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Diamagnetism is a magnetic state where an applied magnetic field creates an induced magnetic field in them in the opposite direction, causing a repulsive force.
Diamagnetism is a quantum mechanical effect that occurs in all materials; when it is the only contribution to the magnetism, the material is called diamagnetic. In paramagnetic and ferromagnetic substances, the weak diamagnetic force is overcome by the attractive force of magnetic dipoles in the material. The magnetic permeability of diamagnetic materials is less than the permeability of vacuum, μ0. In most materials, diamagnetism is a weak effect which can only be detected by sensitive laboratory instruments, but a superconductor acts as a strong diamagnet because it repels a magnetic field entirely from its interior.
Superconductors may be considered perfect diamagnets (χv = −1), because they expel all magnetic fields (except in a thin surface layer) due to the Meissner effect.
Of all the metals, bismuth is the most diamagnetic, with only pyrolitic carbon and superconducting materials being more diamagnetic.