diamagnetism

Diamagnetism is a form of magnetism in which a material develops an induced magnetic moment in a direction opposite to an external magnetic field. The resulting interaction is repulsive, and the effect is true for all materials, though it is typically very weak and often masked by stronger magnetic responses such as paramagnetism or ferromagnetism.

Microscopically, diamagnetism arises from the orbital motion of electrons. When an external field is applied, electrons adjust their orbital motion so as to oppose the change in flux, producing tiny circulating currents that generate a magnetic field in the opposite direction. This response occurs even in materials with all electrons paired and no net magnetic moment; it is commonly described using Landau diamagnetism for conduction electrons and core-electron diamagnetism for bound electrons.

As a bulk property, the magnetic susceptibility χ is negative for diamagnetic materials and very small in magnitude. In SI units the volume susceptibility χv is typically between −10^−6 and −10^−4, meaning M = χH with magnetization opposing the applied field. The diamagnetic response is largely temperature independent compared with paramagnetism, though some small temperature effects can arise from electronic structure. Common diamagnetic substances include bismuth, graphite, copper, quartz, and water.

Superconductors are sometimes described as perfect diamagnets because they expel magnetic fields from their interior via the Meissner effect; in that context the effective susceptibility approaches −1, far beyond ordinary diamagnetism. Diamagnetism is a baseline magnetic response observed across materials, though its practical impact is typically limited by stronger magnetic phenomena.