviour depends on atomic structure, not just the ingredient list.

Real-World Applications

• Ferromagnetic cores in transformers, motors and electromagnets; permanent magnets in speakers and motors.
• Paramagnetic oxygen analysers and temperature-dependent magnetic sensors.
• Diamagnetic levitation demonstrations and magnetic shielding considerations in precision instruments.

Diamagnetism weakly opposes, paramagnetism weakly follows, and ferromagnetism strongly locks onto a magnetic field; the behaviour of every magnetic material you meet is one of these three responses.