diamagnetisme

Diamagnetism is a form of magnetism in which a material develops an induced magnetic moment that opposes an externally applied magnetic field, causing a weak repulsion from the field. It is a universal property of matter, but in most materials it is very weak and often masked by stronger magnetic effects such as paramagnetism or ferromagnetism. The response is described by the magnetic susceptibility χ, where magnetization M = χH; for diamagnetic materials χ is negative (χ < 0).

The effect arises from changes in the orbital motion of electrons when a field is applied. The induced currents generate a magnetic field that opposes the external field, in accordance with Lenz’s law. Quantum mechanically, diamagnetism can be associated with the energy shifts of occupied electron orbitals; it does not require unpaired spins and is present in many nonmagnetic materials.

Characteristics and examples: Diamagnetism is typically very weak, with χ values on the order of 10^-5 to 10^-3 and often roughly temperature independent. It is observed in a wide range of substances, including water, copper, quartz, and carbon-based materials such as graphite. Bismuth is among the strongest natural diamagnets. By contrast, in materials with paramagnetic or ferromagnetic order, their magnetic responses dominate at typical conditions.

Special case: superconductors exhibit a related phenomenon known as the Meissner effect, which expels magnetic fields from their interior and is often described as perfect diamagnetism. This quantum effect is distinct from ordinary diamagnetism but shares the characteristic of opposing applied magnetic fields.