Diamagnetism: The Quiet Repulsion
Imagine you bring a magnet near a piece of copper or water. Nothing dramatic happens — no strong pull like iron, no noticeable push. But if you had a very sensitive scale, you'd detect a tiny repulsion. The material is pushed away, ever so slightly. That's diamagnetism.
Why does this happen? The key is that in a diamagnetic material, every atom has zero net magnetic moment to begin with. All its electrons are paired up — each electron's tiny magnetic field is cancelled by its partner. The atom is magnetically neutral.
Now bring an external magnetic field. Something subtle occurs: the field slightly distorts the motion of the orbiting electrons. According to Lenz's law, the electrons respond by creating a tiny induced magnetic moment that opposes the applied field. This is like a weak electromagnetic shield — the material generates its own field pointing opposite to the external one.
This induced moment is temporary. Remove the external field, and the material returns to being magnetically neutral. No permanent magnetism remains.
The result? The material is weakly repelled. The strength of this repulsion is measured by magnetic susceptibility χ, which for diamagnets is small and negative — typically around −10−5 to −10−6.
The Precise Statement
Diamagnetism is the property of materials whose atoms have no permanent magnetic dipole moment. When placed in an external magnetic field B, the orbital motion of electrons induces a magnetic moment m opposite to B, leading to a negative magnetic susceptibility χ<0.
m=−6mee2⟨r2⟩B
where e is electron charge, me its mass, and ⟨r2⟩ the mean square orbital radius.
The susceptibility is:
- Small in magnitude (∣χ∣∼10−5)
- Negative (repulsion)
- Independent of temperature — because it arises from orbital motion, not from thermal alignment of permanent moments
Unlike paramagnets and ferromagnets, diamagnetism is present in all materials. But it's usually masked by stronger magnetic effects. Only in materials with no unpaired electrons (like copper, bismuth, water, and most organic compounds) does diamagnetism become the dominant response.
Common Examples
| Material | χ (approx.) | Behaviour |
|---|
| Bismuth | −1.66×10−4 | Strongest diamagnet among elements |
| Copper | −9.7×10−6 | Weakly repelled |
| Water | −9.0×10−6 | Pure water is diamagnetic |
| Graphite | −1.6×10−4 | Anisotropic — stronger along certain axes |