Diamagnetic materials create an
induced magnetic field in a direction opposite to an externally
applied magnetic field, and are repelled by the applied magnetic field. In contrast, the opposite behavior is exhibited by
paramagnetic materials.
Diamagnetism is a
quantum mechanical effect that occurs in all materials; when it is the only contribution to the magnetism the material is called a
diamagnet. Unlike a
ferromagnet, a diamagnet is not a permanent magnet. Its
magnetic permeability is less than µ
0, the permeability of vacuum. In most materials diamagnetism is a weak effect, but a
superconductor repels the magnetic field entirely, apart from a thin layer at the surface.