diamagnet

A diamagnet, or diamagnetic material, is a substance that develops a weak magnetic moment in opposition to an externally applied magnetic field, causing a repulsive interaction. In bulk terms, its magnetization M is proportional to the external field H with a negative magnetic susceptibility χm (χm < 0). The effect is typically very small, with susceptibilities on the order of 10^-5 to 10^-4 in magnitude.

Mechanism and characteristics: Diamagnetism arises from the motion of electrons in atoms. When a magnetic field is applied, induced currents in closed electron orbits create a magnetic field that opposes the applied one, in accordance with Lenz’s law. This response is present in all materials to some degree but is usually masked by stronger magnetic effects such as paramagnetism or ferromagnetism. Diamagnetism is largely temperature independent and linear with the field in ordinary conditions.

Materials and examples: All materials exhibit some diamagnetic response, but in many substances it is weak compared with other magnetic effects. Notable diamagnets include bismuth, copper, quartz, water, graphite, and many organic compounds. Graphite, for example, shows anisotropic diamagnetism. In practice, a material is often described as diamagnetic if its net susceptibility is negative in the measured range, even if other magnetic contributions exist.

Observations and related phenomena: Diamagnetism can lead to weak repulsion from a magnetic field gradient, observable with suitable instruments or demonstrations. A extreme and well-known case of diamagnetism is the Meissner effect in superconductors, which expels magnetic fields completely and represents perfect diamagnetism. Diamagnets are used in a variety of scientific contexts and are contrasted with paramagnets and ferromagnets.