Diamagnetism

Diamagnetism is the universal, always-present magnetic response of electrons in orbital motion. Apply a magnetic field to any electron orbit and, by Lenz's law, the orbit adjusts: electrons moving one way around speed up slightly, electrons moving the opposite way slow down. The net change produces a tiny magnetic moment pointing against the applied field. Every atom in every material does this. The susceptibility is small and negative, typically χ ≈ −10⁻⁵.

In most materials diamagnetism is hidden. If the atoms carry permanent magnetic moments — unpaired electron spins or uncompensated orbital angular momenta — then the much larger paramagnetic (or ferromagnetic) response from moment-alignment swamps the diamagnetic counter-current. Diamagnetism becomes visible only in materials whose atoms have fully closed shells: noble gases, water, many organic molecules, copper, gold, bismuth, superconductors. Bismuth is the strongest ordinary diamagnet (χ ≈ −1.7 × 10⁻⁴); water is weakly diamagnetic (χ ≈ −9 × 10⁻⁶), just enough that a levitating-frog demonstration in a 16 T Bitter magnet famously worked in Nijmegen in 1997.

Superconductors are the extreme case: they exhibit perfect diamagnetism, χ = −1, expelling applied flux entirely from their interiors (the Meißner effect). This is qualitatively different from ordinary diamagnetism — an ordinary diamagnet merely reduces the field inside, while a superconductor drives it to zero — but both share the same sign: a magnetization opposing the applied field, a material that weakly (or, in the superconducting case, perfectly) pushes magnetic fields away. The modern quantum picture comes from Landau's diamagnetism theory (1930), which derives the effect from the quantisation of electron orbits in a magnetic field into Landau levels.

Diamagnetism was discovered by Michael Faraday in 1845, when he showed that a piece of bismuth placed between the poles of a strong electromagnet oriented itself perpendicular to the field — the opposite of what iron filings do. He coined the word "diamagnetic" (from Greek dia-, "across") to distinguish this behaviour from "paramagnetic" alignment with the field. Langevin gave the first classical theory in 1905; Landau gave the quantum theory in 1930.