Diamagnetism

By susceptibility: diamagnetic if χ is negative (−1 ≤ χ < 0, 0 ≤ μᵣ < 1, μ < μ₀), paramagnetic if χ is small and positive (μᵣ just above 1, μ > μ₀), ferromagnetic if χ ≫ 1 (μᵣ ≫ 1, μ ≫ μ₀).

A diamagnetic substance tends to move from the stronger to the weaker part of a field: a magnet repels it. In a field, the lines are pushed out of it and the field inside is reduced, usually by about one part in 10⁵.

Explanation: in a diamagnetic atom the orbital moments of the electrons cancel. An applied field slows the electrons whose moments lie along it and speeds up those opposing it (an induced effect, following Lenz's law), leaving a net moment opposite to the field, hence the repulsion.

Examples: water, sodium chloride, nitrogen at STP, and the solids bismuth, lead, copper and silicon.

Diamagnetism is present in every substance, but it is so weak that para- or ferromagnetism usually hides it.

Superconductors, metals cooled to very low temperatures, are perfect conductors and perfect diamagnets: the field is expelled completely, χ = −1 and μᵣ = 0. This is the Meissner effect. A superconductor repels a magnet and is repelled by it; superconducting magnets can be used for magnetically levitated fast trains.

The two opposite behaviours come from a tiny difference in χ: about −10⁻⁵ for diamagnetic materials against about +10⁻⁵ for paramagnetic ones.